That depends, really. If you have an inversion layer going, you can get pretty poor air quality in lots of locations because people use crappy wood-fired stoves that produce a lot of particulates.
That does happen occasionally where I live, last night for example. But it's only a single digit number of days out of a year. The rest of the time air quality is very good.
To be fair, "cities are the exception with bad air" neglects the reality that cities are the default for where people live. So the "air near people" is generally city air.
That said, I live in SF and my AQI is usually <50. Not as great as 5, but we sometimes get down to single-digits. Cities don't have to have bad air.
The filtration is icing on the cake. You want an HRV or ERV in any location where you want heating or cooling for any non-negligible portion of the year so that you can have energy efficient fresh air.
I live in small town on NZ coast. Air is very clean. Mosquitos and neighbor's wood burners don't care. ERV is top 3 item in my house (other than induction cooktop and Japanese toilets).
- auto lid open and close
- heated seat
- water jet to clean buttocks with subsequent warm air blower
- relaxing music speaker (to drown out your defecating sounds and get you started)
just some ideas what some (public)/most (nice home) japanese toilets offer, which might be hard to come by anywhere else
The ERV itself is for removing CO2, not dust. Especially in cold climates where houses are built to be as air-tight as possible, these are a necessity. Even if you lived in a forest cabin you'd want filters to prevent too much dust and pollen. I've got a dual filter on mine, HEPA and activated carbon filter. The HEPA filter removes dust and pollen. I've found that if I don't use the carbon filter I get higher than recommended levels of NOx.
You can add a filter to the TW4, there is an adapter/kit. It's a hepa filter from a car cabin filter system design. You rarely need both anti pollen and also ERV, so you would take the heat exchanger out and just use the filter and fan. As for dust, I recommend a good pc fan filter based appliance, not putting the filter in the flow path of the ERV.
Most people here just open a window when they want to air out a room. That said, that does waste a lot of energy when heating/cooling your home on cold/hot days, so ERVs and HRVs are used to get the clean air in without exchanging heat with the outside world too much. They're quite cheap and effective compared to just running normal ventilation.
I live near a ten lane highway. I prefer to keep the outdoor air out, and the indoor air filtered. I could see this being very useful for other dwellings though.
You are either already failing to keep outdoor air out or you are getting too little ventilation. What you want is something like this device with the H13 filter option. Maybe that plus carbon.
Think of it this way, since you aren't dying of asphyxiation when you sleep at night, you're already getting some air exchange with the outside air.
With an ERV you can control how much air you get, and put whatever filter you want in between the outside air and your room.
In fact the fancier systems will let you overpressure your house by running the fan pushing in slightly higher than the air pushing out, ensure air only leaks out of your house not in.
People breathe, so you need outdoor air to replenish the oxygen and get rid of the carbon dioxide. That's "fresh" air.
Unfortunately, outdoor air has particulate and ozone pollution. Filtering it gives you "clean" air.
In winter and summer, you also heat or cool the indoor air for comfort. If you just pump in outside air, you effectively also pump out the indoor air. This wastes the energy that had gone into heating or cooling it.
These systems save that energy by transfering heat between the air that's getting pumped in and the air that's getting pumped out.
The way I like to describe HRV/ERVs to people who don't know about them is:
"Imagine you could open a window to get fresh air into your house and stale air out, but when you did so, most of the heat/humidity would stay in during the winter, or stay out during the summer, leaving you just the fresh air".
In terms of the effect on environment inside a house, I usually say:
"Imagine it's always a fresh-air spring day inside your house"
This is a great project. One problem with ERV/HRV systems right now is that they are very expensive niche products. While this system doesn't achieve the extremely high heat recovery efficiencies of counter-flow units, the perfect is the enemy of the good, and this seems like it could be orders of magnitude cheaper.
Counter-flow heat exchangers. A parallel-flow heat exchanger would result in the average, as you say; but a counter-flow exchanger means that as the formerly-warm air gets progressively cooler, it is exposed to progressively colder air.
I've got a counter-flow heat exchanger, but it looks like they're using a different design:
> Each OpenERV TW4 module has a very quiet pair of fans, pointed in opposite directions, and a heat exchanger in a 6 inch pipe, that goes through a wall. The hot, polluted air from inside goes out for 30 seconds, and the heat from it is stored in the heat exchanger.
> Then, the fan reverses direction, moving clean air from outdoors to the indoors. On it's way in, it picks up that heat from the heat exchanger. This type of heat exchanger is called a regenerative heat exchanger, or less commonly, a regenerator. The kind shown in the video is a recuperative type, not regenerative. Recuperative types are what most people think of, consisting of a thin layer of material that separates two gas streams. Regenerative heat exchangers are different. They briefly store the energy while air flows in one direction, then release it when the air flow reverses.
> The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss.
the intuition: if the 2 colums flow in the same direction, the final temp is the average. but if the 2 columns flow in the oposite directions it is posible to fully exchange the temperature
The direction of flow is reversed every 30s. The cycle is short enough that due to the thermal mass there is thermal gradient within the heat-exchanger. So it effectively works as counter-flow heat exchanger. Same principle (but continuous flow) is used in rotor ERV: https://en.wikipedia.org/wiki/Heat_recovery_ventilation#Ther...
Heat exchanger there is usually an extruded ceramic grid (ERV) or rolled corrugated aluminum (functions closer to HRV than ERV)
Then it just means that you're overventilating! We emit much more steam (through cooking and showering for instance) than we consume oxygen/emit CO2 so controlling the humidity is the main purpose of ventilation, air renewal comes for free as a byproduct of that.
My system doesn’t over ventilate - we sometimes have too high CO2 in the bed rooms, but humidity is way too low in winters, sometimes below 30%. That means getting sick more often and having irritated airways at times.
For reference, it is currently 84% relative humidity and -20°C outside where I live. This is about the same absolute humidity as 5% relative humidity at 22°C (i.e., inside).
An all you need to bring that up to above 50% is 2cl/cubic meter of water!
A sponge drying up in your kitchen sink is enough to raise your kitchen's air humidity by 10%. Shut down your ventilation and you'll see, you won't suffocate but instead you'll get mold starting to pop-up. Moisture is the reason why houses have ventilation system in the first place.
As I said elsewhere in this thread, though, there's a problem with “dumb” ventilation systems though: they can't really adapt to big variations in outdoor conditions, and as such they tend to suck way too much air out of your house than needed during the cold days.
> A sponge drying up in your kitchen sink is enough to raise your kitchen's air humidity by 10%.
That's simply not correct.
I go through liters of water a day with my two humidifiers just to try to raise humidity by around 20 percentage points. In a small urban apartment that isn't much bigger than some people's whole suburban kitchens.
A damp sponge isn't going to do a thing, and I can't imagine where you would ever have gotten the idea that it would.
Moisture is not the primary reason for ventilation, except above showers -- it's to prevent CO2 buildup along with other toxic gases like CO and VOC's.
> I go through liters of water a day with my two humidifiers just to try to raise humidity by around 20 percentage points. In a small urban apartment that isn't much bigger than some people's whole suburban kitchens.
No surprise, that's because your water gets vented away…
My brother had a broken ventilation for a whole northern England winter in a flat he rented (and the landlord was too busy fixing this shit up), he had massive humidity issues with fungi spores making him sick before he understood what the problem was, and he'd tell you how much discipline it takes in manually venting your house by opening the windows to keep things from molding!
> A damp sponge isn't going to do a thing, and I can't imagine where you would ever have gotten the idea that it would.
Hey you know what, just do the math by yourself, it's just one pV = nRT away ! But of course, this is assuming you're not removing all that water directly as it evaporates.
> Moisture is not the primary reason for ventilation, except above showers -- it's to prevent CO2 buildup along with other toxic gases like CO and VOC's.
Maybe have a look at your local building code and see how the ventilation requirements are made. I've refurbished a house by myself and I did just that, it turns out the regulations are built on water extraction, as CO2 won't realistically kill or harm you, CO only matters in kitchens if/where you have gas stove (and in my country, this is subject to additional ventilation requirements in the kitchen itself independent of the house's ventilation), and VOC are only a recent concern. That's also why there have been hygrometer to pilot ventilation for a while.
Even with our cross counterflow enthalpy exchanger it can get somewhat dry in the bedroom in winter. The device in the OP would probably require an additional humidifier.
My house currently is sitting at 35% humidity while being very poorly ventilated (~900ppm CO2). In the summer, it’s around 50% with the same level of ventilation. This generally has been the case everywhere I’ve lived; in the summer, you’re cooling air, which (all else equal) increases the relative humidity of that air. In the winter, you’re heating air, which decreases the relative humidity of that air.
It's “less than ideal” level of ventilation, but it's also very far from “very poorly ventilated” (it's not even above the target level set by workplace regulation in my country, which is 600ppm above the baseline).
Isn't it the other way around? 50% humidity means that air contains 50% as much moisture as it, at a given temperature. Raising the temperature means that the air can now hold much more moisture.
Bringing in cold air at 50% humidity, then warming it up to room temp makes the humidity fall, leading to dryer air indoors than comfortable.
Yep. Humidity will tank to 25% here in winter. I have two humidifiers fighting the HRV continuously when it gets cold. As I understand it an ERV controls moisture as well, but such a module for my system costs over $4000.
But household activities like cooking, showering, drying laundry, or even just washing the dishes etc. generate tons of moisture, and this moisture is the reason why your home has ventilation in the first place: to get it out and avoid mold!
There's a problem with “dumb” ventilation systems though: they can't really adapt to big variations in outdoor conditions, and as such they tend to such way too much air out of your house than needed during the cold days (and it also tend to be designed to suck cold air into dry room first, and get out from wet rooms, when you want it the other way round when it's very cold).
> In the hot and humid summer you're definitely trying to reduce indoor humidity.
No, you can't do that with ventilation when it's hotter outside than inside actually, that's not how thermodynamics works! But we don't care about that, because in the summer you don't have cold walls or window where water vapor can condense and let mold grow.
> But in the winter when it's bone-dry? A hot shower barely makes a difference.
The reason why it doesn't make a difference is because all the moisture is vented away by your ventilation system! And that's because that's what it's designed to do! Stop it and see how it goes! For the record a single wet sponge drying up in your kitchen is enough to raise humidity by 10%! You barely need 2cL of water per cubic meter to have 50% humidity at 20°C.
As I said the problem is that in winter, ventilation system often ventilate way too much.
Also, they are often designed so the cold and dry air enters in the bedrooms/living room and the warm/moist air is extracted in the kitchen and the bathroom, and because of that the rest of the house doesn't get any of the excess moisture of these places. This is done because the designers wanted to make sure that the humidity level never raise too much in the room, because again humidity will ruin your house and health pretty quick (having air that's too dry isn't very good for your lungs, but having fungi spores in the air is much worse!)
I think you are agreeing with the parent comment with a tone of disagreement.
They say they let cold air in during the winter because they want to lower the humidity.
Then you say that, if you let cold air in (and then let it heat back up again) then you end up with lower humidity.
The only thing you might disagree on is exactly what humidity you would like inside your house. But that's subjective. (As it happens, I agree with them: I often find it too humid inside during the winter, because I've restricted airflow to keep the heat in.)
Does very much depend on your location. In the UK a dehumidifier is still often a good idea in the winter because although our humidity drops, its nowhere near enough. Inside can still be high 50's, in older properties it'll never really go below 60%.
Really a lot of our older homes shoud be retrofitted with a MVHR unit to help things as ventelation is awful in most houses. I'm actually quite surprised a lot of landlords in the UK don't do it as theres always a fight between them and their tenants who don't necesserily want to leave a window open all day in the winter to stop mold.
Positive Input Ventilation (PIV) units seem to be a new thing for landlords to battle this problem.
I think it basically trickles in cold (and therefore drier) air into a central space to reduce humidity, like an (slower, quieter) extractor fan in reverse.
Seems a bit of a conflict of interest still, especially if the tenant is paying for heating.
ERV is apparently the term used for HRVs that also exchange humidity and keep it in/out as required, though I'm not sure that makes much logical sense since E is for Energy and H is for Heat, no mention of humidity.
Possibly just different terms used in different countries where the humidity is a bigger problem (very hot and/or very cold outside air).
This current OpenERV product appears to use dessicants for this purpose but might be an optional add-on?
A perfect ERV will recover the heat (occurs naturally by a temperature difference) and the latent heat (associated with condensation/evaporation).
i.e. water vapour in the outbound flow should be transferred the inbound flow.
This is to avoid energy loss that would occur when moisture condenses (i.e. the latent heat) by using an adsorbant material to capture moisture before it escapes, or a membrane that allows moisture in the outbound flow to pass to the inbound flow.
Air exchange HRVs are quite common here in the Netherlands. Not exactly ubiquitous, but common enough that you can find them in most recently built apartments at the very least. If you're going to have mechanical ventilation installed, you may as well save a buck in the long run on the heat loss.
There's only so much temperature gradient these setups can handle economically, and it's quite possible that the hot LA summers combined with the cool AC air are too much for such an installation not to leak energy at an unacceptable rate.
Then again, just like with ACs that also serve as heat pumps, it could just be a matter of not enough people (or professionals) knowing about these installations to make it viable to build a business around them.
Type in HRV system in to your preferred search provider and hit the shopping tab.
Here in Australia they range from about 1500 Antipodean Dineros for a single room through-wall mounted systems, and around 5500 upward for a centralised unit. Plus installation costs, but HVAC install is one of my paid activities, so I mostly don’t pay that part.
NB heat pump is a more accurate term for refrigeration type air conditioners, as in cooling mode they’re ‘pumping’ the heat out of the inside environment and rejecting it outside.
They have been required by code in all new houses in Minnesota for about 25 years.
I'm sure CA requires them too. Though in older houses they are a waste of money as your house already leaks much air. These are a good thing if you have a well sealed house, but older houses universally are not sealed that well and so they won't give you anything.
> I can't find anyone to install one in Los Angeles.
If you call a random HVAC company, they may not want to deal with "fussy clients" that want something "fancy" like an HRV/ERV. Best to look at folks that perhaps try to adhere to building science more. A quick search for the LA area:
In Finland, many people avoid opening windows altogether, and only rely on various devices for mechanical movement of air. I find it quite alarming actually, as many homes have stale air and subpar ventilation systems, always justified by "never let heat escape the house". The idea of opening windows to refresh the air in the house is basically alien to many Finns, while it's a normal thing to do in other countries; in German, there's a term Stoßlüften.
My entire "medium sized European suburban house" runs on a $2.5k 400m3/h unit with HEPA filters made in Lithuania - and that was the more expensive model that I can directly control over MODBUS / 0-10V signal (even turning it into a "dumb" unit). Most of the expenses were running the ducts. YMMV
It's just awesome. Every single room has fresh-smelling air and after fine tuning all my heating systems with algos implemented in Home Assistant - I'm getting ~60-100ppm over outdoor CO2, perfectly clean air, temperature within 1C of the set value, on-demand humidity extraction after showers etc. All it needs to be properly overengineered now is a bunch of dampers and per-room CO2/humidity feedback :)
Anyone know if there's a good way to control 0-10V dampers? I looked for a solution to control 5 dampers but I didn't find anything, so I started to design my own a couple years ago but never finished the project. I'm having a hard time imagining something so common doesn't have a common solution.
If you're OK with large controllers, the cheap Chinese RS485 stuff seems to work perfectly fine. For "digital inputs" I started out with Polish $150 devices, eventually ended up using the cheapest AliExpress listings for some expansions and so far they've been working exactly the same, with the only difference being the quality of the docs. I'd expect the 0-10V modules to be exactly the same.
How is the noise of these systems? And how often/long need they run to give you clean air and humidity extraction. Is there extra noise when it is windy outside? Is it installed inside the windows somehow or do I need to drill through the wall?
Typically 2 holes, one for air intake and one for air outtake are drilled through the wall. They’re often installed when a home is being built or heavily remodeled. They can be installed after the fact (especially if you have an accessible basement or attic) but it might be a bit invasive running ducts where you need them.
Alternatively, we got one piped into the HVAC ductwork. It's not as optimal as its own ducting, but it's much easier/cheaper than running a bunch of new ductwork.
That's what we did. Our house was built in 1916, but the weather sealing we've done combined with six people breathing in it led to quite high CO2 levels. We have a traditional ducted air conditioning system in the attic (heat is hot water radiant), and added the ERV there. It made a massive difference.
I'll definitely prepare a longer write-up when I have everything figure out, but here's a summary:
I have 4 systems:
- Komfovent HRV for ventilation
- NIBE F-series heat pump for floor and water heating
- Vaillant gas boiler that "supports" the heat pump
- Samsung multi-split AC units
HRV - Komfovent uses the same controllers in all of their units, so you get all the communication goodies you'd want - though it took me a long while to figure out that basic features need to be toggled on :) There are existing YAML presets for their C6/C6M controllers on HA forums. The only caveat is that if you want to feed it a virtual thermostat, you need a stuff a device simulating a 10k NTC inside of the ventilator. Otherwise it's just a single Ethernet cable.
Heat pump - I'm not exactly sure if I'm happy with NIBE, but thanks to the community the integration ended up being quite easy. I wasted a bunch of money on their MODBUS40 just to learn that you need to use a certain MODBUS address in the internal bus to make certain registries writeable (eg. thermostat values) - so I took an ESP32 with Ethernet, a galvanically isolated RS485 dongle, a 12V to 5V converter and used https://github.com/elupus/esphome-nibe. The firmware extracts my templated HA sensor's value and feeds it to the heat pump as a virtual thermostat.
Vaillant uses this weird "eBUS" protocol, there's a bunch of cheap PCBs that you can use to connect to it - I'm using https://github.com/danielkucera/esp-arduino-ebus. That's the last system that I haven't touched :)
Samsung ACs use their MIM-B19N modules installed in the outdoor units. There's some magic around enabling remote control, but once you plug their diagnostics device into their indoor units, you can flash all of them at once. I had to mess around with internal NASA addresses to have all the units appear at once.
For indoor sensors I have 3 types:
- AirGradient units measure CO2, tempeature, humidity, PMx etc. - these are mounted at ~150cm and feed the "current house temperature" template.
- I have like 8 Everything Presence One devices, powered by a custom PCB that converts 12V/24V sent over wired alarm cables to the device. They have built-in temperature, humidity and motion sensors. These are mostly installed for motion sensing and their height makes the temperature measurements quite useless.
- Everything else (and most importantly bathrooms) is done using custom ESP32-C3 devices that use SHT31 sensors to measure humidity and LD2412 for movement sensing. Also using the same adapter PCB for powering.
Thermostats are synchronized across all the devices with HA scripts. The HRV specifically uses its own wired temperature sensor to determine if it should enable heat recovery ("free cooling mode"), since its extracted air temp is always a bit lower than room temp (laziness :-)). "Current temperature" template fed to other heaters is derived from multiple room temperatures (currently using an average), with rooms "ignored" if AC is heating there (or was turned on recently). Ventilation has 2 modes set up - 20% and 80% - with the latter toggled by a bathroom humidity threshold.
There are 3 remaining things I want to set up:
- auto switching to gas heating if it's cheaper / the house is running on batteries - so far I've only imported electricity / gas prices into HA and quickly realized that I'm missing a power monitor on the heat pump circuit
- dampening of air ducts to reduce the temp drop when high humidity extraction boost gets triggered
- using more of the HRV range by auto-adjusting fan speed depending on real CO2 values - there's max 2 ppl at the house most of the time, so even at 20% the HRV is quite wasteful
As a comparison for a "dumb" system, our house has a Nibe F110 that handles all air extraction and energy recovery (only for water heating) and a single Mitsubishi mini-split AC. And the bedrooms have some small electric panel heaters. I just cannot be bothered to fiddle with the smart house stuff, this solution works just fine for us.
I wish this was as common as having a fridge in the house. The productivity gains from people not being sleepy and tired from shit air would be insane.
When having my mini splits installed I pushed for an erv system in the bedroom. The installer had only ever done them in commercial units, and he hemmed and hawed about it, but I had two c02 monitors in my room showing it getting to above 2000 whilst sleeping. I've noticed a big improvement in how groggy I feel in the morning.
If there were good enough ERVs that could be installed by the DIY'er for about the cost of a cheap refrigerator (~$600 or so) then they would be more common.
I get that manufacturing the transfer plates isn't simple or cheap, but other than that one thing they're basically fans, they shouldn't cost that much.
> My entire "medium sized European suburban house" runs on a $2.5k 400m3/h unit with HEPA filters made in Lithuania - and that was the more expensive model
That is just the HRV, not the design of the system, the ductwork, and the installation. All those add up. In new construction those costs can be shared with the regular HVAC system design, but in a retrofit its far more expensive
These units are ubiquitus in northernish Europe, as any new/renovated building needs them to reach A/A++ energy effiency. Brands like Komfovent, SystemAir, offering 200 m³/h ducted units for 2000 Euros, with efficiency like:
Outdoors °C -23 -15 -10 -5 0 25 30 35
After unit, °C 12,9 14,5 15,5 16,5 17,5 22,6 23,6 24,6
You probably cannot do a renovation the tightens up your house enough to matter. Of course you have not specified what you are doing, it is certainly possible to do that, but it is a major effort that makes the house unlivable for a couple months and costs a lot of money. If you don't do that level of renovation your house will have enough leaks that a ERV will not make any difference in air quality (and even that level doesn't always make the house airtight enough to need an ERV). Making a house airtight is very hard - worth doing because of the energy savings, but not easy.
If you are doing that level of renovation is is probably better to just tear down the house and rebuild. The costs will be similar and there are a lot of other things people demand of a new house layout that cannot be retrofitted in the old shell. Often the law will not allow this and so you are forced to renovate just to keep some now illegal feature that is worth keeping, but otherwise a tear down would be better.
That is true, and I guess technically you don't want a house with no leaks, only with leaks that you have complete control over.
Spray foam insulation can seal essentially any structure, and it doesn't require much more than a small hole drilled into the wall between the beams which is easily spackled over.
It can also be put on as insulation in attics and crawlspaces.
Combine that with a full ducting audit if any of your ducting pierces the envelope, a full intrusions audit for power boxes and the like, and new windows/house sheathing/ proper roofing, you can get very close to as good as a new built.
Most of these I would not suggest doing as a DIY, therefore I still say it is expensive, and even moreso if you have lathe and plaster instead of more modern drywall or encounter any of a myriad of issues likely to be uncovered when doing these things to a very old house.
> Spray foam insulation can seal essentially any structure, and it doesn't require much more than a small hole drilled into the wall between the beams which is easily spackled over.
A hard maybe on here. Done right it can of course, but you are depending on it filling the cavity without so much pressure that is breaks the walls. The industry as figured a lot out, but this is still compromise and some guessing and so it won't always work (it usually will). There are also many ways they used to put voids in old houses that would not be reached by this.
You're right in the main, of course, that it takes a major renovation to make an old home tight enough to be worth it, but wrong in your assumptions:
* It's a rowhouse, so there are two party walls on either side.
* Brick.
* The front and back walls are half the length (15') of the party walls.
* The house is small, a footprint of 450 sq ft.
* The renovation will extend the back roof line (pitched roof, front to back).
* The attic (with extended roof) will be renovated with a bedroom and 3/4 bath, plus storage and mechanical.
* Plumbing will be replaced.
* Gas boiler (with radiators) and existing central A/C will be replaced with heat pump.
* We'll be out of the house for months.
* It's in a historic district so it can't be torn down.
* Fortunately most of the windows have been replaced before the historic preservation office started cracking down on replacements. We have light-blocking hex blinds that insulate them nicely at night.
* I will try to sneak in a new front door that's the same design as the old one, and fix up the jambs and sill.
* Historically DC is still more heat dominated than cooling dominated, but climate change is tilting the balance. It may be hard to fight the stack effect in the winter but in the summer I hope to run the house at positive pressure. It's when the A/C's been going for days that the air in the house seems stale.
* Even though there's very little insulation today in the attic (just cellulose strewn between joists), it's not expensive to heat or cool, because it's such a small house.
* I care more about the ERV as a luxury good than as a cost effective appliance. I'm quite sure the cost of it will be negligible when compared to everything else in the reno (we're doing the kitchen, too).
Although the cost will be insane, I expect we'll come close to breaking even on value added to the property, given the neighborhood we're in.
You have also touched on why I oppose historical districts. Historical buildings should be something people are required to learn about in their history class.
Installation is usually the most expensive part, and can easily send the total price into 5 digits of $, especially in a retrofit, depending on the market.
What makes outdoor air "fresh" compared to indoor air? You said that the temperature and humidity of indoor air are preserved. So is it just CO2 concentration? Would installing a chemical CO2 scrubber have an effect similar to an ERV system then?
CO2 ppm is the main number that research studies look at, but it's also a proxy for the "freshness" of the air. All the air quality metrics are correlated. Eg a lot of cheap CO2 meters measure something else like TVOC and convert it to eCO2 using a lookup table.
CO2 scrubbing would be better than nothing, but it's really expensive and won't improve other metrics like TVOC
You have it backwards. Counter flow units have lower efficiency than these "regenerative" type ERVs.
The downside of this is that the high efficiency is limited to small spaces (based on the mass of your core), where counter flow units are great for entire homes.
One point often overlooked with counter flow units, is that you can place exhaust ducts in spaces that you want to purposefully remove air, like bathrooms and kitchens, while providing fresh air to places with little air movement, like closets, basements.
Regenerative core ERVs do little for fresh air circulation.
My bad. Yes, I saw DIY heat exchangers posted in the past and people were worried about Legionella/Legionnaires and mold. It seems scary when we don't know what we are doing.
DIY heat exchangers would be so awesome though. Only new constructions are required to have an heat exchanger here.
I'm confused how the heat retention could be around 80-90% without expending a ton of energy.
Naively, if on average the same amount of air goes in and out, I'd expect the temperature of the heat exchanger (on average in space and time, eventually) to be the average of the outside and inside temperatures. If the outside is hotter, the air coming in would be cooler than the outside air (which is a win), but it couldn't be cooler than the average of the temperatures. So, it would still not be anywhere as cool as the inside air, which doesn't sound like 90% heat retention.
Is the heat exchanger attached to a heater or a cooler? The linked video, https://www.youtube.com/watch?v=CDCu0IbEn8Q , would suggest not, as it talks about saving the energy needed for cooling or heating. Is there another clever trick?
Counter-flow heat exchangers can be very efficient, without a heater or cooler attached. That said, I don't think I've seen a commercial ERV claim to be more than 80% efficient, so I'm skeptical of the 90% measurement.
(I've seen ERVs with heaters attached; but for the purpose of avoiding frost buildup when it's below freezing outside.)
There isn't a uniform temperature across the entire exchanger. There's a smooth gradient extending from one end to the other. If the outside is hotter, then the inbound air gradually cools as it gives up heat to the outbound air which is gradually warming.
Co-current flow (both flows moving in the same direction) work the way you described.
Countercurrent-flow heat exchangers (where the two channels of the fluid/gas) move in opposite directions on both sides of the heat-transfer mechanism maintain a heat flow gradient over the entire length of the heat exchanger. This can result in an almost complete transfer of heat from one current to another.
High efficiency HRV/ERVs use counter-current flow heat exchangers.
What you describe is kind of like a theoretical heat exchanger that only averages temperature at a single point.
You can improve this by exchanging heat across a continuous length along opposing flows. Imagine two parallel pipes thermally bonded where fluids flow in opposite directions. Each point still averages the temperatures, but the average temperature varies across the length and approaches the interior temperature on the interior side and the exterior temperature on the exterior side.
Yeah, I think this makes sense. If you connect many of my heat exchangers in series, the temperature gradient increases; only the middle one will work at the average of the inside and outside temperature (the example of 3 in a row makes sense to me). At the limit, it becomes what you described.
The person you are replying to was discussing "exchanging heat across a continuous length along opposing flows", which is countercurrent exchange. Regenerative exchange is, at least to my understanding, more of a cyclical store and release process.
Yes, which is what OpenERV is. I can understand why they might have thought I was talking about their system, my wording was a bit ambiguous there. So it doesn't hurt that they cleared that up :)
I'm not 100% sure about this. But with ERV (opposed to HRVs), iirc, also the moist of the air is transferred to the incoming air. The moist contains a lot of the energy.
I find the idea of reversing the air flow direction every 30s simpler to understand than two counter-flowing pipe side by side.
Imagine a pipe filled with 3 metallic grid sections (such that the air temperature in the section will equalize with the metal temperature) separated by plastic grids (such that the heat isn't conducted through the metal), and you push air alternatively from one hot side at 20°C to a cold side at 0°C for 30s and in the other direction for 30s.
For symmetry reason, the pipe will passively (we don't count the energy required to move the air) have a gradient of temperature from the hot side to the cold side. The first section will be ~15°C, the second ~ 10°C, the third ~5°C. (Each section temperature is the temporal average of the temperature of the air flowing from previous sections : so because air switch direction, it means it's the average of left and right sections.)
From the point of view of the house, you only lose energy from the first section of the pipe which will be more like 15°C rather than 0°C.
You could do this as an air purifier, but it will primarily just remove particulates and other HEPA-y things. It won't actually bring fresh outside air in.
You could glue a fan to an air filter and then position the thing in a window, to bring in outside air that then gets scrubbed by the filter. But now you're bringing in cold air in the winter, or hot air in the summer.
An ERV brings in fresh air and mostly solves the problem of having cold air rushing in on a winter day, or hot air rushing in on a summer day.
Air filters remove particulates down to some size, varying based on the filter. They do not scrub CO2 or CO, nor do they (generally) remove other things like VOCs - unless they’re really, really expensive filters. Opening a window exchanges all that junk to be roughly equal to the baseline of your environment, which for most people is at least lower CO2 levels.
Or open windows, but outside's cold in many places this time of the year. Energy recovery ventilators run stale outgoing air through a heatsink to pre-heat incoming fresh air to save heating costs. Sounds BS but it's a well established and widely used thing.
I've installed ERV about a year ago. It's great, but kinda overbuilt. The core is some very thin plastic film (some are aluminum). The body around it is 2mm metal. The entire device is 2-3mm metal. There are 2 motors inside that likely need good support, but not reason for this device to cost $1k and weight 30 kg.
It condenses on the heat exchanger, then it evaporates when the airflow reverses direction. That's if you don't have sorbent. If you have sorbent, it gets grabbed out of the air before it can condense.
Me too, when reading "open source" I was expecting some design docs or the like.
Aside from the general confusion of the website, I haven't been able to find some of the most important information. For example, there's no diagram or immediate explanation of the general working principle and airflow path. The heat exchanger itself is published only as-is for those designs, while the author writes that he uses a custom python script tuned for the design size and his 3d printer to generate it.
When i saw this I immediately thought of studying it and reuse some of its designs for my custom use case, which does not appear to be currently possible.
At first glance it appears to be "open source" in the sense that you can buy it, but if and when something breaks you can print/reorder it easily.
Under "source code", there's a link to a GDrive with a ton of design files and documentation, as well as source code.
These are licensed CC BY-NC-SA 4.0, so depending on your personal definitions, they may or may not be "open source" (IMO they're open source but not FOSS but I've seen others equate open source with FOSS).
As a community we need to do some thinking on how open source may sensibly be applied to hardware. Unfortunately Prusa, who used to be a real champion, has departed from the assumed True Path, and they have discussed their reasons, which are largely valid. That said their design at a more fundamental level has also departed from a maintainable, simple and elegant design.
The purpose of the source code is to enable maintenance, not cloning, I say that on the website. That is this context, there are many others. It improves the economics because the machine lasts longer and there is no planned obsolescence. People are welcome to make their own units from the source if they have the skill, but although it would be fun, I don't really have time to make it easy. Some day there may be a kit which is very economical but it will still take a whole day of work to assemble, probably.
I don't think it's FOSS at the moment. I actually would build two or three of these units on my own and could provide some feedback along the way. My definition of open source means that I should be able to do so.
Unfortunately, the gDrive files are not providing enough information for me to build one of these in a DIY manner. I didn't find enough information on the hardware side, no BOM, no hardware documentation. I think that, if the author would like to actually boost DIY adoption, it'd be worth having a step by step assembly guide. At the same time, when reading the page, I had a feeling like it's more supposed to be a way of advertising a future commercial product, not really focusing on the FOSS/DIY side.
The software is provided, but from my experience with such projects, it's maybe half of the minimum information needed to build a full fledged device.
I like the project and would love to build it in the near future though.
Or a very quick way to get your personal email address blocked. Pretty sure this will eventually trigger some sort of automated spam response and that account will promptly get blocked.
Hope they used a throwaway google account with absolutely no link to his domains or main email address.
I have a full system in my house. The unit is in the attic space. It's great although it makes a lot of noise, especially in the bedroom below the unit
In the Netherlands these systems are fairly common in new houses. Mostly because the law mandates a certain level of energy efficiency of new houses. There are other ways of obtaining this required efficiency level, but an ERV unit is pretty cost effective.
I've personally been looking at installing such a system [1]. However since houses in the Netherlands are almost all made out of concrete installing such a system in an existing house is pretty hard.
Same in Denmark, we pretty much had to install one when building our house, to make up for energy loss from the large window area we wanted. We didn't have it properly calibrated at first, but once that was (professionally) done it has worked perfectly and kept a pleasant indoor-climate ever since.
Similar story here in Germany. New energy standards require a ventilation concept. Some people choose to rely on daily ventilation to save some money, but most people nowadays opt for an ERV. At least here in Germany, for some reason there are quite a lot of people who are super against the idea of having an ERV. Personally, I wouldn't want to miss is for having fresh air alone, not having to deal with pollen is an added bonus
This sounds great!
The site left me a bit confused however. Is it open in respect to software/firmware? Or also the hardware?
Can I just build my own with stock components? Something was mentioned about a DIY kit...
The WM12 is basically two TW4 modules ... Um, TW4?
As an ignoramus I need some introduction please...
What’s so good about fresh air? Like I don’t want stinky stuffy air but as someone with central HVAC I had no issues with my indoor air. Are we trying to get outdoor smells? Or is it something else?
In the winter it's cold outside and opening the window cools down the room -> no ventilation most of the time.
In the summer it's not a problem for me, I leave my windows partially open all the time but in the winter especially when working from home this would be quite neat. Also, I live in a small town in germany so the air quality here is comparatively good to many of the city folks here.
High CO2 levels impair cognition and stale air accumulates pathogens, not just smells. The V in your HVAC stands for Ventilation, so you're already getting fresh air, that's probably why you have no complaints. If you live in an air tight apartment with no forced circulation where CO2 levels spike super fast requiring ventilation several times a day, it's a different story.
I'm not understanding if it also filters the outdoor air coming in.
I live in a place where supposedly the air is of good quality, and yet, when I open the windows, the all place gets a thin film of black dust on all surfaces - most probably due to the particle emissions and tire degradation dust from vehicles from the highway nearby.
The solution I've found is to open the windows every day for about half an hour and then put an air purifier to work.
Looks interesting. Especially since it seems to be much cheaper that the closed solutions...
Have looked in this kind of systems, for my parents. The use case was basically, not about energy efficiency, but rather noise protection - to be able to sleep with a closed windows. I think so far I always had two issues (in that usecase).
- First the device by itself - produces a bit noise like 42db might be too much for some people if you want to sleep. Especially some of the devices are using one ventilator, which switches directions and won't produce homogeneous noise.
- Second 60 CFM is fine, but if you want to have the feeling of an open window - it should be much more and most devices can't deliver that. Also the heat exchange thing is kind of cool in the winter for sure. In the summer, you often have the case that in the evening you house is much warmer than the air outside - so you would like to turn the heat exchange off in the winter.
PS: Actually, maybe looking for a complete different use case. But I think what would be very cool, would be some idea to make at least one room 100% quite (with fresh air ) in a cheap way. Guess this would be a huge life changer for a lot of people, who suffer from noise pollution.
I think that summer/winter distinction is really important. In the UK where most houses don't have air conditioning, you really don't want heat recovery for 1/3 of the year. On really hot days you might want to use heat recovery during the peak few hours, but otherwise you are trying to cool the house down with colder outside air.
I would love some sort of intelligent house ventilation system which could do all that. Heat recovery when it makes sense, normal ventilation when it doesn't. All automated based on dT and relative humidities.
I think with this model you can do this as long as you install them in a synced pair (which I think is the baseline assumption).
Normal (heat recovery mode) you have them reverse flow every 60 seconds or so to swap heat.
In cooling mode you just run then continually. One is bringing in fresh air and the other is removing stale air.
The heat store in the intake will soon cool to the outside temp and the heat store in the output is irrelevant (apart from maybe slowing the air flow and creating noise).
If you manually control the system you could combine with a few open windows to create cross breezes even on still evenings.
Looks to me like a low cost version of the same could be designed with 2 CPU fans ($1), and a large 3D print ($1 - for the DIY version) or injection moulding for a commercial version ($0.30). Do time-sync between the units with grid frequency sampling (free), and have the whole thing controlled by a 2 cent microcontroller and a pair of triacs.
The whole thing, designed and made in China could probably come to a BOM under $4, and retail in the USA for $12.
While I agree with your points, $0.30 for injection moulding would need quite the scale, and I have doubts about whether two CPU fans would have enough power to flow enough air even in the absence of a HEPA filter.
Injection moulding is cheaper than you imagine now. The cheapest moulds start from about $250, and there are plenty of companies who will make a mould and make and ship 5k parts within 7 days.
These designs would need quite a few changes to be injection moulding compatible - for one thing the fins are probably going to have to be flat not circular due to draft angle requirements.
Changing the core to a roll of embossed steel foil might be a better bet, and whilst that would add about $1 to the price, it would also make the product work better and be more compact or more efficient due to a higher thermal mass in the core.
Never pay for a mould to be made with plans to ship the mould itself to another manufacturer - it's a common scam. Always make mould+make parts as a single transaction.
Look up the price of a blauberg Vento or a Lunos e2. They are ~$1800 CAD, and they get a fraction of the airflow. Computers have a large ecosystem behind them and have been in development for a very long time. This is still at the start of the deployment curve.
I have a spreadsheet where I track all inputs, and unfortunately it's just the cost of everything. The filament really doesn't help, but it also takes a lot of labor, and paying off the dev cost is also an important factor. Most of all, it's made in Canada where rent is expensive and I have pay that so I have to get paid a living Canadian wage.
It's better compared to a blauberg vento or lunos e2. That product probably gets poor efficiency, I have not checked the technical sheet but if they say 80% that means at the minimal flow levels, which are only 10 cfm or something. The TW4 gets >85% sensible and comparable latent efficiency, at 60 cfm. It also has twice the maximal flow of that device. It's got many other features as well, and is more durable. Ultimately, it's about return on investment. You have to make a spreadsheet and see which one is best, given the actual tested values for efficiency flow, maintenance cost, etc. If that's not possible, it's a shot in the dark.
I so need this, and I so need it to function with Home Assistant.
I would love to ventilate based on values of my Aranet 4 (a bluetooth CO2 sensor).
Also, would be nice if it coordinates with multiple units, ie what this brand does: [0]
EDIT: It does, if you click on "learn more", you'll learn more: "The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss." ...Perfect!
Currently I have two holes in my wall for ventilation, when it is windy it's too much (feel the wind blowing inside), when some people visit and there is no wind, boom, >3000 ppm CO2 in 20 minutes.
I just really hope it is very quiet, although it says ~37 dBa (which is quite a lot imho), I replaced my bathroom ventilator recently, it produces 25 db! [1]). The previous one [2] produced 52 dB (cheapest around), that was pretty annoying, you'd hear it in the bedrooms above the room it was used in. Maybe 37 dB it isn't so bad, especially since you can wind it down and mostly need it when it's busy/noisy (many people) anyway.
Btw, don't buy a CO2 sensor, pretty soon you're a ventilation nerd, or as my wife would call it, a ventilation curmudgeon.
> One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss." ...Perfect!
Absolutely.
> A room is not heated by increasing its internal energy but by decreasing its entropy due to the fact that during heating, the volume and pressure remain constant and air is expelled.
The point about balancing airflow is crucial, but I think underappreciated by non-professionals. Thermodynamics is highly non-intuitive in places, and the enclosed climate-controlled spaces we love to inhabit are certainly included in that.
Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
Oh man I had this discussion with my wife yesterday, we have a small electric heater in a room where a pipe burst and I still need to fix that (no heating means instant fungus problems). It keeps its fan rotating always, that way it determines the input temp for its thermostat more accurately. But wife insists it is sometimes blowing cold air and thus very very bad... I explain what a thermostat is (bimetals and all) and that she experiences "coldness" because a layer of warm air is blown from her skin, it's not blowing cold air... she doesn't follow... I even measure the energy usage and the thing only uses 20 W or so when just blowing, not heating. Even when just blowing it's moving cold moist air from the walls so overall good. It's difficult dealing with her like this.
I'll pay someone to tell me how to deal with someone like this and maintain a positive atmosphere. The thing is, I also do it for things that really are probably not worth discussing... I should pick my battles better, is there ever a good time for some mansplaining? Or should I say... Nerdsplaining?
The problem as it stands now is that she is experiencing something (“it is blowing cold air”) and your claims (“no it’s not”) run counter to that direct experience.
Place and leave a thermometer in front of it and give her the information that you are using to make your own claims.
You went through a process to learn that moving air feels colder than still air at the same temperature. It seems that perhaps she has not. Surfacing the ground truth of the air temperature may help the situation.
Full of enthusiasm I did this whole thing as they did in Veratasium [0]. Didn't impress anybody in my family. Although my son is getting there I think... Maybe I'm just too bad and lengthy at nerdsplaining.
Actually, I didn't put an ice cube on the metal, that is the trick. of course.
> Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
On that note, I'm curious if hybrid heating/cooling solutions will ever take off. Other than this OpenERV product I mean, which I guess technically counts!
Low Tech Magazine mentioned some experiments in their article on compressed air energy storage (CAES). Instead of trying to make that form of energy storage an adiabatic process, the idea is to use the heat produced/required in the compression/decompression steps in the household to improve the energy efficiency (e.g. use heat produced during compression to heat water; do the decompression in a space that should be cold anyway like a basement used for food storage).
37dba is practically nothing. Like a very soft whisper from a couple meters away. Remember the scale is logarithmic, 37 is almost 2 orders of magnitude below 52.
Thank you, this is correct. Using my class 2 sound meter, if I stand in my house in nowhereland cornwall with power shut off to the whole house, it's 38 dBa. 37 dBa is audible in that environment but nearly inaudible in a normal environment where your computer cooling fan is making 45 dBa at 1 meter, etc. 42 dBa is pretty quiet too, my furnace makes 43 dBa at 1 meter from the duct when it turns on. And that 42 dBa is a full 60 cfm, full blast. That's more than twice the airflow of competing units like the blauberg vento. You don't turn it up that high when you are sleeping.
You have to look closely at the actual test protocol, which they rarely share. And in reality noise is hard to measure and hard to understand. There is also SPL and SWL, which are not the same. The marketing department has no clue and convert back and forth with approximations, adding or losing decibels, and nobody ever checks or really knows or, usually, cares. We need third party reviewers with actual sound meters or the numbers mfrs give us are practically useless. This is why I am prioritizing selling beta units to people who can actually test the units.
Check out Komfovent units if you want a ready solution. My setup is Komfovent HRV (over MODBUS TCP), NIBE heatpump (over MODBUS UDP + esphome-nibe), Vaillant gas boiler (over eBUS-WiFi) + a bunch of AirGradients scattered around the house. Nothing has access to the internet, everything is glued together with HA. Works surprisingly well :)
Why not just buy an ERV? It's available, comparable in costs, and in 5-10 years, there will still be parts, unlike this project where the author hasn't actually shared the most crucial component, the exchanger.
There's some comments here suggesting that the incoming and outgoing air pass each other in a heat exchanger. But this is a different model that passes he two streams in turn through a heat sink:
> Recuperative types are what most people think of, consisting of a thin layer of material that separates two gas streams. Regenerative heat exchangers are different. They briefly store the energy while air flows in one direction, then release it when the air flow reverses.
I have to admit I am slightly more dubious about this type as they are new to me, though I did see a YouTube video about a commercial one recently and they seem to be a hot new thing.
Possibly this is something sensible that only becomes practical with software and wireless communication? Rather than running ducts to a central location.
Though then fitting two side by side in a window seems odd. Why not use the traditional type in that case?
I have two equivalent regenerative commercial units (HRV, no vapour handling) fitted at opposite sides of a mostly open plan ground floor. They use a heavy ceramic core, and sync for opposite or coordinated flow (optional). They go up to 60m3/h (~35CFM) which is extractor fan level for me, 60CFM (~100m3/h) is quite a step up. They were under €200 a unit about 18 months ago.
They are rated 90% recovery at low speed. Today it's 11C 75%RH outside, 18C 65%RH inside, at low speed (15m3/h rated at 1.2W) there's barely a difference: 17.8-17.9C air intake temperature.
They keep the air noticeably fresh, drier and also keep the CO2 down (<600ppm right now). I'm running them below the "recommended" 50% air-change per hour (ACH about 35%), and boost when needed.
There's a recuperative ducted type in the attic for the first floor, when I checked last month it was 4C outside, 18C at the outlet vent, and 17C at the inlet vents. That runs at 50% ACH.
The reasoning for the paired up window model isn't obvious, maybe a simple increase in capacity. The website is quite clear you need a push/pull pair to be efficient, and an immediately adjacent such pair is not going to work so well.
I have traced the air with a smoke generator, there is a youtube video of me doing this on my youtube channel. There is not significant re-inhalation (short circuiting) of the air. The reason for the window mount is that a lot of people rent or otherwise cannot punch holes in their walls. However the reason the window mount is nearly the same as the TW4 (through wall) type, is that I cannot invest the time and money to re-design a whole new machine for a window. The primary value is in the TW4, but a lot of people wanted window mount ones so I helped some people out by deving a quick window adapter and then they helped me out by testing some of the components/the fundamentals of the system in the real world. Unfortunately the window mount units nobody is willing to really pay the cost of manufacture for, so they have to use grade B parts etc. and they have to serve the purpose of testing the TW4 or they aren't worth making.
Great project. Hrv and erv should be a lot more common. Is there a diagram of the airflow in that unit? It looks like the intakes and exhaust on both sides might be very close to each other.
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[ 4.0 ms ] story [ 425 ms ] threadIs the air outdoor or clean? It can't be both!
Marketing bullshit aside, this looks great!
Cities and industrial sites are the exception with bad air quality, not the default.
That said, I live in SF and my AQI is usually <50. Not as great as 5, but we sometimes get down to single-digits. Cities don't have to have bad air.
Haven't heard of them from grand-designs and those kind of youtubers)
just some ideas what some (public)/most (nice home) japanese toilets offer, which might be hard to come by anywhere else
They are $200-300 online. Not Toto quality, but plenty good enough.
With an ERV you can control how much air you get, and put whatever filter you want in between the outside air and your room.
In fact the fancier systems will let you overpressure your house by running the fan pushing in slightly higher than the air pushing out, ensure air only leaks out of your house not in.
Low-CO2 outdoor air vs high-CO2 indoor air, if you prefer. Important for how air-tight modern energy-efficient construction is.
Unfortunately, outdoor air has particulate and ozone pollution. Filtering it gives you "clean" air.
In winter and summer, you also heat or cool the indoor air for comfort. If you just pump in outside air, you effectively also pump out the indoor air. This wastes the energy that had gone into heating or cooling it.
These systems save that energy by transfering heat between the air that's getting pumped in and the air that's getting pumped out.
"Imagine you could open a window to get fresh air into your house and stale air out, but when you did so, most of the heat/humidity would stay in during the winter, or stay out during the summer, leaving you just the fresh air".
In terms of the effect on environment inside a house, I usually say:
"Imagine it's always a fresh-air spring day inside your house"
This is a great project. One problem with ERV/HRV systems right now is that they are very expensive niche products. While this system doesn't achieve the extremely high heat recovery efficiencies of counter-flow units, the perfect is the enemy of the good, and this seems like it could be orders of magnitude cheaper.
Naively allowing the air columns to thermally mix would result in the average of the inside and outside temp. So how does this do better?
> Each OpenERV TW4 module has a very quiet pair of fans, pointed in opposite directions, and a heat exchanger in a 6 inch pipe, that goes through a wall. The hot, polluted air from inside goes out for 30 seconds, and the heat from it is stored in the heat exchanger.
> Then, the fan reverses direction, moving clean air from outdoors to the indoors. On it's way in, it picks up that heat from the heat exchanger. This type of heat exchanger is called a regenerative heat exchanger, or less commonly, a regenerator. The kind shown in the video is a recuperative type, not regenerative. Recuperative types are what most people think of, consisting of a thin layer of material that separates two gas streams. Regenerative heat exchangers are different. They briefly store the energy while air flows in one direction, then release it when the air flow reverses.
> The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss.
https://www.openerv.ca/learn-more
Heat exchanger there is usually an extruded ceramic grid (ERV) or rolled corrugated aluminum (functions closer to HRV than ERV)
Getting rid of humidity in winter is the main reason why you want to bring fresh air in a house though!
A sponge drying up in your kitchen sink is enough to raise your kitchen's air humidity by 10%. Shut down your ventilation and you'll see, you won't suffocate but instead you'll get mold starting to pop-up. Moisture is the reason why houses have ventilation system in the first place.
As I said elsewhere in this thread, though, there's a problem with “dumb” ventilation systems though: they can't really adapt to big variations in outdoor conditions, and as such they tend to suck way too much air out of your house than needed during the cold days.
That's simply not correct.
I go through liters of water a day with my two humidifiers just to try to raise humidity by around 20 percentage points. In a small urban apartment that isn't much bigger than some people's whole suburban kitchens.
A damp sponge isn't going to do a thing, and I can't imagine where you would ever have gotten the idea that it would.
Moisture is not the primary reason for ventilation, except above showers -- it's to prevent CO2 buildup along with other toxic gases like CO and VOC's.
No surprise, that's because your water gets vented away…
My brother had a broken ventilation for a whole northern England winter in a flat he rented (and the landlord was too busy fixing this shit up), he had massive humidity issues with fungi spores making him sick before he understood what the problem was, and he'd tell you how much discipline it takes in manually venting your house by opening the windows to keep things from molding!
> A damp sponge isn't going to do a thing, and I can't imagine where you would ever have gotten the idea that it would.
Hey you know what, just do the math by yourself, it's just one pV = nRT away ! But of course, this is assuming you're not removing all that water directly as it evaporates.
> Moisture is not the primary reason for ventilation, except above showers -- it's to prevent CO2 buildup along with other toxic gases like CO and VOC's.
Maybe have a look at your local building code and see how the ventilation requirements are made. I've refurbished a house by myself and I did just that, it turns out the regulations are built on water extraction, as CO2 won't realistically kill or harm you, CO only matters in kitchens if/where you have gas stove (and in my country, this is subject to additional ventilation requirements in the kitchen itself independent of the house's ventilation), and VOC are only a recent concern. That's also why there have been hygrometer to pilot ventilation for a while.
Aren't you missing a zero of something? Because 900ppm isn't “very poorly ventilated”.
I notice I'm not as mentally focused once it gets to 800 or so.
I have a CO2 monitor to keep it below 600 for productivity and concentration.
Remember, fresh air is around 420.
Bringing in cold air at 50% humidity, then warming it up to room temp makes the humidity fall, leading to dryer air indoors than comfortable.
There's a problem with “dumb” ventilation systems though: they can't really adapt to big variations in outdoor conditions, and as such they tend to such way too much air out of your house than needed during the cold days (and it also tend to be designed to suck cold air into dry room first, and get out from wet rooms, when you want it the other way round when it's very cold).
In the hot and humid summer you're definitely trying to reduce indoor humidity.
But in the winter when it's bone-dry? A hot shower barely makes a difference.
I keep two humidifiers running all winter long just to bring indoor humidity up to 35% or 40% where it's healthy.
Otherwise it often goes down to 15% or even 10% on cold winter days, which is terribly unhealthy.
> In the hot and humid summer you're definitely trying to reduce indoor humidity.
No, you can't do that with ventilation when it's hotter outside than inside actually, that's not how thermodynamics works! But we don't care about that, because in the summer you don't have cold walls or window where water vapor can condense and let mold grow.
> But in the winter when it's bone-dry? A hot shower barely makes a difference.
The reason why it doesn't make a difference is because all the moisture is vented away by your ventilation system! And that's because that's what it's designed to do! Stop it and see how it goes! For the record a single wet sponge drying up in your kitchen is enough to raise humidity by 10%! You barely need 2cL of water per cubic meter to have 50% humidity at 20°C.
As I said the problem is that in winter, ventilation system often ventilate way too much.
Also, they are often designed so the cold and dry air enters in the bedrooms/living room and the warm/moist air is extracted in the kitchen and the bathroom, and because of that the rest of the house doesn't get any of the excess moisture of these places. This is done because the designers wanted to make sure that the humidity level never raise too much in the room, because again humidity will ruin your house and health pretty quick (having air that's too dry isn't very good for your lungs, but having fungi spores in the air is much worse!)
They say they let cold air in during the winter because they want to lower the humidity.
Then you say that, if you let cold air in (and then let it heat back up again) then you end up with lower humidity.
The only thing you might disagree on is exactly what humidity you would like inside your house. But that's subjective. (As it happens, I agree with them: I often find it too humid inside during the winter, because I've restricted airflow to keep the heat in.)
Really a lot of our older homes shoud be retrofitted with a MVHR unit to help things as ventelation is awful in most houses. I'm actually quite surprised a lot of landlords in the UK don't do it as theres always a fight between them and their tenants who don't necesserily want to leave a window open all day in the winter to stop mold.
I think it basically trickles in cold (and therefore drier) air into a central space to reduce humidity, like an (slower, quieter) extractor fan in reverse.
Seems a bit of a conflict of interest still, especially if the tenant is paying for heating.
Possibly just different terms used in different countries where the humidity is a bigger problem (very hot and/or very cold outside air).
This current OpenERV product appears to use dessicants for this purpose but might be an optional add-on?
This is to avoid energy loss that would occur when moisture condenses (i.e. the latent heat) by using an adsorbant material to capture moisture before it escapes, or a membrane that allows moisture in the outbound flow to pass to the inbound flow.
There's only so much temperature gradient these setups can handle economically, and it's quite possible that the hot LA summers combined with the cool AC air are too much for such an installation not to leak energy at an unacceptable rate.
Then again, just like with ACs that also serve as heat pumps, it could just be a matter of not enough people (or professionals) knowing about these installations to make it viable to build a business around them.
Type in HRV system in to your preferred search provider and hit the shopping tab.
Here in Australia they range from about 1500 Antipodean Dineros for a single room through-wall mounted systems, and around 5500 upward for a centralised unit. Plus installation costs, but HVAC install is one of my paid activities, so I mostly don’t pay that part.
NB heat pump is a more accurate term for refrigeration type air conditioners, as in cooling mode they’re ‘pumping’ the heat out of the inside environment and rejecting it outside.
But have you heard about brown?
If you call a random HVAC company, they may not want to deal with "fussy clients" that want something "fancy" like an HRV/ERV. Best to look at folks that perhaps try to adhere to building science more. A quick search for the LA area:
* https://www.jmsacandheating.com/indoor-air-quality/heat-ener...
* https://www.aircomfortexperts.com/additional-products/ervs/
* https://www.azaircond.com/indoor-air-quality/energy-recovery...
* https://www.socalclimatecontrol.com/ervs-and-hrvs-energy-eff...
Or do a dealer search from a manufacturer, e.g.,:
* https://broan-nutone.com/en-us/home/dealer-locator
> Is there a particular climate these are suited for?
Any climate. Modern ones can even handle IECC Zones 6 and 7:
* https://basc.pnnl.gov/images/iecc-climate-zone-map
My entire "medium sized European suburban house" runs on a $2.5k 400m3/h unit with HEPA filters made in Lithuania - and that was the more expensive model that I can directly control over MODBUS / 0-10V signal (even turning it into a "dumb" unit). Most of the expenses were running the ducts. YMMV
It's just awesome. Every single room has fresh-smelling air and after fine tuning all my heating systems with algos implemented in Home Assistant - I'm getting ~60-100ppm over outdoor CO2, perfectly clean air, temperature within 1C of the set value, on-demand humidity extraction after showers etc. All it needs to be properly overengineered now is a bunch of dampers and per-room CO2/humidity feedback :)
I think this is the one I'm using for ~200-ish sqm.
I'd be very interested in hearing the details of this.
I have 4 systems:
- Komfovent HRV for ventilation
- NIBE F-series heat pump for floor and water heating
- Vaillant gas boiler that "supports" the heat pump
- Samsung multi-split AC units
HRV - Komfovent uses the same controllers in all of their units, so you get all the communication goodies you'd want - though it took me a long while to figure out that basic features need to be toggled on :) There are existing YAML presets for their C6/C6M controllers on HA forums. The only caveat is that if you want to feed it a virtual thermostat, you need a stuff a device simulating a 10k NTC inside of the ventilator. Otherwise it's just a single Ethernet cable.
Heat pump - I'm not exactly sure if I'm happy with NIBE, but thanks to the community the integration ended up being quite easy. I wasted a bunch of money on their MODBUS40 just to learn that you need to use a certain MODBUS address in the internal bus to make certain registries writeable (eg. thermostat values) - so I took an ESP32 with Ethernet, a galvanically isolated RS485 dongle, a 12V to 5V converter and used https://github.com/elupus/esphome-nibe. The firmware extracts my templated HA sensor's value and feeds it to the heat pump as a virtual thermostat.
Vaillant uses this weird "eBUS" protocol, there's a bunch of cheap PCBs that you can use to connect to it - I'm using https://github.com/danielkucera/esp-arduino-ebus. That's the last system that I haven't touched :)
Samsung ACs use their MIM-B19N modules installed in the outdoor units. There's some magic around enabling remote control, but once you plug their diagnostics device into their indoor units, you can flash all of them at once. I had to mess around with internal NASA addresses to have all the units appear at once.
For indoor sensors I have 3 types:
- AirGradient units measure CO2, tempeature, humidity, PMx etc. - these are mounted at ~150cm and feed the "current house temperature" template.
- I have like 8 Everything Presence One devices, powered by a custom PCB that converts 12V/24V sent over wired alarm cables to the device. They have built-in temperature, humidity and motion sensors. These are mostly installed for motion sensing and their height makes the temperature measurements quite useless.
- Everything else (and most importantly bathrooms) is done using custom ESP32-C3 devices that use SHT31 sensors to measure humidity and LD2412 for movement sensing. Also using the same adapter PCB for powering.
Thermostats are synchronized across all the devices with HA scripts. The HRV specifically uses its own wired temperature sensor to determine if it should enable heat recovery ("free cooling mode"), since its extracted air temp is always a bit lower than room temp (laziness :-)). "Current temperature" template fed to other heaters is derived from multiple room temperatures (currently using an average), with rooms "ignored" if AC is heating there (or was turned on recently). Ventilation has 2 modes set up - 20% and 80% - with the latter toggled by a bathroom humidity threshold.
There are 3 remaining things I want to set up:
- auto switching to gas heating if it's cheaper / the house is running on batteries - so far I've only imported electricity / gas prices into HA and quickly realized that I'm missing a power monitor on the heat pump circuit
- dampening of air ducts to reduce the temp drop when high humidity extraction boost gets triggered
- using more of the HRV range by auto-adjusting fan speed depending on real CO2 values - there's max 2 ppl at the house most of the time, so even at 20% the HRV is quite wasteful
I get that manufacturing the transfer plates isn't simple or cheap, but other than that one thing they're basically fans, they shouldn't cost that much.
That is just the HRV, not the design of the system, the ductwork, and the installation. All those add up. In new construction those costs can be shared with the regular HVAC system design, but in a retrofit its far more expensive
If you are doing that level of renovation is is probably better to just tear down the house and rebuild. The costs will be similar and there are a lot of other things people demand of a new house layout that cannot be retrofitted in the old shell. Often the law will not allow this and so you are forced to renovate just to keep some now illegal feature that is worth keeping, but otherwise a tear down would be better.
Use a qualified professional. Get multiple inputs.
It will cost money, and more money as you approach perfection, but it is doable.
Spray foam insulation can seal essentially any structure, and it doesn't require much more than a small hole drilled into the wall between the beams which is easily spackled over.
It can also be put on as insulation in attics and crawlspaces.
Combine that with a full ducting audit if any of your ducting pierces the envelope, a full intrusions audit for power boxes and the like, and new windows/house sheathing/ proper roofing, you can get very close to as good as a new built.
Most of these I would not suggest doing as a DIY, therefore I still say it is expensive, and even moreso if you have lathe and plaster instead of more modern drywall or encounter any of a myriad of issues likely to be uncovered when doing these things to a very old house.
A hard maybe on here. Done right it can of course, but you are depending on it filling the cavity without so much pressure that is breaks the walls. The industry as figured a lot out, but this is still compromise and some guessing and so it won't always work (it usually will). There are also many ways they used to put voids in old houses that would not be reached by this.
* It's a rowhouse, so there are two party walls on either side.
* Brick.
* The front and back walls are half the length (15') of the party walls.
* The house is small, a footprint of 450 sq ft.
* The renovation will extend the back roof line (pitched roof, front to back).
* The attic (with extended roof) will be renovated with a bedroom and 3/4 bath, plus storage and mechanical.
* Plumbing will be replaced.
* Gas boiler (with radiators) and existing central A/C will be replaced with heat pump.
* We'll be out of the house for months.
* It's in a historic district so it can't be torn down.
* Fortunately most of the windows have been replaced before the historic preservation office started cracking down on replacements. We have light-blocking hex blinds that insulate them nicely at night.
* I will try to sneak in a new front door that's the same design as the old one, and fix up the jambs and sill.
* Historically DC is still more heat dominated than cooling dominated, but climate change is tilting the balance. It may be hard to fight the stack effect in the winter but in the summer I hope to run the house at positive pressure. It's when the A/C's been going for days that the air in the house seems stale.
* Even though there's very little insulation today in the attic (just cellulose strewn between joists), it's not expensive to heat or cool, because it's such a small house.
* I care more about the ERV as a luxury good than as a cost effective appliance. I'm quite sure the cost of it will be negligible when compared to everything else in the reno (we're doing the kitchen, too).
Although the cost will be insane, I expect we'll come close to breaking even on value added to the property, given the neighborhood we're in.
You have also touched on why I oppose historical districts. Historical buildings should be something people are required to learn about in their history class.
Most building codes in US/CA mandate them since about 2015, so I'm not sure how niche they are (at least in new construction).
Depending on the (air) volumes involved, ERVs can be had for under CA$ 2000:
* https://gasexperts.ca/product-category/air-exchangers/lifebr...
* https://bphsales.ca/collections/high-quality-erv-air-exchang...
HRVs for less, but it's probably worth the extra few hundred for better humidity management.
CO2 scrubbing would be better than nothing, but it's really expensive and won't improve other metrics like TVOC
The downside of this is that the high efficiency is limited to small spaces (based on the mass of your core), where counter flow units are great for entire homes.
One point often overlooked with counter flow units, is that you can place exhaust ducts in spaces that you want to purposefully remove air, like bathrooms and kitchens, while providing fresh air to places with little air movement, like closets, basements.
Regenerative core ERVs do little for fresh air circulation.
DIY heat exchangers would be so awesome though. Only new constructions are required to have an heat exchanger here.
Naively, if on average the same amount of air goes in and out, I'd expect the temperature of the heat exchanger (on average in space and time, eventually) to be the average of the outside and inside temperatures. If the outside is hotter, the air coming in would be cooler than the outside air (which is a win), but it couldn't be cooler than the average of the temperatures. So, it would still not be anywhere as cool as the inside air, which doesn't sound like 90% heat retention.
Is the heat exchanger attached to a heater or a cooler? The linked video, https://www.youtube.com/watch?v=CDCu0IbEn8Q , would suggest not, as it talks about saving the energy needed for cooling or heating. Is there another clever trick?
(I've seen ERVs with heaters attached; but for the purpose of avoiding frost buildup when it's below freezing outside.)
Imagine two air streams counter-flowing. They "swap places" within your heat exchanger, so you can (theoretically) get 100% heat recovery.
This principle is used by animals to minimize the heat waste, by counter-flowing warm and cold blood: https://en.wikipedia.org/wiki/Rete_mirabile
Countercurrent-flow heat exchangers (where the two channels of the fluid/gas) move in opposite directions on both sides of the heat-transfer mechanism maintain a heat flow gradient over the entire length of the heat exchanger. This can result in an almost complete transfer of heat from one current to another.
High efficiency HRV/ERVs use counter-current flow heat exchangers.
You can improve this by exchanging heat across a continuous length along opposing flows. Imagine two parallel pipes thermally bonded where fluids flow in opposite directions. Each point still averages the temperatures, but the average temperature varies across the length and approaches the interior temperature on the interior side and the exterior temperature on the exterior side.
Thanks!
https://en.wikipedia.org/wiki/Countercurrent_exchange
Imagine a pipe filled with 3 metallic grid sections (such that the air temperature in the section will equalize with the metal temperature) separated by plastic grids (such that the heat isn't conducted through the metal), and you push air alternatively from one hot side at 20°C to a cold side at 0°C for 30s and in the other direction for 30s.
For symmetry reason, the pipe will passively (we don't count the energy required to move the air) have a gradient of temperature from the hot side to the cold side. The first section will be ~15°C, the second ~ 10°C, the third ~5°C. (Each section temperature is the temporal average of the temperature of the air flowing from previous sections : so because air switch direction, it means it's the average of left and right sections.)
From the point of view of the house, you only lose energy from the first section of the pipe which will be more like 15°C rather than 0°C.
You could glue a fan to an air filter and then position the thing in a window, to bring in outside air that then gets scrubbed by the filter. But now you're bringing in cold air in the winter, or hot air in the summer.
An ERV brings in fresh air and mostly solves the problem of having cold air rushing in on a winter day, or hot air rushing in on a summer day.
When i saw this I immediately thought of studying it and reuse some of its designs for my custom use case, which does not appear to be currently possible.
At first glance it appears to be "open source" in the sense that you can buy it, but if and when something breaks you can print/reorder it easily.
Correct me if i'm wrong
These are licensed CC BY-NC-SA 4.0, so depending on your personal definitions, they may or may not be "open source" (IMO they're open source but not FOSS but I've seen others equate open source with FOSS).
The purpose of the source code is to enable maintenance, not cloning, I say that on the website. That is this context, there are many others. It improves the economics because the machine lasts longer and there is no planned obsolescence. People are welcome to make their own units from the source if they have the skill, but although it would be fun, I don't really have time to make it easy. Some day there may be a kit which is very economical but it will still take a whole day of work to assemble, probably.
Unfortunately, the gDrive files are not providing enough information for me to build one of these in a DIY manner. I didn't find enough information on the hardware side, no BOM, no hardware documentation. I think that, if the author would like to actually boost DIY adoption, it'd be worth having a step by step assembly guide. At the same time, when reading the page, I had a feeling like it's more supposed to be a way of advertising a future commercial product, not really focusing on the FOSS/DIY side.
The software is provided, but from my experience with such projects, it's maybe half of the minimum information needed to build a full fledged device.
I like the project and would love to build it in the near future though.
Hope they used a throwaway google account with absolutely no link to his domains or main email address.
I've personally been looking at installing such a system [1]. However since houses in the Netherlands are almost all made out of concrete installing such a system in an existing house is pretty hard.
[1] https://www.duco.eu/uk-ie/products/mechanical-ventilation/ve...
The old-and-trusted brand here is https://www.genvex.com/en (ours was supplied by Ecovent though https://ecovent.dk/?lang=en )
In the summer it's not a problem for me, I leave my windows partially open all the time but in the winter especially when working from home this would be quite neat. Also, I live in a small town in germany so the air quality here is comparatively good to many of the city folks here.
I live in a place where supposedly the air is of good quality, and yet, when I open the windows, the all place gets a thin film of black dust on all surfaces - most probably due to the particle emissions and tire degradation dust from vehicles from the highway nearby.
The solution I've found is to open the windows every day for about half an hour and then put an air purifier to work.
Unfortunately, due to cars and dirty heating systems around where I live, outdoor air tends to be not great.
There's lots of text on that web-site, but details of the actual design of the thing are pretty scant.
Have looked in this kind of systems, for my parents. The use case was basically, not about energy efficiency, but rather noise protection - to be able to sleep with a closed windows. I think so far I always had two issues (in that usecase).
- First the device by itself - produces a bit noise like 42db might be too much for some people if you want to sleep. Especially some of the devices are using one ventilator, which switches directions and won't produce homogeneous noise.
- Second 60 CFM is fine, but if you want to have the feeling of an open window - it should be much more and most devices can't deliver that. Also the heat exchange thing is kind of cool in the winter for sure. In the summer, you often have the case that in the evening you house is much warmer than the air outside - so you would like to turn the heat exchange off in the winter.
PS: Actually, maybe looking for a complete different use case. But I think what would be very cool, would be some idea to make at least one room 100% quite (with fresh air ) in a cheap way. Guess this would be a huge life changer for a lot of people, who suffer from noise pollution.
I would love some sort of intelligent house ventilation system which could do all that. Heat recovery when it makes sense, normal ventilation when it doesn't. All automated based on dT and relative humidities.
Normal (heat recovery mode) you have them reverse flow every 60 seconds or so to swap heat.
In cooling mode you just run then continually. One is bringing in fresh air and the other is removing stale air.
The heat store in the intake will soon cool to the outside temp and the heat store in the output is irrelevant (apart from maybe slowing the air flow and creating noise).
If you manually control the system you could combine with a few open windows to create cross breezes even on still evenings.
Great work.
I cannot help to wonder what brings the total cost to $600, the price of a modern, powerful computer.
And yes, I am familiar with the economy of scale :)
The whole thing, designed and made in China could probably come to a BOM under $4, and retail in the USA for $12.
These designs would need quite a few changes to be injection moulding compatible - for one thing the fins are probably going to have to be flat not circular due to draft angle requirements.
Changing the core to a roll of embossed steel foil might be a better bet, and whilst that would add about $1 to the price, it would also make the product work better and be more compact or more efficient due to a higher thermal mass in the core.
Maybe I could use them in my own project :)
https://zhongshengmould.en.alibaba.com/
Never pay for a mould to be made with plans to ship the mould itself to another manufacturer - it's a common scam. Always make mould+make parts as a single transaction.
What does really inflate the BOM ?
And this is in no way to discount your effort or your results, I'm genuinely curious.
And imagine how higher the cost would be if everyone involved in the chain was paid a living wage, from mining to processing to logistics to to to..
This product cost around 500$ and also has heat exchange. A friend of mine has installed it and is very happy with it.
EDIT: It does, if you click on "learn more", you'll learn more: "The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss." ...Perfect!
Currently I have two holes in my wall for ventilation, when it is windy it's too much (feel the wind blowing inside), when some people visit and there is no wind, boom, >3000 ppm CO2 in 20 minutes.
I just really hope it is very quiet, although it says ~37 dBa (which is quite a lot imho), I replaced my bathroom ventilator recently, it produces 25 db! [1]). The previous one [2] produced 52 dB (cheapest around), that was pretty annoying, you'd hear it in the bedrooms above the room it was used in. Maybe 37 dB it isn't so bad, especially since you can wind it down and mostly need it when it's busy/noisy (many people) anyway.
Btw, don't buy a CO2 sensor, pretty soon you're a ventilation nerd, or as my wife would call it, a ventilation curmudgeon.
[0] https://blaubergventilatoren.de/en/series/vento-expert-a50-1...
[1] https://www.filterfabriek.nl/ventilatoren/badkamerventilator...
[2] https://www.hornbach.nl/p/rotheigner-toilet-badkamerventilat...
Absolutely.
> A room is not heated by increasing its internal energy but by decreasing its entropy due to the fact that during heating, the volume and pressure remain constant and air is expelled.
https://pubs.aip.org/aapt/ajp/article-abstract/79/1/74/10418...
The point about balancing airflow is crucial, but I think underappreciated by non-professionals. Thermodynamics is highly non-intuitive in places, and the enclosed climate-controlled spaces we love to inhabit are certainly included in that.
Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
Oh man I had this discussion with my wife yesterday, we have a small electric heater in a room where a pipe burst and I still need to fix that (no heating means instant fungus problems). It keeps its fan rotating always, that way it determines the input temp for its thermostat more accurately. But wife insists it is sometimes blowing cold air and thus very very bad... I explain what a thermostat is (bimetals and all) and that she experiences "coldness" because a layer of warm air is blown from her skin, it's not blowing cold air... she doesn't follow... I even measure the energy usage and the thing only uses 20 W or so when just blowing, not heating. Even when just blowing it's moving cold moist air from the walls so overall good. It's difficult dealing with her like this.
I'll pay someone to tell me how to deal with someone like this and maintain a positive atmosphere. The thing is, I also do it for things that really are probably not worth discussing... I should pick my battles better, is there ever a good time for some mansplaining? Or should I say... Nerdsplaining?
Place and leave a thermometer in front of it and give her the information that you are using to make your own claims.
You went through a process to learn that moving air feels colder than still air at the same temperature. It seems that perhaps she has not. Surfacing the ground truth of the air temperature may help the situation.
Actually, I didn't put an ice cube on the metal, that is the trick. of course.
[0] https://www.youtube.com/watch?v=vqDbMEdLiCs
On that note, I'm curious if hybrid heating/cooling solutions will ever take off. Other than this OpenERV product I mean, which I guess technically counts!
Low Tech Magazine mentioned some experiments in their article on compressed air energy storage (CAES). Instead of trying to make that form of energy storage an adiabatic process, the idea is to use the heat produced/required in the compression/decompression steps in the household to improve the energy efficiency (e.g. use heat produced during compression to heat water; do the decompression in a space that should be cold anyway like a basement used for food storage).
[0] https://solar.lowtechmagazine.com/2018/05/ditch-the-batterie...
> Recuperative types are what most people think of, consisting of a thin layer of material that separates two gas streams. Regenerative heat exchangers are different. They briefly store the energy while air flows in one direction, then release it when the air flow reverses.
I have to admit I am slightly more dubious about this type as they are new to me, though I did see a YouTube video about a commercial one recently and they seem to be a hot new thing.
Possibly this is something sensible that only becomes practical with software and wireless communication? Rather than running ducts to a central location.
Though then fitting two side by side in a window seems odd. Why not use the traditional type in that case?
They are rated 90% recovery at low speed. Today it's 11C 75%RH outside, 18C 65%RH inside, at low speed (15m3/h rated at 1.2W) there's barely a difference: 17.8-17.9C air intake temperature. They keep the air noticeably fresh, drier and also keep the CO2 down (<600ppm right now). I'm running them below the "recommended" 50% air-change per hour (ACH about 35%), and boost when needed.
There's a recuperative ducted type in the attic for the first floor, when I checked last month it was 4C outside, 18C at the outlet vent, and 17C at the inlet vents. That runs at 50% ACH.
The reasoning for the paired up window model isn't obvious, maybe a simple increase in capacity. The website is quite clear you need a push/pull pair to be efficient, and an immediately adjacent such pair is not going to work so well.
It effectively heats the fins slightly in one direction, then cools them again slightly in the opposite direction.
Same with wetting and drying for recovering moisture.
I am surprised that the fins appear to be plastic - one would imagine that steel fins would have far better thermal capacity