The point is: you open those windows to let the humidity out. Warm air holds more water than cold air. The warmer it gets inside, the more humidity. If you don’t open your windows, the humidity escapes through least isolated areas, very often window sills. Sills get naturally colder, at below 12 degrees celsius you get mold in the sills. So, open your windows to blow the humidity out of the apartment but don’t open for too long so the sills don’t cool down too much, especially not below 12 degrees celsius.
Yes, the device is a Heat Recovery Ventilator[1]. As you increase the insulation and airtightness of the windows to conserve energy, you need to bring fresh air in steadily (and pump air out). The HRV handles that, and passes the incoming air through a heat exchanger with the outgoing air, which is useful whether you're trying to keep warm air inside (in the winter) or cool air inside (in the summer.
Insulation against heat transfer and insulation against mass transfer are also two different things. Although in a trivial setup the latter inevitably leads to the former, that doesn't need to be the case: Counterflow heat exchangers are a thing.
> Why not change the windows to open outward/upward?
Opening outward is much harder to handle (as it requires something to pull the window back in to close it); upward would need a robust mechanism that prevents the window from violently smashing down due to gravity.
Sliding windows have neither downside, but are much, much worse at noise and thermal insulation.
Related and currently on the frontpage with 80 comments: "Stoßlüften: Shock Ventilation" "This word (and concept) is especially found in public places and schools all around Germany." https://news.ycombinator.com/item?id=38813856
Based on the title I first thought there is an open source operating system used in Germany I was not aware of.
But is really true „Lüften“ is something we do in every room every morning here.
Luckily modern Bauhaus architecture style homes have windows down to the floor so that opening them is always possible because there is no sideboard with plants on top in front of the window.
It is actually kind of common in Germany that landlords attach copies of news articles about the need for correct ventilation or similar "notes" to rental contracts when handing them over to a new resident. Not exactly part of the rental contract (because it's not legally possible to dictate the exact ventilation cycle desires of the landlord in a rental contract), but still a very clear hint at what's expected of you.
I've never assumed anyone to actually do all of this excess ventilation, though...I've always been too lazy to "open all windows three times a day for five minutes each" (where the heck do people find the time to do that each and every day?), and the "solution" of just opening the window a little over a long period is so obviously expensive in terms of heating costs that I rather keep most windows closed most of the time (except bathroom after showers for obvious reasons).
The average person spends like 3-5 hours in front of a screen for entertainment only, I'm fairly sure spending 30s here and there to open windows is in the realm of possibilities
Yes, water vapor in the air causes mold, but here is the thing: the water only sticks to cold walls.
It is the combination of a cold wall plus water in the air that is the problem. Ventilation alone cannot make your wall warmer, in fact, excessive ventilation may cool your walls down even faster. As expensive as it might sound, the only reliable solution is to properly insulate the wall and keep it warm.
I'm renting an apartment in Germany in a building from around 1900 and I do have a clause requiring me to open my windows at least once a day for at least 5 minutes. Not sure that if it's legally enforceable, but if clauses about keeping some minimum temperature indoors are binding (judging from the fact that the government made them explicitly invalid at some point in 2022 when the country got left on thin ice after Russian gas got shut off), then I wouldn't be surprised if that one were binding as well.
Afair the rules about minimum temperatures that got loosened are all about requiring the landlord to provide sufficient heating that the home is livable (it must be possible to heat the home to 20°C during the day and 18°C at night with the provided heating). The tenant is only required to not damage the apartment by letting mold grow or pipes freeze.
OK, although "stupid Germans keep opening windows to defeat stupid green goals" is of course a take, in reality, a modern energy-efficient Euro-style building will:
-Indeed be 'airtight', in the sense that limited air (and noise, as a bonus) will enter or leave the building except via defined ventilation routes.
-Offer primary 24/7 ventilation (mostly for CO2 management) via ingress/egress vents as part of a balanced ventilation system, which performs 'comfort' regulation of incoming air temperature using heat recovered from outgoing air.
-Heat or cool the building via under-floor convection connected to a reversible heat pump. This will slowly but surely bring most of the building mass to a comfortable temperature regardless of season.
-In addition to basic ventilation and heating/cooling offer very-well-insulated yet securely operable windows for additional temporary ventilation. Window openings are very much defined ventilation routes as part of the 'airtight' plan.
The important things to keep in mind here are that the balanced ventilation system prevents things like mold already, and that opening windows for brief periods of time (as in: measured in hours) for (perceived) comfort won't impact energy efficiency very much at all.
The most relevant criticism here is that retrofitting existing buildings for the above scheme is a significant challenge, but that doesn't quite seem to be the point the article wants to be making...
I highly recommend installing a decentralized ventilation system with enthalpy exchanger in often used rooms like the sleeping room.
It's super efficient and improves air quality a lot when your building is highly insulated. The enthalpy part is important because winter air is pretty dry after heat exchange. Bonus points when you can fit good air filters (ePM1 50%+). Also, some systems offer a summer bypass for cooling in summer without AC as well as "smart functions" like orchestration of multiple devices or automatic control via CO2/humidity sensor.
Central ventilation is unfortunately very hard to retrofit...
If there’s actual poor air quality (smoke, etc), this is inadequate. 50% is fine for a recirculating system — fine particles will be reduced by half on each pass. For for a single pass system, if you have 50 μg/m3 outdoors (quite common in a lot of places, sadly) you get 25 μg/m3 coming in through your fancy ventilation system, which is not great.
What you want is a great honking filter, 95% or better, with a prefilter. You’ll pay several hundred dollars for that big filter, but you’ll be running it at low velocity (maybe 150cfm of flow in a 24”x24” filter housing), so it will have minimal pressure drop and will last a long time. Even an actual HEPA filter wouldn’t be out of place here.
But even the really fancy European ERV/HRV brands use those inadequate ePM1 50% filters. I’ve even measured them with a handheld particle counter — they work every bit as poorly as one would expect from the specs. And those filters are small, and they gunk up quickly.
Yeah, it's far from optimal, sadly. The best I found for my device were (allegedly) ePM1 70%. We get quite a lot of smoke exposure from badly behaved fireplaces/unsuitable (palette) wood right now and it reeks.
Got any suggestions how I could retrofit a better filter solution for my device (Zehnder ComfoSpot50). It has "slotted" filters like these https://cleanfilter.eu/en/product/zehnder-comfospot-50-filte.... Maybe something to chain outside in front of the intake or inside after the outlet (though I'm afraid I could run the Comfospot out of specs..)?
Or I guess I could kind of suboptimally get a standalone one.
These handheld particle counters look intriguing. Unfortunately a bit on the expensive side... Maybe I can borrow one, might give me some hard data for contacting my municipality about this.
Already ordered a Levoit Vital 200S Pro. Levoit EverestAir unfortunately is unavailable here.
IQAir Atem X and Allermed devices, that I've read good things about, are too expensive right now (and too large as well, considering the ~20m^2 room is relatively crammed).
Hope the device is a sane choice. As usual I needed to prevent myself from overanalyzing while battling spam blogs and broken corporate sites.
The IKEA things apparently are "only" EPA12, but totally ok for my case... Might still evaluate one if the Levoit doesn't perform well for some reason.
I think IKEA is on the right track with their lower efficiency filter. For a standalone air filter, you care about the rate of removal of contaminants, which is roughly (concentration of contaminants) · (flow rate) · (filter efficiency), and you probably end up caring the most about the particle size at which the filter is least efficient. So a real HEPA filter has .9995 for that last factor, and .99 is barely worse. Even MERV 13 at 75% or so isn’t so bad.
For a given amount of power consumption, you end up with (concentration of contaminants) · (flow rate) · (filter efficiency) / (power), which is (at fixed contaminant level and a fixed quality of fan) roughly proportional to (filter efficiency) / (pressure drop at design flow rate) and this is where IKEA is making the right tradeoff: those non-HEPA filters have considerably less pressure drop and only fail to filter a percent or so of the air going through.
Yeah, the Starkvind seems a better long term choice wrt energy consumption (and perhaps filter availability, but definitely filter price (performance?)). Thanks for the input and enjoy the new year.
The thing about HRV/ERV filters is that it is effectively a pre-filter, because the real filtering is supposed to be done by the HVAC system that the HRV/ERV dumps into.
You still get your HEPA filtering, because that's on your HVAC system.
Yeah I'm surprised how ventilation system is hardly seen in German houses. I suffer from terrible headaches in winter in Germany. On the other hand when I spend winter in France with closed windows and heating on, I have no issues. The only difference is in France where I stay, there is a modern ventilation system.
But you don't understand! It's simply impossible to sleep in a room without an open window! Every German visitor has insisted on this. I was born in Germany myself,and of course slept with the window open, that's just what one did. But have long ago come to the land of heating and air conditioning, sometimes both on the same day, and HEPA filtration, God forbid that dirty, unsanitary nature might make inroads in your germ-free sanctum.
The irony is that the windows that Germans insist on leaving open at night put our crank casement things to shame. Double gasketed? No, triple gasketed! Guaranteed airtight to god-knows-how-many psi, and without even a center post. And open horizontally or tilt vertically depending on which way you turn the handle.
They’re not exactly limited to Germany, although sash windows are more popular here. If you want them, just go to a nearby home improvement store and buy them. Search for “Tilt and Turn” casement windows.
Not one mention of a heat recovery system. You can have your house very airtight and still be able to turn the air over without loosing as much energy in the air.
Related to leaving windows open...My friend went to Germany for work last summer and was housed in a new apartment. While she mostly enjoyed it, she complained a lot about the lack of screens on windows to keep out the fruit flies. That could be one reason for not wanting to leave your windows open.
germans have the best engineers in the world and are the most clever people i have ever met. if they do something like aerating their homes, i will bet they are, again, more clever than us who don't do it as much... indoor pollution is something we don't detect and suffer, and again germans beat us there.
32 comments
[ 5.9 ms ] story [ 92.9 ms ] thread> The key may be to have less insulation, because the current system seems to be too airtight to accomodate for residents' differing preferences
Why not change the windows to open outward/upward?
1: https://en.wikipedia.org/wiki/Heat_recovery_ventilation
Yeah, that's an absurd suggestion.
Insulation against heat transfer and insulation against mass transfer are also two different things. Although in a trivial setup the latter inevitably leads to the former, that doesn't need to be the case: Counterflow heat exchangers are a thing.
> Why not change the windows to open outward/upward?
Opening outward is much harder to handle (as it requires something to pull the window back in to close it); upward would need a robust mechanism that prevents the window from violently smashing down due to gravity.
Sliding windows have neither downside, but are much, much worse at noise and thermal insulation.
But is really true „Lüften“ is something we do in every room every morning here.
Luckily modern Bauhaus architecture style homes have windows down to the floor so that opening them is always possible because there is no sideboard with plants on top in front of the window.
I've never assumed anyone to actually do all of this excess ventilation, though...I've always been too lazy to "open all windows three times a day for five minutes each" (where the heck do people find the time to do that each and every day?), and the "solution" of just opening the window a little over a long period is so obviously expensive in terms of heating costs that I rather keep most windows closed most of the time (except bathroom after showers for obvious reasons).
Something about that rubs me the wrong way. Doesn't it matter that the task is menial and doesn't seem all that effectual?
Yes, water vapor in the air causes mold, but here is the thing: the water only sticks to cold walls.
It is the combination of a cold wall plus water in the air that is the problem. Ventilation alone cannot make your wall warmer, in fact, excessive ventilation may cool your walls down even faster. As expensive as it might sound, the only reliable solution is to properly insulate the wall and keep it warm.
-Indeed be 'airtight', in the sense that limited air (and noise, as a bonus) will enter or leave the building except via defined ventilation routes.
-Offer primary 24/7 ventilation (mostly for CO2 management) via ingress/egress vents as part of a balanced ventilation system, which performs 'comfort' regulation of incoming air temperature using heat recovered from outgoing air.
-Heat or cool the building via under-floor convection connected to a reversible heat pump. This will slowly but surely bring most of the building mass to a comfortable temperature regardless of season.
-In addition to basic ventilation and heating/cooling offer very-well-insulated yet securely operable windows for additional temporary ventilation. Window openings are very much defined ventilation routes as part of the 'airtight' plan.
The important things to keep in mind here are that the balanced ventilation system prevents things like mold already, and that opening windows for brief periods of time (as in: measured in hours) for (perceived) comfort won't impact energy efficiency very much at all.
The most relevant criticism here is that retrofitting existing buildings for the above scheme is a significant challenge, but that doesn't quite seem to be the point the article wants to be making...
It's super efficient and improves air quality a lot when your building is highly insulated. The enthalpy part is important because winter air is pretty dry after heat exchange. Bonus points when you can fit good air filters (ePM1 50%+). Also, some systems offer a summer bypass for cooling in summer without AC as well as "smart functions" like orchestration of multiple devices or automatic control via CO2/humidity sensor.
Central ventilation is unfortunately very hard to retrofit...
If there’s actual poor air quality (smoke, etc), this is inadequate. 50% is fine for a recirculating system — fine particles will be reduced by half on each pass. For for a single pass system, if you have 50 μg/m3 outdoors (quite common in a lot of places, sadly) you get 25 μg/m3 coming in through your fancy ventilation system, which is not great.
What you want is a great honking filter, 95% or better, with a prefilter. You’ll pay several hundred dollars for that big filter, but you’ll be running it at low velocity (maybe 150cfm of flow in a 24”x24” filter housing), so it will have minimal pressure drop and will last a long time. Even an actual HEPA filter wouldn’t be out of place here.
But even the really fancy European ERV/HRV brands use those inadequate ePM1 50% filters. I’ve even measured them with a handheld particle counter — they work every bit as poorly as one would expect from the specs. And those filters are small, and they gunk up quickly.
Got any suggestions how I could retrofit a better filter solution for my device (Zehnder ComfoSpot50). It has "slotted" filters like these https://cleanfilter.eu/en/product/zehnder-comfospot-50-filte.... Maybe something to chain outside in front of the intake or inside after the outlet (though I'm afraid I could run the Comfospot out of specs..)?
Or I guess I could kind of suboptimally get a standalone one.
These handheld particle counters look intriguing. Unfortunately a bit on the expensive side... Maybe I can borrow one, might give me some hard data for contacting my municipality about this.
https://www.hvacquick.com/products/residential/Air-Filters/P...
Haven’t actually finished connecting it yet. It’s nicely constructed, though.
I'll look around a bit but I'll probably get a standalone indoor one.
IQAir Atem X and Allermed devices, that I've read good things about, are too expensive right now (and too large as well, considering the ~20m^2 room is relatively crammed).
Hope the device is a sane choice. As usual I needed to prevent myself from overanalyzing while battling spam blogs and broken corporate sites.
The IKEA things apparently are "only" EPA12, but totally ok for my case... Might still evaluate one if the Levoit doesn't perform well for some reason.
For a given amount of power consumption, you end up with (concentration of contaminants) · (flow rate) · (filter efficiency) / (power), which is (at fixed contaminant level and a fixed quality of fan) roughly proportional to (filter efficiency) / (pressure drop at design flow rate) and this is where IKEA is making the right tradeoff: those non-HEPA filters have considerably less pressure drop and only fail to filter a percent or so of the air going through.
You still get your HEPA filtering, because that's on your HVAC system.
The irony is that the windows that Germans insist on leaving open at night put our crank casement things to shame. Double gasketed? No, triple gasketed! Guaranteed airtight to god-knows-how-many psi, and without even a center post. And open horizontally or tilt vertically depending on which way you turn the handle.