> But heat pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures.
This claim defies thermodynamics (and I expect better from IEEE).
Heat pumps are bound to be less efficient as the spread between inside and outside temperature grows. (They might be just as effective or they might be just as efficient as current heat pump tech, but they will not be as efficient at -20°C outside air temp as they are at +10°C.)
No, they're talking about air source ones (which is what's popular these days):
> At about 4 °C, most air-source heat pumps currently on the market start operating at less than their full capacity, and at some point (usually around -15 °C), they can no longer do the job at all. At that point, an auxiliary heat source kicks on, which is less efficient.
> But advancements in compressor technology over the last five years have addressed that issue. By controlling the compressor motor’s speed, and improving the timing of when vapor is injected into the compressor, engineers have made heat pumps efficient in colder temperatures.
Yes on rereading it definitely appears phrased the way you interpreted. But as you said that's nonsensical so my brain must have dumped the nonsensical interpretation. :)
I've found that marketing material takes significant liberties with their claims, with the understanding that the audience isn't actually interested in the gory details, just the general question of "should I buy option A or option B?" I've been asked to critique statements that were flat-out wrong, only to be told yeah, thanks, nobody cares about that.
If you want to get nitpicky: Cycle losses aren't the only source of inefficiency and in fact the end-to-end efficiency curves of most domestic heat pumps tend to be pretty flat through their useful range. Basic thermodynamics, provided a refrigerant that is well matched to the cycle (which is what the article is actually about), isn't really the core design point.
I've never seen a CoP curve that was not downward sloping with decreasing outside temperature.
Do a google image search for "heat pump cop curve" and glance at the results. All of them are significantly downward sloping because the underlying thermodynamic cycle effect is very strong (agree not the only) limit.
My father has a heat pump that works fine in alpine climate (half of the winter snow outside) for more than a decade now. Not sure about the precise english terminology here, but his hear pump works using the deep ground water next to our house. The temperature of that water slightly dips in winter, but it is no more than a degree.
It is a very old house that before had a oil heating system and I think the replacement worked quite well.
That’s a ground-source heat pump (often called geothermal heat pump). These have been around for decades in the U.S. but haven’t been very popular due to the 2-3x installed cost vs. an air source heat pump.
This is common in some parts of the world (which is great!), but not in the US. They're quite a bit more expensive, especially compared to modern mini-split air-source heat pumps that can cost just a couple thousand dollars and don't require any digging or duct installations, just a small hole in the wall.
I guess my father was just lucky, because 1m through the wall from our oil cellar we had an old well that could be used, so there was no need for much more digging (except 1m sidways).
He added a ton of solar as well, which is a nice combination.
It can be. Would you take an even-money wager that the CoP curve will be flat [or increasing] with decreasing outside temp vs my wager that it will be downward sloping with decreasing outside temp?
The claim is true.
You've transformed their claim into something else.
What they wrote is " But heat pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures."
That is about whether they gained the technology to make the finished units as efficient in bitter cold as they are currently efficient in mild winter.
Given they were inoperative in bitter cold, and fairly inefficient in mild winter, this is almost certainly trivially true.
If we all mild winter 32F, and bitter cold -10F, they would only have to improve efficiency by 2x at bitter cold to make it as efficient as current heat pumps as mild winter.
That is easily possible.
As for whether they are strictly flat/decreasing, they are not.
You can see capacity, input power, and efficiency do not have a strictly linear (flat or decreasing) relationship with outdoor temperature. This is an artifact of it being impossible to good at all things at once.
It is also obvious to every reader you can not make a "maximally efficient in all" cases heat pump as efficient in bitter cold as in mild winter. So rather than assume that is what they are claiming by adding a lot of implicit things to it, i would just read it in a straightforward manner and when I do, it seems pretty easily true.
In Kelvin the percentage difference looks a lot smaller than -20°C to +10°C, more towards a rounding error for an ideal heat pump.
So if they can operate at the same percentage efficiency away from ideal, the extra difference from the actual source/sink temperature could be very minimal right?
Is there a way to get your house (currently with hydronic gas heat) retrofitted for heat pumps that doesn’t result in either a ton of demo or ugly lines all over the exterior? I tried to switch over last year only to find the savings were not that great in my climate, and that it would have an unacceptable aesthetic cost.
I think probably the ugly lineset covers characteristic of lowest possible budget (and therefore overwhelmingly most common) installation of air-to-air heat pumps.
It is -- it looks just like an air conditioner plopped on a pad. There's two lines connected to it and leading into the house; one is an insulated copper pipe, and the other is the mains power connected to an electrical disconnect box.
Air to water heat pumps exist that can efficiently supply flow/LWT* of 135°F, so if you can turn down your existing boiler to 135°F flow and maintain temperature in the house, you can retrofit to an A2W heat pump, which does not result in your home being festooned with linesets and covers running around the outside like plastic termite tunnels.
I also tried to switch in 2022 and found that it would literally never pay off due to the dramatically higher upfront project cost, and so a combi gas boiler now hangs on my basement wall.
* - LWT = "leaving water temp" aka "supply" aka "flow" depending on where you are in the world
No, I think you got it. You either have a lot of work to install ductwork or you're going to be doing lots of mini splits with ugly lines all over the exterior.
I had heat pumps installed in my house last year, to replace oil-based heat from baseboards. We have no drainage lines on the the front of the house, and only one bundle on the back. It follows the roof line, and then down a corner of the side of the house. So, yes, you will need drainage lines, but no, they shouldn't have to be all of the exterior. They should be able to be placed in an acceptable way. (Similar to, say, gutters.)
On the savings side: I think we're about even compared to oil. But I'm okay with that: we didn't have any air conditioning at all in the house, so we had no duct work. (We bought the house at the end of 2022 like this.) Since we were already doing significant work to install the duct work and get a new air conditioning system, I wanted to make them heat pumps to get off of oil. I think in the long run, we'll save money. I eventually plan on installing solar panels. And even if it's a wash in terms of money, I'm care about climate change, and I want to do what I can to electrify my house.
edit: After looking at other comments, I should clarify what we have. We have two condensers outside, and three air handlers inside. It acts like "central air", for the most part. We do have one mini-split, for the main bedroom, as we could not get ducts to it. My point here is that "heat pumps" do not necessarily mean "mini splits." My house is almost entirely centrally heated and cooled through duct work.
> Is there a way to get your house (currently with hydronic gas heat) retrofitted for heat pumps that doesn’t result in either a ton of demo or ugly lines all over the exterior?
It's literally the same physical design as an air conditioner. So... yes, unless you hate seeing AC units too, I guess.
A house with hydronic gas heat probably doesn’t have ducts right now. An owner of a house asking how to retrofit a heatpump without a ton of demo increases the likelihood that their house doesn't have ducts.
Adding ducts to an existing house qualifies as a ton of demo in virtually every case.
I have my doubts about heat pumps ever being viable for Canadian and other high-latitude countries -40 degree winters. Everywhere else though, they probably should be the standard.
What fraction of the population lives with -40 degree winters? That's the only reason they might not be viable, in that they require customization that is only useful to a tiny tiny tiny fraction of the population.
A huge chunk of canadas population in Quebec, Ontario, prairies, and BC see -20 for weeks on end. -40 is more rare but happens every year. Huge chunks of the mid-western USA see those as well. At -30 my furnace is basically on continuously.
A bit late, but maybe someone will read this. I live in Toronto and installed a heat pump this year. We've been through the worst of Winter, I believe, and although the cost ramps up on the coldest days, the heat pump worked perfectly at -15C (it claims a COP of 2 at that temperature). I watch the auxiliary circuit, that is triggered if the heat pump fails to heat to the required temperature, and it never ran for the whole season. Yes, Winter can still get a lot worse and a lot cooler, but my house also has insulation and there is no scenario where I will freeze to death.
In terms of cost, it's a lot cheaper than gas + electricity (it evens-out on the coolest days, and is cheaper otherwise). It will not amortize the cost of the heat pump itself for a while, but that's a cost I'm happy to eat right now.
This would be nice. It's not the end of the world to convert them myself but given the large readership base that uses Freedom I'd like to see these with both. It's much easier for them to add our units than it is for us to adopt metric here anytime soon.
As someone who switched to a heat pump recently, you should know "your mileage may vary." Don't confuse physical efficiency for cost efficiency. In many places, natural gas is still more cost efficient, even if it isn't as efficient in terms of physics.
Natural gas is dirt cheap in most of the U.S., so it does make it hard for heat pumps to compete, especially if electricity is expensive. Just checked my most recent gas bill and I only paid $1.09/therm (plus 7% sales tax and $10 fixed fee).
OTOH if you’re heating with propane or fuel oil, heat pumps are probably a good alternative.
Thats kind of high. I only pay $0.78 a therm here in Wisconsin. 1 therm is around 30kwh so im paying like 2.5c a kwh vs 17c for electricity. So unless I can get a CoP of 700% from a air source heatpump im literally lighting money on fire. That really is the main issue, as much as I care about emissions I don't care enough to spend money on a heat pump to then spend more money on utilities. And the calculus gets worse and worse when your poorer.
seems pretty comparable to the other colder parts of the country. (besides the east coast) The warmer parts of the US i'd imagine are less sensitive to residential gas prices.
Last year I had to replace my HVAC. I was set on getting a heat pump, being a huge fan of efficiency. I was averaging a $1.05/therm delivered for gas, and about 17.5 cents/kwh for electricity delivered. A therm is about 29 kwh, so I pay the equivalent of $5.07/therm in electricity.
The best whole house air-source heat pumps had a SEER2 of 18.5, or a COP of just under 5.0. I need a COP of over 5 to just break even in terms of cost efficiency. So a heat pump would actually cost me more on an ongoing basis. And the price for a heat pump installation was $10k more than a furnace/AC.
I just couldn't do it. To pay more up front and then ongoing was just too much. If you showed me even a super long range projection of saving money, I would have been down for it, but it just was not there.
If I can get solar on my house it may make sense, I will cross that bridge in a few years as my roof is almost end of life, but that has its own headaches as I have limited space (brownstone, flat roof), and am in a historic district.
Next go around, as heat pump tech improves and hopefully gets cheaper, it will probably make sense, but for me right now, it just didn't work.
As someone who switched four years ago to a heat pump plus conventional furnace, I can state from experience that my thermostat takes electricity cost per kWh and gas cost per therm (other units are available, these are just the ones that match my utility bills) and efficiency of both the heat pump and furnace and runs whichever is cheaper.
Gas cost has varied a bunch over the years I’ve had it, electricity less so. But the furnace has only run a few days per year and only when when it’s been (depending on gas cost) below 10 to -5 degrees Fahrenheit.
My heat pump (trane xv19) kicks ass and if I were doing the hvac upgrade again I’d skip the furnace entirely and let the heat pump work a little harder/longer on the coldest days.
It’s damned near always more cost-efficient to run the heat pump. And my shiny new high efficiency furnace will never, ever pay itself off saving me a few bucks a day the three or four days per year that the math works in its favor.
So your heat pump's COP has to exceed the ratio between natural gas and electricity in your area to make it cost-effective (or you can get solar + battery backup).
Electricity prices can vary quite a bit, too... in Cali, for example, it's gone up from 14¢ to 24¢ over 16 years, which is nearly double the rate of inflation.
> As someone who switched four years ago to a heat pump plus conventional furnace, I can state from experience that my thermostat takes electricity cost per kWh and gas cost per therm […]
What is the make and model of the thermostat you are using?
Is it from the heat pump OEM (Trane?) or a third party?
Heat pump is a Trane XV19 and the thermostat is an xl 1050 (supposedly trane but my understanding is it’s a rebranded Nexia)
The process to update the costs (which has to be done a few times per year when my utility changes their rates) is not very user friendly - have to get into the technician menus and change scary looking stuff) but it works quite well.
Especially if you get your electricity from PG&E, who charges ridiculously high transmission and distribution fees.
It makes financial sense in a lot of places to switch to solar plus battery for nearly all energy use. Especially if you can charge your EV directly from solar. Heat pumps at night and in the evening need the batteries though.
Fracking has made natural gas extremely cheap in the U.S over the past several years. Apparently, right now its at the lowest inflation-adjusted price since 1990 per the WSJ: https://archive.is/8wEi7
A good HVAC contractor will create a design based on your building and location, and it will cover 99% of average days efficiently for heating and cooling:
For the remaining 1% it will basically have to run all the time to keep up. (In any given year you may not even get below/above the average outlier 1%.)
Gas powered heat pumps also exist, using the same principle as gas powered refrigerators… from what I’ve heard they are already used in some countries but not on in the USA.
When did you last see a kerosene-powered fridge? It's about 40 years since I last saw one. Hence my 'obsolete technology' label.
Reverse-cycle (heating) heat-pumps rely on fans to extract the heat away from the machine and send warm air into the room. If you have electricity for the fans, you might as well use that electricity to power the heat-pump too.
modern RVs still widely use propane powered absorption refrigeration, although that is starting to move over to solar and battery arrays.
The fans are a tiny part of the energy used by a heat pump, most of the energy is for running the compressor. Electric heat pumps are more efficient and economical than electric heat, but still generally more expensive than gas heat, and impractical compared to gas if that electricity itself is made from gas in the first place.
If you power a heat pump with gas directly, you are effectively getting over 100% efficiency: you keep all of the heat generated from the gas, plus are 'pumping' heat from outside. It's going to be even more economical than regular gas heat, which is already more economical than anything else.
We decided to install a cold climate central ducted heat pump system, operating down to -20 F and swapped from wood to pellet stoves in our fireplaces. We didn't have AC previously, and got too old to haul wood around. We can run the heat pump on the coldest days but burning biomass is cheaper and has a nice ambiance. Does wonders for humidity in the summer time too!
I just got one installed last month in middle of winter at my home in New England.
The air from heat pump feels more comfortable than the dry air from the furnace.
I got installed with a the furnace back-up option, that way if the heat-pump can't generate the heat, it switches to propane (it becomes very cold here in New England in Winters). From my experience, the heat pump is working well in 30F and above weather (-1.C)
Heat pumps burn a lot of electricity, luckily I have solar to off-set that cost, although Solar generation is low in Winter, it adds up well over the year.
I was poking through my Thermostat's App a couple of weeks ago, and found a setting that said "Use Gas", "Use Heat Pump", and "Let system decide". I had thought our furnance was running the heat pump automatically... for the last 5 years. I switched, and now the display says "Heating with Heat Pump" and I'm a little upset that it wasn't more obvious before now.
I feel you! There’s a lot of specifics that are hard to know without digging in and making mistakes!
I just found out my heat pump was configured incorrectly in the Nest Thermostat bc I have a Rheem and their heat cool valves are reversed from typical.
TLDR my January heating bill was $500 bc heat pump never came on!
I recently had a Mitsubishi hyper heat installed to replace my natural gas furnace. The air handler is connected to the ducts so it was an easy transition. I live in Chicago so I also had 10kW resistive heat strips installed as a backup. This winter they never needed to come on, and I was able to keep the house at 70 degrees F even when we hit around 0 degrees F outside.
I haven't crunched all the numbers but so far my electricity bill has gone up by about as much as my gas bill has gone down, so there really isn't much savings there which is too bad. I mainly did this though for the environmental benefits, as I want to slowly phase out all natural gas in my home. Next step will be to replace my gas range with an induction cooker, and the last will be a heat pump water heater.
If you're considering a heat pump, be sure to check the minimum operating temperature before you buy, and/or make sure you have a backup heat source.
I live in a rented apartment with a mini-split, and it worked great for 363 of the last 365 days. Those two days, we had a sudden cold spell and temperatures went down to -6 F (which isn't that cold), but we found out the hard way that our unit was only rated to -4 F.
Once it went down to -5, the heat pump didn't just work less efficiently, it stopped working altogether. It was the only real heat source in the house. We tried turning on the kitchen oven for a while, but it couldn't keep up. We had to leave the apt for those days and crash with a friend.
Other than that, though, it's been wonderful! SO much nicer than other heat sources (furnaces, space heaters, etc.) It's comfortable, cheap, quiet, and has its own thermostat. We just leave it on and it does its thing and it's magical.
I don’t think it’s normal for a heat pump to just stop working completely like that vs. just getting more and more inefficient. Wonder if perhaps it iced up and the defrost wasn’t working.
That's because the heat-exchanger is not efficient enough to prevent too much frost from building up. The answer there is a better (probably bigger) heat-exchanger.
Yep, correct. As many have pointed out in this thread though, HVAC is full of trade offs. I COULD get a bigger heat exchanger.. Although my furnace is already a 5 ton unit. The question is, in the Ozarks, where I am, are the 1-4 days a year where it's too cold to run the heat pump worth paying the extra money for the larger system?
I added an efficient wood stove instead. There's a good tax rebate that covers installation as well - I took advantage of that last year.
It's entirely possible? Mitsubishi makes a bunch of different models, and ours is a cheaper one. Our area rarely gets under 0, so it's understandable why the landlords chose this model (and it's also an older model, for an older apartment). They have other models rated down to -15 or lower.
But even if it's because the defroster stopped working, or it's just an auto-shutoff feature to keep the equipment safe... the end effect is the same: There is no more heat when you most need it, and you'll freeze. We were wrapped up in multiple layers that day and tried to warm the house however we could, but ended up just needing to stay with a friend cuz it was so cold.
In some areas, "two sources of home heating" is mandated by law, but not where I live now. It still seems like good advice, specially if you can do that with two separate fuel sources (like electricity vs natural gas vs propane vs heating oil), in case one infrastructure goes down, as they often do during the worst storms.
My heat pump is 12 years old with a CoP of only 3 so when it gets down to -15C, it's less efficient than a resistive heater so it shuts off and the display shows P3. It was designed that way so you can have a secondary heat source set a few degrees below the temp you normally want so that kicks in when the heat pump shuts off. My backup is the electric baseboards I used before I got the heat pump.
Some heat pumps have a resistive heater built into them which kicks in when it's too cold for the heat pump to have a CoP of greater than 1.
As someone who lives where natural gas isn't available, virtually everyone here had either EBBs or an oil furnace. A heat pump uses as little as 1/4 the electricity but uses more as you get into lower temps. A heat pump remains more efficient than EBBs down to around -25C. Heating oil is more expensive than EBBs, so very expensive compared to the electricity needed for a heat pump, and switching is a 'no brainer'.
So far, 35% of households in my province have installed heat pumps, most keeping their EBBs or oil furnace as a backup. Every home that switches from EBBS is cutting their heating electricity usage by 50% (annually), freeing up electricity for the homes switching from oil furnaces to use for their heat pumps so our grid isn't being overloaded.
I very recently replace my forced air gas furnace with a forced air heat pump (with 10 KW of auxiliary restive heater as well) in southern Ontario.
Over the last 11 days or so (February with a combination of cold and unseasonably warm days) I've used 1047.15 kWh of electricity, of which I estimate half to two-thirds is used on the heat pump. Taking the new ultra-low overnight pricing into account I'm paying an average of 9¢ (CAD) per kWh. I've been trying to pre-heat my home at the tail end of the ultra-low overnight rates (2.8 ¢/kWh) leading up to the 7 a.m. cutoff time.
My gas furnace was my only gas appliance, so I can remove now remove it and save on the month connection charge which is about $30 a month, even in the summer when I don't use gas.
Canada also, at least for now, has a pigouvian carbon tax, so I'll be saving on that as well.
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[ 3.8 ms ] story [ 171 ms ] threadThis claim defies thermodynamics (and I expect better from IEEE).
Heat pumps are bound to be less efficient as the spread between inside and outside temperature grows. (They might be just as effective or they might be just as efficient as current heat pump tech, but they will not be as efficient at -20°C outside air temp as they are at +10°C.)
> At about 4 °C, most air-source heat pumps currently on the market start operating at less than their full capacity, and at some point (usually around -15 °C), they can no longer do the job at all. At that point, an auxiliary heat source kicks on, which is less efficient.
> But advancements in compressor technology over the last five years have addressed that issue. By controlling the compressor motor’s speed, and improving the timing of when vapor is injected into the compressor, engineers have made heat pumps efficient in colder temperatures.
The way I read it was "just as efficiently as a conventional furnace". IOW, a COP of 1 at bitter cold temperatures.
Do a google image search for "heat pump cop curve" and glance at the results. All of them are significantly downward sloping because the underlying thermodynamic cycle effect is very strong (agree not the only) limit.
It is a very old house that before had a oil heating system and I think the replacement worked quite well.
He added a ton of solar as well, which is a nice combination.
The claim can be literally true.
It may be they are at x% efficiency in both cases, where x is optimal for bitter cold but not for mild winter.
It can be. Would you take an even-money wager that the CoP curve will be flat [or increasing] with decreasing outside temp vs my wager that it will be downward sloping with decreasing outside temp?
What they wrote is " But heat pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures."
That is about whether they gained the technology to make the finished units as efficient in bitter cold as they are currently efficient in mild winter.
Given they were inoperative in bitter cold, and fairly inefficient in mild winter, this is almost certainly trivially true.
Here is a real curve: https://hvac-talk.com/vbb/attachment.php?s=f7eb5f83efc880440...
If we all mild winter 32F, and bitter cold -10F, they would only have to improve efficiency by 2x at bitter cold to make it as efficient as current heat pumps as mild winter.
That is easily possible.
As for whether they are strictly flat/decreasing, they are not.
Here is more data: https://connect.fujitsugeneral.com/3cfb13a7-a386-4697-aa3e-5...
You can see capacity, input power, and efficiency do not have a strictly linear (flat or decreasing) relationship with outdoor temperature. This is an artifact of it being impossible to good at all things at once.
It is also obvious to every reader you can not make a "maximally efficient in all" cases heat pump as efficient in bitter cold as in mild winter. So rather than assume that is what they are claiming by adding a lot of implicit things to it, i would just read it in a straightforward manner and when I do, it seems pretty easily true.
So if they can operate at the same percentage efficiency away from ideal, the extra difference from the actual source/sink temperature could be very minimal right?
Or do you have existing ductwork (which a house with hydronic gas heat probably does not have, or else they'd probably not be asking the question)?
I understand the situation -- I had searched to see if hydronic gas heating used ducts, and it can but it seems that this is not common.
Their issue is not an issue of heat pumps themselves, but rather the lack of an air handler.
I also tried to switch in 2022 and found that it would literally never pay off due to the dramatically higher upfront project cost, and so a combi gas boiler now hangs on my basement wall.
* - LWT = "leaving water temp" aka "supply" aka "flow" depending on where you are in the world
On the savings side: I think we're about even compared to oil. But I'm okay with that: we didn't have any air conditioning at all in the house, so we had no duct work. (We bought the house at the end of 2022 like this.) Since we were already doing significant work to install the duct work and get a new air conditioning system, I wanted to make them heat pumps to get off of oil. I think in the long run, we'll save money. I eventually plan on installing solar panels. And even if it's a wash in terms of money, I'm care about climate change, and I want to do what I can to electrify my house.
edit: After looking at other comments, I should clarify what we have. We have two condensers outside, and three air handlers inside. It acts like "central air", for the most part. We do have one mini-split, for the main bedroom, as we could not get ducts to it. My point here is that "heat pumps" do not necessarily mean "mini splits." My house is almost entirely centrally heated and cooled through duct work.
It's literally the same physical design as an air conditioner. So... yes, unless you hate seeing AC units too, I guess.
Adding ducts to an existing house qualifies as a ton of demo in virtually every case.
In terms of cost, it's a lot cheaper than gas + electricity (it evens-out on the coolest days, and is cheaper otherwise). It will not amortize the cost of the heat pump itself for a while, but that's a cost I'm happy to eat right now.
OTOH if you’re heating with propane or fuel oil, heat pumps are probably a good alternative.
The best whole house air-source heat pumps had a SEER2 of 18.5, or a COP of just under 5.0. I need a COP of over 5 to just break even in terms of cost efficiency. So a heat pump would actually cost me more on an ongoing basis. And the price for a heat pump installation was $10k more than a furnace/AC.
I just couldn't do it. To pay more up front and then ongoing was just too much. If you showed me even a super long range projection of saving money, I would have been down for it, but it just was not there.
If I can get solar on my house it may make sense, I will cross that bridge in a few years as my roof is almost end of life, but that has its own headaches as I have limited space (brownstone, flat roof), and am in a historic district.
Next go around, as heat pump tech improves and hopefully gets cheaper, it will probably make sense, but for me right now, it just didn't work.
Gas cost has varied a bunch over the years I’ve had it, electricity less so. But the furnace has only run a few days per year and only when when it’s been (depending on gas cost) below 10 to -5 degrees Fahrenheit.
My heat pump (trane xv19) kicks ass and if I were doing the hvac upgrade again I’d skip the furnace entirely and let the heat pump work a little harder/longer on the coldest days.
It’s damned near always more cost-efficient to run the heat pump. And my shiny new high efficiency furnace will never, ever pay itself off saving me a few bucks a day the three or four days per year that the math works in its favor.
Electricity can range from 11¢/kWh to 29¢/kWh in California or higher: https://www.energybot.com/electricity-rates/
Natural gas ranges from (converted) ~ 2¢/kWh to 8¢/kWh in Florida: https://www.eia.gov/energyexplained/natural-gas/prices.php (using conversion factors from https://www.nrg.com/resources/energy-tools/energy-conversion..., which I think assumes 1000 ft^3 ~= 1 therm ~= 29.3 kWh)
So your heat pump's COP has to exceed the ratio between natural gas and electricity in your area to make it cost-effective (or you can get solar + battery backup).
Electricity prices can vary quite a bit, too... in Cali, for example, it's gone up from 14¢ to 24¢ over 16 years, which is nearly double the rate of inflation.
What is the make and model of the thermostat you are using?
Is it from the heat pump OEM (Trane?) or a third party?
It’s a rebranded Nexia
* https://www.bayareaservice.com/wp-content/uploads/2020/02/18...
The process to update the costs (which has to be done a few times per year when my utility changes their rates) is not very user friendly - have to get into the technician menus and change scary looking stuff) but it works quite well.
It makes financial sense in a lot of places to switch to solar plus battery for nearly all energy use. Especially if you can charge your EV directly from solar. Heat pumps at night and in the evening need the batteries though.
As page where you can enter some numbers:
* https://www.heatpumpswork.com/cop-target
Also, given a US zip code, how often the temperature may drop below 15F / -10C (which is a cut-off for many units):
* https://www.heatpumpswork.com/cold-days
A good HVAC contractor will create a design based on your building and location, and it will cover 99% of average days efficiently for heating and cooling:
* https://www.greenbuildingadvisor.com/article/design-temperat...
* https://www.airequipmentcompany.com/2021/what-does-design-da...
For the remaining 1% it will basically have to run all the time to keep up. (In any given year you may not even get below/above the average outlier 1%.)
When did you last see a kerosene-powered fridge? It's about 40 years since I last saw one. Hence my 'obsolete technology' label.
Reverse-cycle (heating) heat-pumps rely on fans to extract the heat away from the machine and send warm air into the room. If you have electricity for the fans, you might as well use that electricity to power the heat-pump too.
The fans are a tiny part of the energy used by a heat pump, most of the energy is for running the compressor. Electric heat pumps are more efficient and economical than electric heat, but still generally more expensive than gas heat, and impractical compared to gas if that electricity itself is made from gas in the first place.
If you power a heat pump with gas directly, you are effectively getting over 100% efficiency: you keep all of the heat generated from the gas, plus are 'pumping' heat from outside. It's going to be even more economical than regular gas heat, which is already more economical than anything else.
The air from heat pump feels more comfortable than the dry air from the furnace. I got installed with a the furnace back-up option, that way if the heat-pump can't generate the heat, it switches to propane (it becomes very cold here in New England in Winters). From my experience, the heat pump is working well in 30F and above weather (-1.C)
Heat pumps burn a lot of electricity, luckily I have solar to off-set that cost, although Solar generation is low in Winter, it adds up well over the year.
Heat Pumps + Solar is the way to go.
I just found out my heat pump was configured incorrectly in the Nest Thermostat bc I have a Rheem and their heat cool valves are reversed from typical.
TLDR my January heating bill was $500 bc heat pump never came on!
I haven't crunched all the numbers but so far my electricity bill has gone up by about as much as my gas bill has gone down, so there really isn't much savings there which is too bad. I mainly did this though for the environmental benefits, as I want to slowly phase out all natural gas in my home. Next step will be to replace my gas range with an induction cooker, and the last will be a heat pump water heater.
I live in a rented apartment with a mini-split, and it worked great for 363 of the last 365 days. Those two days, we had a sudden cold spell and temperatures went down to -6 F (which isn't that cold), but we found out the hard way that our unit was only rated to -4 F.
Once it went down to -5, the heat pump didn't just work less efficiently, it stopped working altogether. It was the only real heat source in the house. We tried turning on the kitchen oven for a while, but it couldn't keep up. We had to leave the apt for those days and crash with a friend.
Other than that, though, it's been wonderful! SO much nicer than other heat sources (furnaces, space heaters, etc.) It's comfortable, cheap, quiet, and has its own thermostat. We just leave it on and it does its thing and it's magical.
I added an efficient wood stove instead. There's a good tax rebate that covers installation as well - I took advantage of that last year.
But even if it's because the defroster stopped working, or it's just an auto-shutoff feature to keep the equipment safe... the end effect is the same: There is no more heat when you most need it, and you'll freeze. We were wrapped up in multiple layers that day and tried to warm the house however we could, but ended up just needing to stay with a friend cuz it was so cold.
In some areas, "two sources of home heating" is mandated by law, but not where I live now. It still seems like good advice, specially if you can do that with two separate fuel sources (like electricity vs natural gas vs propane vs heating oil), in case one infrastructure goes down, as they often do during the worst storms.
Some heat pumps have a resistive heater built into them which kicks in when it's too cold for the heat pump to have a CoP of greater than 1.
So far, 35% of households in my province have installed heat pumps, most keeping their EBBs or oil furnace as a backup. Every home that switches from EBBS is cutting their heating electricity usage by 50% (annually), freeing up electricity for the homes switching from oil furnaces to use for their heat pumps so our grid isn't being overloaded.
Over the last 11 days or so (February with a combination of cold and unseasonably warm days) I've used 1047.15 kWh of electricity, of which I estimate half to two-thirds is used on the heat pump. Taking the new ultra-low overnight pricing into account I'm paying an average of 9¢ (CAD) per kWh. I've been trying to pre-heat my home at the tail end of the ultra-low overnight rates (2.8 ¢/kWh) leading up to the 7 a.m. cutoff time.
My gas furnace was my only gas appliance, so I can remove now remove it and save on the month connection charge which is about $30 a month, even in the summer when I don't use gas.
Canada also, at least for now, has a pigouvian carbon tax, so I'll be saving on that as well.
The cost of a heat-exchanger is a compromise between cost and efficiency.