The Achilles’ Heel of Heat Pumps
Air source heat pumps are a great way to keep a home comfortable. On the cooling side, they’re exactly the same as an air conditioner. In heating mode, though, they have a limitation that other types of heating equipment don’t have. That is, the heating capacity of most air source heat pumps decreases as the outdoor temperature goes down. That’s not the Achilles’ heel of heat pumps I’m talking about, but it’s related.
If the heat pump can’t provide as much heat as the house needs, we just have to figure out how to get the extra heat we need. The most common way it’s done is to add electric resistance, or strip heat, to the system. When the heat pump isn’t quite carrying the load, it kicks on and adds extra BTUs to the air stream. And this is where the Achilles’ heel of heat pumps comes in. Let’s dive in.
Electric resistance vs. compressor heat
Electric resistance auxiliary heat is just a big toaster element that gets installed in the airstream of a heat pump. The photo above shows one I saw recently. It’s downstream of the coil because otherwise hot air would blow over the coil and cause it not to work.
Now, the next statement is going to drive some people crazy, but I’m gonna say it anyway: Electric resistance heat is 100 percent efficient. All of the electrical energy getting used there becomes heat. OK, a there might be a bit of light, noise, and vibration, but that all turns into heat, too.
And this will drive those people even crazier: By the same measure, a heat pump is about 300 percent efficient. That is, the amount of heat energy delivered to the home is about three times more than the electrical energy used to operate the heat pump. Yes, the efficiency of an air source heat pump decreases as the outdoor temperature drops, but it’s still more than 100 percent in all but the coldest weather.
The takeaway here is that you want to use the heat pump as much as you can and rely on the resistance heat only when necessary.
[For the curious, the reason talking about the efficiency of strip heat and heat pumps drives some people crazy is that it’s the wrong quantity. We really need to talk about coefficient of performance (COP), which is defined as the ratio of the amount of useful heating provided to the amount of energy used to provide that heat. So technically, I should say electric resistance has a COP of 1.00 and a heat pump has a COP of about 3.]Incorrect thermostat advice
I’m not going to say much here because I’ve covered it in an article that you can read for a full explanation. Basically, though, the problem here is that some bad advice about how to set the mode for the heat pump keeps getting spread around.
The thermostat for a heat pump gives you two modes for heating: Heat and Emergency Heat. When you choose Heat mode, it’s supposed to rely on the compressor for as much heat as it can provide and then call on the strip heat when that’s not enough. It’s a way of providing just enough strip heat to keep the house comfortable and energy bills reasonable.
If you select Emergency Heat, you’ve just turned off a working compressor and now will get all of your heat from the electric resistance strips. That means you’ll be paying two to three times as much for that heat as if you were using the compressor plus the strip heat. Read more about it here.
Incorrect settings or wiring
The goal in using strip heat is to use the compressor to provide as much heat as it can and then supplement with the strip heat when necessary. But whether that happens or not depends on the controls. One way to control the auxiliary heat is to base it on the difference between the thermostat setpoint and indoor temperature. If the gap is larger than the setting dictates—say, 2 °F (~1 °C)—the strip heat comes on to assist.
Another method is to have an outdoor temperature sensor. Then the installer sets the outdoor temperature that triggers the strip heat. If it’s set for 40 °F (4.5 °C), you’ll use more strip heat, most likely unnecessarily. If the system is sized correctly, the heat pump’s balance point should be about 30 °F (-1 °C) or maybe even lower.
Then there’s something called lockout. It’s an outdoor temperature setting at which the heat pump compressor is locked out. Generally, that should be used only when your auxiliary heat comes from a furnace. If you have electric resistance auxiliary heat, you’ll end up with higher bills, again unnecessarily.
Finally, it’s not that uncommon for the thermostats and auxiliary heat to be wired incorrectly. That can make the strips come on when they’re not needed. The worst case here is when they come on in cooling mode. Yes, that really happens.
Alternatives to electric resistance
One way to avoid having problems with electric resistance auxiliary heat is not to use it. No, you won’t freeze. You just need to have another way of getting supplemental heat when the outdoor temperature is below the balance point and emergency heat when the heat pump stops working.
Here are a few ways to do that:
- Cold-climate heat pump. Some heat pumps can maintain full capacity down to -5 °F (-20 °C) and still get about 80 percent at -15 °F (-26 °C). That’s usually the kind that lets you avoid strip heat, but it only avoids the need for supplemental heat. You still need a plan for emergency heat.
- Furnace as backup. This one’s not just supplemental heat. It’s replacement heat because the furnace and heat pump don’t run at the same time. At the switchover temperature, the heat pump goes off and the furnace kicks on. This is called a dual fuel system.
- Hydronic coil. Instead of having strip heat after the heat pump coil, you could have a hydronic coil providing the supplemental or emergency heat. (Details here.)
- Redundancy. If you have multiple indoor units to heat your home, you can still get heat even if one unit goes out. This helps more with cases where you need emergency heat, but it may help with the need for supplemental heat, too, if you have excess capacity in some zones that can help heat other areas. But you have to have multiple outdoor units for this to work. (See my article on redundancy for more.)
- Space heaters. For homes with low loads, a space heater or two may be a perfectly adequate source of auxiliary heat. Gary Nelson heats his Minneapolis home with a heat pump that has no auxiliary heat. He told me that if it’s so cold the heat pump can’t keep up, he can get the extra heat he needs with a space heater…or even by baking cookies.
- Woodstove. This one’s controversial in the world of building science, but it’s what I’m planning to use as my source of emergency heat.
- Ground-coupled heat pump. I know I’ve focused on air-coupled heat pumps here, but another way to avoid electric resistance heat could be to go with a ground-coupled heat pump. They don’t lose heating capacity the way an air-coupled heat pump does when the outdoor air temperature drops, but that’s not a guarantee you won’t need some kind of auxiliary heat. (More here.)
Electric resistance is often necessary
Every house and every project is different. Some people (like me) are OK with undersizing their heat pumps and letting the indoor temperature drop below the setpoint during extreme weather. Others want to be able to keep the house at 75 °F (24 °C) on the coldest days. Then there’s the matching of the equipment capacity to the loads, the type of equipment selected, and the climate zone. All these things go into deciding what to do about auxiliary heat.
In our HVAC design projects at Energy Vanguard, probably half or more of our projects have strip heat. It’s the easiest way to assure that they’ll be covered when it’s cold outdoors or when the heat pump breaks.
Final thoughts
Electric resistance heat isn’t a bad thing to be avoided at all costs. It’s fine to install it. You just want to make sure it’s called on only when necessary.
Achilles, you probably know, was quite the accomplished warrior. But he had that one weak spot on the heel his mother held when she dipped him into the river Styx. Likewise, heat pumps are great at doing their job, but the Achilles’ of heat pumps is getting set up improperly with electric resistance auxiliary heat. It’s a weak spot that can cause higher upfront cost (especially if you need a panel upgrade) and higher electricity bills.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.
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“…the amount of heat energy delivered to the home is about three times more than the electrical energy used to operate the heat pump…”
The first law of thermodynamics is a fundamental principle that states that energy can be converted from one form to another, but it cannot be created or destroyed. It is essentially the law of conservation of energy applied to thermodynamic systems. The key points about the first law of thermodynamics are:
The total energy of an isolated system remains constant. It is said to be conserved.
Energy can be transferred between a system and its surroundings in the form of heat (Q) and work (W).
The change in the internal energy (ΔU) of a system is equal to the sum of the heat (Q) supplied to the system and the work (W) done on the system.
This is typically expressed as the equation:
ΔU = Q + W
Heat (Q) is the transfer of thermal energy due to a temperature difference. Work (W) is the energy transferred due to other processes like pushing, pulling, or electromagnetic forces.
If work is done by the system, W is considered negative. If work is done on the system, W is positive.
If heat flows into the system, Q is positive. If heat flows out of the system, Q is negative.
In essence, the first law accounts for the transfer and conservation of energy in thermodynamic processes involving heat and work interactions. It forms the basis for studying the relationships between heat, work, and internal energy changes in systems.
All that is when you have a sealed system. The electricity is not the only source of heat with a heat pump. The pump moves heat from outside into the house. That is not subject to the laws you quote.
All true, but I think I missed your point?
The laws of Thermodynamics are not broken with heat pumps and it is also true that the amount of heat energy pumped into a house with a heat pump is greater than the energy used by the pump almost all the time (12kw of heating effectively using 3kw of power). Heat pumps do not convert electrical potential to heat, they move heat from outside to inside like a reverse air conditioner. The energy the heat pump uses to operate is actually energy lost by imperfect thermal movement.
Great piece on resistance heating backup. The one thing that wasn’t covered is thermostat setbacks that cause the resistance heaters to come on every time the thermostat “sets up” . Many of us like it cooler for sleeping. This phenomenon(resistance kicking in at set up) can really add to the electrical consumption of a heat pump system.
Some thermostats can have enough set point changes to allow you to ramp back up without kicking in the dreaded resistance heaters.
Another solution is to keep them turned off completely. I did that with mine and eventually the heat pump didn’t keep up, but it was at the 1% temperature for my climate. With that knowledge one could simply only turn it resistance heaters back on when it got really cold, or in my case simply ride through with a few degrees cooler temperature. (the outdoor sensor could do this if the system allows for it)
Granted this doesn’t work for everyone since a lot of people just want it to do everything without ones interaction.
Wayne: Thanks for bringing that part of it into the conversation. It’s related to what I said about monitoring the difference between the thermostat setpoint and indoor air temperature. With a big setback, the strip heat might be called on to do more heating than necessary. If you’re doing those setbacks manually, it most likely will ramp up the strip heat. If you program it into a thermostat that has adaptive recovery, it will start early enough to bring the indoor temperature up by the time it’s needed, thus reducing the use of strip heat.
Another factor that results in low efficiency during sub freezing Temps is the heat pump will frequently go into defrost mode if the outdoor coil gets too cold. This effectively turns it into an air conditioner. I live in Southern AZ and have a very efficient trane heat package heat pump with strip heaters that generally does a good job when winter Temps are above 30 f. If it gets colder (mornings can get to the low 20s) I use space heaters as much as possible. My winter heating bills are typically 30% higher in the coldest months vs the hottest months.
The real Achilles heel is the more you need it the less efficient it becomes so like if the temperature difference is 40 50° from what you need it to be so not worth it
The other need for heat strips wasn’t mentioned in the article is when the heat pump goes into defrost mode. Essentially the heat pump goes into cooling mode and the heat strips are turned on to avoid cold air coming from the vents during defrost. The outdoor coils can accumulate frost which diminishes capacity during heating mode. Basic defrost modes operate every 30-60-90 minutes as selected for a set period of time of 5-15 minutes. Newer boards monitor the time since last defrost and the temperature of the outdoor coils and only run the defrost cycle if needed. They also terminate the defrost based on the temperature of the outdoor coils with a tmer as a failsafe.
Almost all except the very best heat pumps reverse modes to the cooling cycle to heat the condenser during defrost.
Auxiliary heat during defrost is primarily used to maintain warm air for human comfort during defrost reverse mode cycles.
Time based defrost is from the seventies and is used on the lowest cost heat pumps.
Demand defrost based on logic and temperatures became common twenty years ago and is the current standard for conventional split systems.
Most minisplits reverse without any sort of auxiliary heat. Some conventional split systems are installed without aux heat.
Dual fuel heat pumps use conventional furnaces during defrost while the highest end inverter systems use hot gas bypass defrost. This method sends hot refrigerant routed in a separate circuit to the condenser while still maintaining inside refrigerant based heat. Hot gas is the most complex from a device required perspective.
In our “pretty good house” we have sealed combustion gas logs as our “backup”. Yes, I know they produce CO2, but I want a backup source that works in case the power is out and sometimes it is just nice to cuddle by the fire. 🙂
Norman: The only pushback you’ll get from me on that is if the gas logs aren’t vented to the outdoors. But I imagine yours are.
Yep.
The biggest issue was getting a set that will not drive you out of the house and still be aesthetically pleasing. If i remember correctly, mine is rated at 45,000 BTU which is more than enough to heat the entire house at its lowest design temp. Normally, the flames are at their lowest possible setting and it can still get warm on a fall evening. 🙂
Great article! Can you elaborate on 1) cold climate heat pumps “you still need a plan for emergency heat” If you selected a heat pump that has adequate capacity at your design condition then what is the purpose for emergency heat? Is it just for when the heat pump fails? Or just for those 0.4% of days when it gets below design conditions?
Eli: Yes, emergency heat is mainly for mechanical failure of the heat pump. That way you still have some heat in the house. Items 2 through 6 in that list can help with emergency heat.
I’m not understanding these heat pumps
We have a mini split on lobby of bldg
Which is directly under my floors
It always seems so cold in my apartment
Are these pumps set for cooling?
I appreciate your expertise this Hvac mess
Tks
Benie: I’m guessing the mini-split in the lobby below your apartment isn’t the problem. It’s probably either lack of insulation and air sealing, thermal bridging, poor windows, or your apartment’s heating system. You might want to hire an energy auditor or home performance assessment pro to have a look.
What about the electrical supply upgrades needed to run the strips?
As part of my HVAC replacement saga with contractors, it seems the strip heat kits are forcing a much larger electrical connection to the inside unit resulting in a required service panel upgrade.
Are others seeing the same thing in their quotes?
In my case, I think we will be skipping the strips entirely and going to baseboard or small in- room heaters to supplement if needed. It appears to be a much cheaper option than spending multi-thousands on an service upgrade to handle the strips.
Jberger: Yes, some homes may need a panel upgrade when adding strip heat. Home Energy Analytics studied the electrical power needs of >22,000 homes in California and found that 100 amps was enough for 98 percent of them. Make sure your electrician assesses your electrical loads accurately. Don’t automatically assume they did.
Strip heat conversion of electricity to heat might be 100% efficient, but doing that conversion outside the space to be heated..in outside winter temperatures then pushing heat into the house isn’t.
Allison – first time, long time😂
I’m in NY (Zone 4) and we had Bosch Heat Pumps installed in 2021, blowing through existing AC vents in the ceiling or high walls (cathedral ceiling). Our goal was to get rid of the old natural gas boiler (baseboard heat) but when it gets below 35-40°, we have to turn off the heat pumps and turn on the boiler because the heat pumps don’t produce enough heat. Our gas heat is on a separate thermostat, not the Em Heat setting of HP Thermostat.
Should/Can I:
1) Replace these heat pumps (not the air handler) with newer “cold weather” heat pumps.
2) Add something to air handler, like coils or heat strips I read about or
3) Stick with my gas boiler (until it fails in the next few years) and connect it to my Em Heat option
Or is there a better/different option?
Thanks Allison and to the many other thoughtful responders on these posts!!
Michael: I’d hate to see you have to replace a three year old heat pump. But why do you turn it off when you turn the boiler on? They should be able to run simultaneously. Then the boiler will provide just the supplemental heating you need, not all of it.
It always seemed like we’d be doubling up on bills, but of course once the set temp is reached, both will shut off. 🤔. What about when my 40 year old boiler finally goes? Is there no way to make the heat pump self-sufficient in my climate?
I am in the process of spray foaming my roof deck with closed cell and dense-packing cathedral ceilings and exterior walls with cellulose. That will keep more heat in, but won’t help the heat pump produce more heat in below 40° temps!
Michael: I’m not sure about your heat pump. Seems like it should do more heating than it you said it does. It could be undersized or maybe there’s something wrong with it.
No, a better building enclosure won’t make your heat pump move more heat, but it should help it keep the house warmer at lower temperatures since you won’t need as much heat.
A couple years ago I had an issue with my brand new heat pump not working when the outside temp got below freezing. A gecko had climbed into the inner workings of the heat pump, touched two spots he should not have, fried himself, exploded all over the place, and burned just a small part of the circuit board. The truly bizarre thing is that the system still ran. All that was effected was the part where the coils reversed long enough to let the heat pump work in colder weather and the electric furnace. Aside from that one problem, I have had no issues and no need for emergency heat. I live in North Carolina and the pump alone works fine down into the 20s (F). That said, is it not standard to include an electric furnace (or whatever other furnace type available to that home) instead of just a few resistance strips? Or is that what you are/were referring to? The electric furnace seems to be standard around where I live. Thanks!
Joe: An electric furnace is an air handler with an AC coil and strip heat, but it’s not a heat pump. All the heat comes from the strips all the time. It’s the same thing, just used differently. Georgia banned electric furnaces as primary heat over a decade ago.
I’ve got LG and Mitsubishi mini splits that are from 2010 (now considered OLD from technical viewpoint, lol) . I’m comfy at 70F inside with temps being 0F outside in a 1979 house that isn’t insulated and sealed even close to a modern home. These aren’t even cold rated Heat Pumps and they’re doing fine at 5F and 14F below their rating respectively. Presuming the house isn’t too leaky air wise and insulated reasonably well, the only time need aux heat is when using old school (non inverter based) heat pumps (which are awful for winter and shouldn’t be sold anymore, but are and is what this article refers to), or non cold rated modern heat pumps when get up into Canada and Alaska. Then parts of Canada and Alaska will always need supplemental regardless. But for the lower 48? It’s fine.
Great article, Allison. Am I mixed up though? In your sentence, “If the gap is larger than the setting dictates—say, 2 °F (3.6 °C) …”, should this be 2 deg C (3.6 deg F)? Isn’t 1 deg C worth 1.8 deg F? Thanks.
Sam: Thanks for the correction! I’ve fixed it now. It’s 2 °F (~1 °C).
Achilles heal? No heat during power failures. or when something goes wrong with the heat pumps. Equipment issues include complicated low ambient controls, ecm blowers and elaborate refrigeration circuits.
Techs with knowledge and parts is an increasing concern as low ambient heat pumps with vapor injection, electronic metering devices and communicating modules become common.
Power failures are the biggest concern because heat pump redundancy fails as well.
In Texas we have decades of experience with heat pumps and know hard freezes statewide strain or break the grid. Texas governance makes this worse since Texas does not connect to adjacent grids by design.
Fuel based heat can be operated with a small generator and affordable dual fuel generators are ideal for the purpose, particularly if you already have natural gas or propane.
Wood stoves or fireplaces do provide heat without electricity or when generator power can not run low ambient hp loads and or heat strips.
Dual fuel heating, eg heat pump and ng/lpg backup is a smart way to use low ambient heat pumps. Proper controls are still essential and dual fuel systems often require better Honeywell or similar thermostats.
Your statement below is really the Achilles heel of heat pumps…
“In Texas we have decades of experience with heat pumps and know hard freezes statewide strain or break the grid. Texas governance makes this worse since Texas does not connect to adjacent grids by design.”
I live 1000 miles or so north of you. In Alberta our max electrical loads are during Arctic outbreaks. This coincides with low to no wind, short days for solar with some cloud, fog or snow cover on the solar…..
Alberta is a gas heating market. New natural gas furnaces or boilers are 90+% efficient, even the old ones 80 Percent…… As natural gas fired electricity is about 50 effective at turning the energy from NG into electricity the break-even point for heat pump on a C02 basis would be a cop of 2
Short point of the story is that Alberta would need to double it’s electrical output on cold days (when renewables are useless) if we were to switch to 100 percent air source heat pumps today.
All a pretty complex system that would cost a lot more for some “carbon reduction”.
I heat primarily with wood, a Hearthstone soapstone stove. The heat pump is our backup for when we are away from the house for more than a 24 hour period. The heat pump has never let us down.
If I have one complaint it’s the slowness of bringing the house to temperature after returning from a long absence. The oil furnace was fast and intense.
What you didn’t explain is when a heatpump goes into a defrost cycle the white wire will be energized from the board to turn on the heat strips. The board also has settings 30/60/90/120, because the outdoor line sensors will activate. Always hire a licensed HVAC company.
In 2018 I installed a 3 ton 16 SEER Goodman at our 1125 sq ft mfd house in Tucson , AZ. It’s not a cold climate unit and came with 8 kW of strip heat (2 4 kW coils). The coldest temperature I’ve recorded is only 25F and when the strip heat comes on the air temp at registers tops 140F. This seems oversized to me. Could I get more compressor run time and therefore better overall efficiency if I reduced the strip heat to 4 kW?
Thanks for your many years of highly informative articles and your recent book.
In your article you conclude with “the Achilles’ of heat pumps is getting set up improperly with electric resistance auxiliary heat”. So I would have to assume that the 50% of you projects with electric resistance auxiliary heat must be in places with a design temperature lower than -15F. Otherwise wouldn’t good design preclude the need for electric resistance or any backup heat? Even if a Cold Climate Heat Pump (CCHP) only produces 80% of its rated capacity at -15F, you can design for that or you can have a customer agree to allow a little lower indoor temperature on those really cold days. That’s just a tradeoff and a good designer can quantify that.
You go on to further muddle the topic with “Then there’s something called lockout.” and “You still need a plan for emergency heat.” Your argument has some of the hallmarks of the fear mongering and upselling of backup systems so prevalent in the HVAC industry. Nobody ever talked about emergency heat in the bad old days of fossil fuel systems. Boilers broke, electricity went off, you had to call your service company.
I am very concerned this type of thinking will filter down to homeowners and property owners and we will end up with everybody using their heat pumps as glorified air conditioners with a little heat when its not too cold out. We will end up not even close to carbon neutral and our grandkids will pay the price.
Please, there is so much we all need to know and new habits we need to develop as we transition from full on/off fossil fuel systems to these complex and sophisticated VRF systems. Help us with the building science of that.
Thank you Tim!
I was screaming inside that the Achilles heel of heatpumps is improper planning.
Many HVAC installers do not know how to determine the necessary heat demand. Unfortunately, there is a habit in the industry to just look at what exists and replace it with what exists vs. taking measurements and performing the calculations to determine the actual system size that is necessary.
I understand that old building methods are to blame as well. New high performance construction that is much tighter on air exchanges and reduced thermal bridging will help. Reduce energy needs for future homes; but, most builders today don’t even do blowdown tests to determine a homes ACH value. They just put the code minimum insulation bats in the wall and then drill a bunch of holes in it to run pipes, conduit and chases through them.
TIMOTHY: No, good design does NOT preclude the need for strip heat. Some installers deliberately undersize and add strip heat for better comfort and dehumidification. And it depends on how close the equipment capacity is to the loads. With heat pumps, you’re trying to optimize for both heating and cooling.
Old info. Newer modulatng heat pumps work better and do not require resistance heat.
Robert: There are plenty of modulating systems that still need auxiliary heat. It all depends, as I said above, on the loads, the type of equipment selected, and the climate zone. And also the homeowner. Some contractors deliberately undersize and add strip heat for better comfort and dehumidification.
Allison: Can you explain how strip heat provides dehumidification?
Roy: Thanks for asking that, since I didn’t explain the connection. The strip heat doesn’t provide better dehumidification directly…most of the time anyway. But when you try to avoid auxiliary heat by sizing a heat pump to the heating load, you often end up oversized for cooling. By sizing to the cooling load, you get more runtime and better dehumidification in cooling mode.
I think I know what you’re going to say next: At design conditions, even an oversized system runs often enough that when the unit comes back on after a pause between cycles, most of the water is still on the coil. And I believe that to be true, too. But a smaller heat pump does better at part-load conditions, and that’s where you really feel the lack of dehumidification capacity.
I’ve also heard of contractors who use strip heat as reheat after overcooling the air, but I don’t know much about that.
Robert. A common problem is a house that needs 36k btu cooling and 70k btu heating. Such as a Texas home constructed almost anytime but certainly those built before year 2000. Even an aggressive solution like a 48k btu modulating system won’t cover the heating load which is often days at a time every year.
Dual fuel is the lowest operating cost while 15k of strip heat and a heat pump the lowest upfront cost. Dual fuel has the added advantage of low cost generator backup during frequent grid outages which can be dangerous in winter.
Houston has ten of thousands without power since last week but 90 degree heat and high humidity is just uncomfortable, but not life threatening for most.
In my case, with a high efficiency gas furnace, I will go with a dual fuel system when my AC crashes (not just a bad condenser). The ideal switchover point will depend on the COP of the heat pump at the specific outdoor temperature and the costs of electricity and gas at the time. I know there is at least one thermostat that will do this automatically, based on real time energy rates and the COP vs temperature curve for the heat pump. It will switch back and forth based on the which fuel is the least expensive heat source at any given time. I’m in no rush, however, since even with this type of control my savings will be quite low at this time since in my area (Ontario Canada) electrical rates are 3x the cost of gas on a heat delivered basis.
The author made a mistake here…the thermostat also has two internal sensors …when the stat is on heat the primary is the compressor.the second sensor is for your electric heat.it is set 3 degrees below set point..when it’s cold out the compressor cannot make a delta t high enough to make temp…the thermostat will wait until the temp has dropped three degrees and then bring on heater.the heater then has to raise temperature but will cut off 1 degree before setpoint..this make your compressor run 24/7during cold days..setting your heater to emergency will make heater the primary thus making it more comfortable and heater runs less time…save machine and energy to turn to emergency heat or put telp control outside so asunder 35 heater becomes primary
Resistance heating with a COP of 100% is AN ELECTRIC FIRE.
Ive had a Bosch 2 ton variable for a couple years for my 1200 sqft early 70s house in northern wisconsin. Its rated down to like -5f, but is virtually useless below 20-25 deg, depending on the wind. At 20 deg, it will run constantly, and lose maybe a degree an hour. Using 3kw at 16 cents during peak, and 8 cents at offpeak, the switchover to natural gas depends on when its running.
Last winter was weird, and never got below 10f for any amount of time, so i mostly ran the pump.
One thing that no one seems to consider regarding overall costs is compressor life. A compressor used only for cooling should last 25 years or longer. If used for a heat pump it will probably run 5 to 6 times more often (based on New Jersey climate). Therefore, you should figure on replacing your compressor every 5 to 6 years and factor in that cost.
Also, since your condenser coil, heat pump mode, does not produce temperatures as high as other forms of heating appliances, the air being circulated into your room may actually feel slightly cool.
My opinion, having been an HVAC contractor for 25 years, is great for the south, not so much for the north.
What about VRF condensers and utilizing 4 smaller air handlers instead of 2 large ones for a 3 bedroom 2500 s.f. home??? The Mitsubishi Hyper-Heat condensers are rated to -20°F and a higher COP than your listed value of 3…. that way the, shaded side of the house in the winter, units will provide the heat and the sunny side doesn’t need as much…
My house does not have air conditioning. If I install a heat pump for heat 6 months of the year and then use A/C for the hot months, how does that “save energy” and make the world a better place?
I think I’ll keep burning 80 – 100 year old carbon in my wood stove in the winter and open the windows in the evening during the summer.
Since you need back up heat anyway, why not just install the gas furnace and forget the heat pump. Then you only have one system to maintain.
The present push for heat pumps is now more for decarbonization than saving money. What’s at stake is more than just rising temperatures and sea levels, it’s the entire ecosystem for food production. I could go on.
https://www.nrdc.org/stories/what-are-effects-climate-change#humans
When I installed my Mitsubishi hyperheats in 2017 I intentionally had 2 outside compressors in the hope that if they could heat my house on the coldest days ( suburbs of Philly) then I’d be able to get rid of my oil boiler as 2 compressors would provide redundancy. After a couple of winters never using the oil heat and only using it for domestic hot water I removed the boiler and the tank and installed a HPWH. I’ve never had an issue heating in the winter since but admittedly the house isn’t huge and it’s pretty well insulated and sealed.
So, would you say using setback might actually COST rather that saving money? 🧐😏
Also missing is demand defrost, and defrost without strip if it doesn’t cause comfort issues.
Finally, completely missing and very important is reheat dehumidification. Add those and you’ve got a great article!
The 600lb gorilla in this room is that nobody talks about the cost of maintenance and potential service of Inverter Heat Pumps and it comes as a big surprise to owners when something goes wrong and it does. That’s when all of the talk of high COP goes out the window so to speak. A HP has to run almost continually and most are made in less than a stellar fashion. The electronics are very sensitive and expensive even in warranty provided you can find someone capable to service them. Until I see more honesty about this I will call this a sham. Gary
IDK, everything breaks eventually including oil and gas boulders. My Mitsubishi hyperheats are 7 years old and, knock on wood, the only thing that’s failed was a line set not made by Mitsubishi.
I live in Michigan and have had a Mitsubishi Hyperheat system since fall of 2019 with backup resistance heat. I sized the system so the resistance heaters were there more for a backup than needed for undersizing. We like a night setback for sleeping. Basically I have the resistance heaters shut off with the breaker so they won’t come on and that has worked well. We do get high humidity high temperature days in the summer and the system seems to reduce the humidity enough. The variable output may take care of it. What many people don’t realize is that these systems can drop their output to about half of their rated output, so in effect they can look like an undersized unit.
They even run at a little higher efficiency in the minimum load operation (@ 95 outside minCOP 4.08 and max COP 3.88) (in heating @17F min COP 2.36 and maxCOP=3.6)
My Bosch 2 ton does an amazing job at cooling. It gets hot in my area, but usually not for extended periods, and rarely over 90F. My thermostat report said i used it for 95 hours last month. In cooling mode, it averages about 1.5 kW. Adjusting for on/off peak times, it probably cost us $20 to cool the house for a month. The neighbor has 2 window units installed, that run almost constantly. It probably costs them $75 and still isnt comfortable other than the rooms they are in
I just had a Lennox hybrid system installed: a heat pump with a gas furnace. The S-40 thermostat control system has two balance points: the high balance point (default 50 degrees) above which the gas furnace will not be used and the low balance point (default 25 degrees) below which the heat pump will not be used. How can the appropriate balance point settings be determined to minimize operating cost?
You really cant, unless your fuel rates remain constant and outside wind is predictible. i made a youtube video explaining my spreadsheet showing breakeven points. i have on and off peak rates, and the natural gas and electric rates tend to vary, if you have solar panels, that will factor in too. The heat pump is cheaper on off peak rates to a lower temp, but strong winds outside the house means the heat pump cant keep up like it does on calmer days.
seems the video link didnt get posted, here it is https://youtu.be/gwS20OK_Xzk
Thanks for the spreadsheet. When I have more time I’ll set it up for my situation.