Solar with Heat Pump for Heating/Cooling House

[sunnyfield]

My wife and I will be building a house later this year, and we're very interested in using solar to power as much of it as possible (ideally 100% after factoring in net metering). As we'll be building in southern Maine, we're very concerned about the cost to heat the home. Both our builder and the solar company we're looking into have recommended using a heat pump to heat and cool the home. Obviously heat pumps are not as efficient in cold temperatures, but they're saying that recent improvements (even in just the last couple of years) have made them highly efficient even as low as -13ºF.

The real issue is that we have no idea how much energy usage to expect from a heat pump for heating or cooling. The house will be around 2400 square feet, and we'll make sure it's as efficient as reasonably possible (triple pane windows and proper insulation). Does anyone have any experience using a heat pump to heat/cool a similar home? Even if it's not similar, how much energy does it typically pull at any given moment when running? What's the duty cycle like?

I realize these answers will vary quite a bit from home to home, but we just need to get a ballpark so we can get an idea of what to expect. Thanks so much in advance!

 

[arnolddeleon]

If you know what the heating load is with some standard units and you know COP of the heat pump then you can convert. An HVAC professional is supposed to be able to calculate your heating and cooling load. Your building contractor presumably has a sub-contractor for that part. If your are just trying to get a rough ballpark before you get there then you can use energy usage from any source and just do the conversions.

Heating with a heat pump is a more gentle process compared to say a gas heater. So it will be on more (possibly at lower levels if it's a multi-stage). If you have experiences with the draw of an air conditioner then the heat pump will be about the same when it is heating.

 

[boaterva]

Right, the good part about using a heat pump is that you can dump the load to solar and Powerwalls since it’s all electric. And I definitely agree with that (see my sig).

However, if you are one of those people (like myself ) that needs to stand over a heating vent in the winter (sometimes) and feel HOT air come out of it, then a heat pump isn’t for you.

Something to keep in mind if you’ve never had one before. You can use the emergency electric heat of the heat pump if your solar net metering will ‘pay for it’. But ‘hot’ vs ‘warm’ air is a factor for many people.

 

[arnolddeleon]

One of the secrets to using a heat pump is not set back as far. As @boaterva says it doesn't put out HOT air, it puts out warm air. I live in CA, so take this with a huge grain of salt, I only set back 2 degrees with heat pump setup (68 to 66). I believe this dramatically makes a difference in the "comfort" of a given temperature. I'm more comfortable in my house at 68 that other places where the air temperature is set higher but where the objects (walls/furniture) we allowed to be cold soaked. My theory is that since I don't let the interior can colder than 66, the objects in the house will be around that temperature.

 

[dhrivnak]

I live in NE Tennessee and at 7 degrees the HP has worked fine for us. It is simple and the new ones are much better. Not sure about -17 but in the single digits they do work well.

 

[boaterva]

One of the secrets to using a heat pump is not set back as far. As @boaterva says it doesn't put out HOT air, it puts out warm air. I live in CA, so take this with a huge grain of salt, I only set back 2 degrees with heat pump setup (68 to 66). I believe this dramatically makes a difference in the "comfort" of a given temperature. I'm more comfortable in my house at 68 that other places where the air temperature is set higher but where the objects (walls/furniture) we allowed to be cold soaked. My theory is that since I don't let the interior can colder than 66, the objects in the house will be around that temperature.

This is a good point. Since you don’t have the heat capacity to warm up quickly, you can’t ‘afford’ to set back to 64 from 70 or whatever during the day.

If you get/are used to 68 and never change it, you may be fine with this setup.

 

[ewoodrick]

Just get your builder or the solar company to give you a referral to others using the same unit in the same size house. If one of them
has a smart thermostat, they may be able to show you actual usage last year.

 

[aesculus]

I have used heat pumps in two homes over the last 40 years. Probably would never use them again if I had my choice. Here are my problems with them:

• They are noisy. You had better find a place to put the outside compressors that is isolated because they will run almost continuously in heating mode. Generally when used in AC mode they cycle a lot less and you are probably also awake then
• They fail miserably in moist and cold climates. Dry and cold they are better, The reason is that they will ice up on the outside coils and then have to run as an AC unit with heating strips on to melt the buildup and also not freeze you out of the house at the same time
• As @arnolddeleon points out you really don't want to set them back much because the whole house acts as a thermal flywheel. This means that you cannot have your bedrooms cold at night to sleep if that is important to you. It is to us.
• If you do get the home "behind the heating curve" then the system will monitor it's progress on heating and will most likely have to turn on very expensive heat strips to make up the difference in temperature in a reasonable timeframe
• Be careful when you compare the energy ratings on the units. Somewhere down the line they changed how SEER was done and it's no longer apples to apples when comparing old and new units. None the less I am sure a 2018 heat pump is more efficient than a 2008 heat pump, but not necessarily by the ratings they will show you.
• When they do the energy calculations its a lot more complicated than square feet. You have to include volume (ie ceiling height), thermal mass in the home ie tile floors, window amount and exposure, whether you use drapes or shades extensively, setback use, what temp you are comfortable with ... Many HVAC vendors get by with as little of Title 21 as they can and it might be OK for an average house with an ideal family, but if you are not an average house or fit the ideal model, beware.

Also be careful on planning on using heat pumps with a Powerwall. I doubt the output is sufficient to provide enough energy to run a heat pump. So at the end of the day you will really have to use power from the grid for much of your heating and hopefully bank enough credits in the summer to offset your winter use. Here in Calif they have recently enacted NEM 2.0 which you pay between 2 and 3 cents for every kWh that comes across the meter no matter how much energy you have "banked" during the summer. So using a heat pump in this case is not going to be as attractive as it was before when you could dip into your account for free.

 

[Big Earl]

One of the secrets to using a heat pump is not set back as far. As @boaterva says it doesn't put out HOT air, it puts out warm air. I live in CA, so take this with a huge grain of salt, I only set back 2 degrees with heat pump setup (68 to 66). I believe this dramatically makes a difference in the "comfort" of a given temperature. I'm more comfortable in my house at 68 that other places where the air temperature is set higher but where the objects (walls/furniture) we allowed to be cold soaked. My theory is that since I don't let the interior can colder than 66, the objects in the house will be around that temperature.

Correct. Doing a deep setback will make your furnishings cold, which take a while to heat back up. This can make a big significant difference in your comfort level.

 

[EVSteve]

Use a ground source heat pump for northern climates. This will reduce or eliminate any issues, use less power and the entire installation -yes including the vertical drilling- qualifies for the 30% renewable energy tax incentive. My house is roughly the same size but horribly inefficient from a thermal standpoint. It was built in 1942 and still has original windows. This is my HVAC usage over the past few years.

 

[Barklikeadog]

Use a ground source heat pump for northern climates. This will reduce or eliminate any issues, use less power and the entire installation -yes including the vertical drilling- qualifies for the 30% renewable energy tax incentive.

x2 on this. Maine is pretty far north for a heat pump. pay the price and go with ground source heat.
Pay a premium and get a good unit(carrier, trane) multiple speeds(that will reduce the noise)
the right installer is more important than the pump. find one that covers the unit and air handler 100% for 10 or more years as long as you keep a maintenance contract with them.

 

[dhanson865]

I have used heat pumps in two homes over the last 40 years. Probably would never use them again if I had my choice. Here are my problems with them:

• They are noisy. You had better find a place to put the outside compressors that is isolated because they will run almost continuously in heating mode. Generally when used in AC mode they cycle a lot less and you are probably also awake then
• They fail miserably in moist and cold climates. Dry and cold they are better, The reason is that they will ice up on the outside coils and then have to run as an AC unit with heating strips on to melt the buildup and also not freeze you out of the house at the same time
• As @arnolddeleon points out you really don't want to set them back much because the whole house acts as a thermal flywheel. This means that you cannot have your bedrooms cold at night to sleep if that is important to you. It is to us.
• If you do get the home "behind the heating curve" then the system will monitor it's progress on heating and will most likely have to turn on very expensive heat strips to make up the difference in temperature in a reasonable timeframe
• Be careful when you compare the energy ratings on the units. Somewhere down the line they changed how SEER was done and it's no longer apples to apples when comparing old and new units. None the less I am sure a 2018 heat pump is more efficient than a 2008 heat pump, but not necessarily by the ratings they will show you.
• When they do the energy calculations its a lot more complicated than square feet. You have to include volume (ie ceiling height), thermal mass in the home ie tile floors, window amount and exposure, whether you use drapes or shades extensively, setback use, what temp you are comfortable with ... Many HVAC vendors get by with as little of Title 21 as they can and it might be OK for an average house with an ideal family, but if you are not an average house or fit the ideal model, beware.

Also be careful on planning on using heat pumps with a Powerwall. I doubt the output is sufficient to provide enough energy to run a heat pump. So at the end of the day you will really have to use power from the grid for much of your heating and hopefully bank enough credits in the summer to offset your winter use. Here in Calif they have recently enacted NEM 2.0 which you pay between 2 and 3 cents for every kWh that comes across the meter no matter how much energy you have "banked" during the summer. So using a heat pump in this case is not going to be as attractive as it was before when you could dip into your account for free.

• depends on the heat pump, the one I have I can't even hear run unless I'm in the room nearest it and I listen for it on purpose. I can hear the air handler when it blows at 100% but not at 70% and below.

• true, my outside coils do ice up completely if it is raining and below 35 but that is a rare combination of weather/temperature. Maybe happens once a winter on average. Some winters 2 or 3 times, some 0 times, some 1 time. When that happens in the extreme case I have to turn off the heat and wait for it to warm up outside or take a hair dryer to the outside unit. But the majority of the time it isn't the humidity that runs up cost, its just the cold.

• I use a smart thermostat with at least 6 time of day settings and draw a curve, If 68 is the middle the curve would be something like (66,67,68,69,70,69,68,67, repeat) it's not a simple setback return, I set the high point of the curve as close as possible to the expected hottest parts of the day (with attic temp prolonging the afternoon warmth and direct solar exposure you can't go directly by outdoor temp). The thermostat keeps a heating curve and a cooling curve in those time of day settings and as the seasons change I leave the temps alone but change the time of the peak and trough (and to some extent the slope in between). It'd take 8 or 10 time periods to do it proper but 6 gets it done well enough.

 

[nwdiver]

Definitely go with ground source if you can afford it. If temps get <0F on a regular basis I think the expense of ground source is probably worth it.

For Air Sources some of the newer split systems are getting really good. IMO the top two are LG and Mitsubishi. They have ~73% of their capacity at -13F but I'd be surprised if the COP is >1.5...

I've got an older LG split system and love it. IMO a VFD compressor is a 'must have' especially if you want backup power. I use a 4kW Magnum inverter connected to a bank of golf-cart batteries for backup and I can keep a couple rooms cool or warm the entire night. Because of the VFD soft-start I don't even notice the AC cycles... unlike the refrigerator.

Another option for an Air Source Heat Pump would be Sanden which uses CO2 as a refrigerant. It's more efficient than R410a based heat pumps but not as good as ground source. It's primarily for water heating but you can probably plumb it to a radiator or something too.

 

[Jigglypuff]

Ground source for cold climates. If you want powerwalls, you'll need several for whole home backup. Consider maybe a hybrid system too.

 

[Zythryn]

My wife and I will be building a house later this year, and we're very interested in using solar to power as much of it as possible (ideally 100% after factoring in net metering). As we'll be building in southern Maine, we're very concerned about the cost to heat the home. Both our builder and the solar company we're looking into have recommended using a heat pump to heat and cool the home. Obviously heat pumps are not as efficient in cold temperatures, but they're saying that recent improvements (even in just the last couple of years) have made them highly efficient even as low as -13ºF.

The real issue is that we have no idea how much energy usage to expect from a heat pump for heating or cooling. The house will be around 2400 square feet, and we'll make sure it's as efficient as reasonably possible (triple pane windows and proper insulation). Does anyone have any experience using a heat pump to heat/cool a similar home? Even if it's not similar, how much energy does it typically pull at any given moment when running? What's the duty cycle like?

I realize these answers will vary quite a bit from home to home, but we just need to get a ballpark so we can get an idea of what to expect. Thanks so much in advance!

Holy cow, you are building our house
We aren’t in southern Maine, but central Minnesota (Minneapolis suburb).

Our house is 2450 sq ft.
Solar, geothermal, powerwalls and two Teslas.

Our house is 100% electric, no natural gas, propane, etc.
Our heat is infloor heat. Extremely comfortable. Extremely inexpensive to heat, cool, and charge cars

As others have noted though, the system has a lot of inertia. So temperatures don't change rapidly. We have it one degree cooler at night than during the day. You could run it at a larger difference, however the AC runs a lot harder that way. We do it simply because we are comfortable with it and it uses less energy.

A blog about my house is at Driven to Net Zero

Feel free to ask anything you like

 

[sunnyfield]

Thanks for the insight everyone! It definitely seems like a lot of people suggest geothermal over an air source heat pump. This was actually my preference too (especially because right now there are great rebates in Maine on geothermal, so it’s about the same cost as air source).

However, whenever I google the actual energy usage of geothermal, I see two types of results:
1) Blog posts / websites from companies that install geothermal
2) Crazed forum posts about how they are using exorbitant amounts of electricity ($1000+) because of it. They always say they’ve had the installer come out and look at it, but they of course say it’s working properly.

Now obviously there’s bound to be lemons or people who just don’t understand their electric bill will go up if they were using fossil fuels to heat there home before, but there’s a lot of horror stories out there. It’s disconcerting to say the least, especially when you factor in the cost to fix it if something were to ever go wrong.

Thanks for the link Zythryn! I haven’t looked through all the data yet but it looks like you have a lot there! Were you able to work out how much electricity was used by geothermal in the winter months?

 

[sunnyfield]

View attachment 313696 Use a ground source heat pump for northern climates. This will reduce or eliminate any issues, use less power and the entire installation -yes including the vertical drilling- qualifies for the 30% renewable energy tax incentive. My house is roughly the same size but horribly inefficient from a thermal standpoint. It was built in 1942 and still has original windows. This is my HVAC usage over the past few years.

Thanks for posting these numbers! They’re amazing, especially considering the house still has the original windows. It’s situations like these that make me wonder how people can struggle so much with geothermal. That said, are you extremely careful with it? Or do you use it fairly comparably to how you would a traditional heating system?

 

[Zythryn]

Thanks for the insight everyone! It definitely seems like a lot of people suggest geothermal over an air source heat pump. This was actually my preference too (especially because right now there are great rebates in Maine on geothermal, so it’s about the same cost as air source).

However, whenever I google the actual energy usage of geothermal, I see two types of results:
1) Blog posts / websites from companies that install geothermal
2) Crazed forum posts about how they are using exorbitant amounts of electricity ($1000+) because of it. They always say they’ve had the installer come out and look at it, but they of course say it’s working properly.

Now obviously there’s bound to be lemons or people who just don’t understand their electric bill will go up if they were using fossil fuels to heat there home before, but there’s a lot of horror stories out there. It’s disconcerting to say the least, especially when you factor in the cost to fix it if something were to ever go wrong.

Thanks for the link Zythryn! I haven’t looked through all the data yet but it looks like you have a lot there! Were you able to work out how much electricity was used by geothermal in the winter months?

The biggest issue with ground source heat pumps is the installer.
Tons of “HVAC” companies say they can install a heat pump. They then do a lousy job sizing it to your house. This results in poor performance, and huge energy use. We had this issue with our previous house.

Make sure you find a contractor that knows what they are doing.

The table showing our use over the last two winters includes our in floor heat, air handler (which does some work pre-heating our domestic hot water).

A well designed system installed and balanced by someone that knows what they are doing is incredibly efficient.

 

[EVSteve]

Thanks for posting these numbers! They’re amazing, especially considering the house still has the original windows. It’s situations like these that make me wonder how people can struggle so much with geothermal. That said, are you extremely careful with it? Or do you use it fairly comparably to how you would a traditional heating system?

I’d say I use it more than I did when we had oil heat but that may be because we have solar and the old oil furnace was so inefficient.

In my opinion the discrepancy between reports stems from people who try to save money using shallow slinky loops instead of paying for vertical boreholes. Other issues arise from homeowners retrofitting to systems designed for conventional HVAC with inadequate ductwork.

 

[Xenoilphobe]

Good paper to read
Introductory Overview of Ground Source Heat Pump Technologies

https://www.aeei.bse.vt.edu/wp-content/uploads/2015/06/Introduction-GSHP-Olgun.pdf