Analyzing and improving heat pump energy usage

[offandonagain]

I'm trying to better understand and improve our heat pump's energy usage. We got our Tesla Solar system soon after moving into the new house and since I hadn't used heat pumps before, I clearly underestimated how power hungry it would be in the winter. I'm hoping to get advice on how to run it more efficiently while keeping the home comfortable.

Here's some info:

• We have a 3000 sqft single floor house.
• The heat pump is a 4-ton Bryant Evolution system (variable speed, air-source, no backup heat). We have 3 zones configured.
• We're in SF South Bay, so moderate climate. Temperatures between 35-55 in the winter.
• We set the heat setpoint conservatively, 67F or 69F depending on the zone (and constant setpoint throughout the day). The family would prefer to set them higher

Energy usage: for the last 4 months, heat pump energy usage has averaged over 800kWh per month. Some days it used as much as 50kWh. This is especially painful since we're on the PG&E EV2A plan and will likely be net consumers (NEM2).

Things I've done so far:

• Got an energy audit to make sure there are no house insulation problems and the system is sized appropriately. While they found some smaller issues with insulation, we still got a top score on energy efficiency.
• I set all the zones to a fixed setpoint after reading about heat pump efficiency.
• I'm now logging data from Bryant so I have daily energy usage, frequent data on active zones, temperatures, etc. I also have overall house load info from the Tesla solar system. I can crunch these numbers to get more info.

I'm not sure where to go from here. Should I get an HVAC expert to evaluate the whole system? Or are these usage numbers reasonable?

Thanks!

 

[BGbreeder]

Unfortunately, or fortunately, that looks about right, with the caveat that most homes in the Bay Area are, in my opinion, woefully under insulated. 50kWh/day is about 170,000BTUs/day, which is pretty good for a Bay Area home.

Is your attic at R-60 or better? Generally, it is the most cost effective step to take. After that the ROI may not be there for many upgrades, though some may be covered under the new IRA.

Did your energy audit include a whole house air leakage test? If not, that may provide some actionable information. Windows are particularly poorly insulated, and can be a great place to gain some energy savings. Window coverings, especially insulated window covers in tracks can make a big difference.

All the best,

BG

 

[offandonagain]

Is your attic at R-60 or better? Generally, it is the most cost effective step to take. After that the ROI may not be there for many upgrades, though some may be covered under the new IRA.

They measured at R-21 in the attic. They did recommend improving that, however the attic only covers a part of the house -- the rest is cathedral ceilings and improving insulation there is a much bigger project.

Did your energy audit include a whole house air leakage test? If not, that may provide some actionable information. Windows are particularly poorly insulated, and can be a great place to gain some energy savings. Window coverings, especially insulated window covers in tracks can make a big difference.

They did the blower door test and measured 0.32 NACH (6.81 ACH50), which I think is good.

We do have a lot of windows and skylights, but interestingly the zone with the most windows is doing better than others (i.e. it is mostly inactive and temperature is always a couple degrees higher than the rest of the house). So I think the windows actually help with solar heat. It's the north-facing bedrooms that seem to be keeping the heat pump busy.

 

[ThomasD]

I have a Heat Pump. Below a certain temperature it goes into emergency heat mode You can also set it into emergency heat mode manually. When in emergency heat mode the heat strips in the air handler turn on and use a lot more energy to heat the house than just the heat pump running

 

[BGbreeder]

They measured at R-21 in the attic. They did recommend improving that, however the attic only covers a part of the house -- the rest is cathedral ceilings and improving insulation there is a much bigger project.


They did the blower door test and measured 0.32 NACH (6.81 ACH50), which I think is good.

We do have a lot of windows and skylights, but interestingly the zone with the most windows is doing better than others (i.e. it is mostly inactive and temperature is always a couple degrees higher than the rest of the house). So I think the windows actually help with solar heat. It's the north-facing bedrooms that seem to be keeping the heat pump busy.

Congratulations on the blower test results!

Our home design is similar, and going to R60 in the attic made a definite difference to the areas below, but the real comfort boost was getting to R12 over the cathedral ceilings. Afterwards, we realized that having a cold ceiling acted like a cold sink and makes the room feel less comfortable at the same temp. The house was originally built with R 2.5 in the cathedral sections of house....

South facing windows are great for solar heating in the winter, but then they are an issue in the summer unless you have a roof overhang that shades them in the summer. I would consider pricing insulated curtains/drapes, just as an alternative. They made a huge difference in comfort for me two prior homes.

If you have a crawl space, you might look into sealing it, or insulating the floor. Sealing it and conditioning it gets the bonus potentially reducing infiltration even more.

Given the locally high construction costs, and the impact of NEM3.0, figuring out the ROIs on various mitigation strategies may take awhile. Good luck!

All the best,

BG

 

[offandonagain]

I have a Heat Pump. Below a certain temperature it goes into emergency heat mode You can also set it into emergency heat mode manually. When in emergency heat mode the heat strips in the air handler turn on and use a lot more energy to heat the house than just the heat pump running

Ah yes, I read that's a common issue. Ours doesn't have a backup heat source.

Our home design is similar, and going to R60 in the attic made a definite difference to the areas below, but the real comfort boost was getting to R12 over the cathedral ceilings.

Curious, how big of a project was improving the insulation in the cathedral ceilings? The energy auditor suggested to redo the insulation when reroofing, but we won't have to replace the roof any time soon.

Given the locally high construction costs, and the impact of NEM3.0, figuring out the ROIs on various mitigation strategies may take awhile. Good luck!

Thanks for all the help!

 

[BGbreeder]

Ah yes, I read that's a common issue. Ours doesn't have a backup heat source.


Curious, how big of a project was improving the insulation in the cathedral ceilings? The energy auditor suggested to redo the insulation when reroofing, but we won't have to replace the roof any time soon.


Thanks for all the help!

I agree with your auditor, it is a production no matter how you do it. I had wanted to do it below the roof, I.e. add a second ceiling, but the cost had an ROI out many, many years, as in 20+years. We put as much as we could on the roof to the limit of what was allowed when we reroofed.

 

[bruce4000]

My son has Tesla Solar and has a heat pump. His house is fairly well insulated, and he has found it best to heat the house to say 71F during the afternoon when solar is high and the outside temp is higher (and the heat pump is most effective), and then at night turn down to 61F and then needs very little heating at night when coldest out (and heat pump least efficient) using the thermal mass of the house.

 

[SanDiegoM3]

My son has Tesla Solar and has a heat pump. His house is fairly well insulated, and he has found it best to heat the house to say 71F during the afternoon when solar is high and the outside temp is higher (and the heat pump is most effective), and then at night turn down to 61F and then needs very little heating at night when coldest out (and heat pump least efficient) using the thermal mass of the house.

Probably the best short-term solution. Like boosting the AC in super-off or off-peak further than you would normally go to carry the house forward.

 

[wwu123]

Our home design is similar, and going to R60 in the attic made a definite difference to the areas below, but the real comfort boost was getting to R12 over the cathedral ceilings. Afterwards, we realized that having a cold ceiling acted like a cold sink and makes the room feel less comfortable at the same temp. The house was originally built with R 2.5 in the cathedral sections of house....

I suspect it's the cold ceiling, like old single-paned windows, not only losing heat directly, but the temp gradient also convects the room air slightly, making it feel that much colder against bare skin. We remodeled including R30 in attic and R21 in the vaulted ceilings, plus double-paned windows. Even though say the bathroom is not heated any higher than before the remodel, it doesn't feel cold or drafty stepping out of the shower, for example.

Attic insulation also helps in the summer heat. I have an additional temp sensor in the attic. After the hot summer days, the attic can be radiating 90+ deg heat down into the conditioned space for hours even after the ambient temps have cooled down at night. And my roof (and solar panels) is fully shaded by 2-3 pm, so those with western sun-exposed attics are probably at least 20+ deg hotter at night.

So with my lack of central A/C, in hot summer evenings, I'll turn on the whole-house fan when it drops to 85 outside, even if inside my inside thermal mass is still below 80. I'll sacrifice the window in just the closest one room to let the warm air in, just to start flushing out the 90-100 deg air in the attic, so it's not radiating down all night long.

 

[CWhite]

You don’t have a backup heat source but Inam pretty sure that all heat pumps have the emergency heat strips in case the heat pump fails. I do have a gas furnace along with the heat pump so my air handler does not have the electric heat strips. I use to let the house cool off at night but that caused issues. The heat pump took several hours to heat the house back up. You are trying to heat the house during the coldest time of the day, which is bad for efficiency. Also, if I crank up the heat from 62° to 68° in one step the system thinks it is having a hard time keeping the house warm and it kicks in the gas furnace. You might want to make sure that your system does not do the same thing with the emergency heat, that uses a lot of power. I do agree that it is good to hear the house up during the afternoon when there is more heat available in the outside air.

 

[aesculus]

^^^^ This
All ducted heat pumps have resistive heat strips in the air handler. They need them when the unit has to go into defrost mode for the outside coils. They will also use them if the user calls for emergency heat. And most newer units will turn them on if the delta from the actual temperature and selected temperature is too great or if the unit does not seem to be making sufficient progress in obtaining the desired temperature in a reasonable time. Often you can see this as a Second Stage heating, or Aux heating.

Here is Lennox's high end iComfort thermostat which kicked on Aux heating on the heat pump after more than an hour of trying to get to it's set point naturally.

 

[offandonagain]

^^^^ This
All ducted heat pumps have resistive heat strips in the air handler. They need them when the unit has to go into defrost mode for the outside coils. They will also use them if the user calls for emergency heat. And most newer units will turn them on if the delta from the actual temperature and selected temperature is too great or if the unit does not seem to be making sufficient progress in obtaining the desired temperature in a reasonable time. Often you can see this as a Second Stage heating, or Aux heating.

I don't think that's the case for my indoor unit. The model is: FE4ANF005L00
F = Fan Coil
E = Evolution / Infinity
4 = Multipoise
A = Original Series
N = 208/230-1-60
F = Single piece Cabinet
005 = 30-48k Btu
L = Aluminum Coil
00 = No Heating

I checked and you can order it online without heat strips. My thermostat also shows "Electric heat: NONE" and I've never seen the heat pump use above ~5kW, so I'm pretty sure it doesn't have resistive heating. (Probably because of the mild climate here.)

 

[charlesj]

It's too late to consider a gas heating system but as an exercise the above usage of 50kWh/day for your heat pump can be compared to 170k BTU which is 1.7 therms on a gas bill. My gas bill showed a PG&E gas price of $3.19/therms in tier 2 charge ($2,75 in tier 1) in February, most of the winter months.
How would that compare to electric usage of 50kWh/day? At .20cents/kWh would be $10.

 

[aesculus]

I checked and you can order it online without heat strips. My thermostat also shows "Electric heat: NONE" and I've never seen the heat pump use above ~5kW, so I'm pretty sure it doesn't have resistive heating. (Probably because of the mild climate here.)

Interesting. This is probably a rarity, and as you said, given by your mild climate.

In any place with temps that get into the mid 40's or worse and dew points to match, the heat pump would ice up and have no way to defrost without blowing cold air into your home.

 

[BGbreeder]

Interesting. This is probably a rarity, and as you said, given by your mild climate.

In any place with temps that get into the mid 40's or worse and dew points to match, the heat pump would ice up and have no way to defrost without blowing cold air into your home.

That's not necessarily the case. If the heat pump has an internal refrigerant bypass, it can route the warm, post compressor refrigerant to the external coils to defrost them. It isn't uncommon. Auxiliary heat can aid when it is really cold, or to speed the defrost cycle, but I think that is not required in most applications with newer designs.

So, I don't know how rare it actually is in Northern California. I do know that none of the quotes we have gotten for heat pumps included auxiliary heating. FWIW: Newer heat pumps, with higher efficiency ratings tend not to have them, if you look over at the DOE energy website.

All the best,

BG

 

[offandonagain]

It's too late to consider a gas heating system but as an exercise the above usage of 50kWh/day for your heat pump can be compared to 170k BTU which is 1.7 therms on a gas bill. My gas bill showed a PG&E gas price of $3.19/therms in tier 2 charge ($2,75 in tier 1) in February, most of the winter months.
How would that compare to electric usage of 50kWh/day? At .20cents/kWh would be $10.

This is not taking into account the efficiency of an air-source heat pump though. You're assuming each kWh will give 3,412 BTUs of heat, but a heat pump with 10 HSPF will give out 10,000 BTUs per kWh.

 

[brkaus]

As long as you don’t have backup/emergency heat kicking on, you can moderately set back your temp when you are away.

 

[aesculus]

That's not necessarily the case. If the heat pump has an internal refrigerant bypass, it can route the warm, post compressor refrigerant to the external coils to defrost them. It isn't uncommon. Auxiliary heat can aid when it is really cold, or to speed the defrost cycle, but I think that is not required in most applications with newer designs.

So, I don't know how rare it actually is in Northern California. I do know that none of the quotes we have gotten for heat pumps included auxiliary heating. FWIW: Newer heat pumps, with higher efficiency ratings tend not to have them, if you look over at the DOE energy website.

All the best,

BG

Well. I can be wrong about "most", but in my application, at about 1200 feet in elevation, with 2022 Lennox's top of the line heat pump and air exchanger, they all came with auxiliary heating, and would not work without it. I put in two of their most recent systems last year, a 2 ton and a 4 ton.

These things are constantly defrosting with our humid/cold air, and often have to kick in the Aux heat, just to catch up with a 1 degree offset.

 

[brkaus]

The easiest "tell" for emergency heat is how many breakers feed the inside unit.