My Electricity Use for my Heat Pump

[OffGridInTheCity]

I have a 4-Ton Lennox XP25 heat-pump - https://www.lennox.com/residential/products/heating-cooling/heat-pumps/xp25 - whole house, forced-air with natural gas backup companion furnace = installed new in 2019. Efficiency Rating (SEER) up to 23.5 and Efficiency Rating (HSPF) up to 10.2

House is 2600sq ft, 1997 construction/insulation, and Zone 4 climate (Southern Oregon) with avg lows of 35 Dec/Jan/Feb core months of winter. Run 73F during day and 68F at night. *The Lennox Heat Pump struggles <30F and functions best >40F. Anything <20F sustained, it cannot raise the temp from 68F to 73F even given all day.

I've varied year to year between 100% heat pump for winter and allowing 'default efficiency settings' that use natural gas for winter. Using 29.31kwh per therm yields almost identical results whether I do 100% heat-pump or a mix of heat-pump + natural gas assist.
Here are some numbers.....


One surprise to me is that cooling is waaaay less power than heating and of course in summer, 15kw PV generates plenty of power for 100% solar based cooling. The winter power numbers are just PAINFUL - Yikes.

If I ever replace this Lennox, I'll be focused on 25F+ efficiency rather than just 40F+ efficiency for heat. The Lennox really struggles (and likely consumes double power) <40F compared to >40F. The HVAC company was clueless and I was too when I installed this - live and learn for sure. Note that 4-Ton for 2600sq ft is right on the edge and in fact cannot keep the house cool when the temp >110F which is occurring more often now. I wonder if 5-Ton would be better efficiency overall - but it's not clear.

Clearly with 15kw PV = 500kwh/month in winter months I'm not even in the ball park of heating the house with solar. My rose colored glasses are off (Sigh). To reach the goal of truly being OffGrid In The City I'm looking into wood forced air or wood boiler as a winter alternative for heat - but not sure the city will allow the chimney/wood-burning.

 

[Supervstech]

Dont get me wrong.
The new 5 stage inverter heat pumps are great.
Seer 2 around 20, BUT... the ductwork has to be leak free, and well insulated, or run inside conditioned space to be most effective.

Ductless minisplits eleminate the energy lost from poor ductwork.

 

[ksmithaz1]

Dont get me wrong.
The new 5 stage inverter heat pumps are great.
Seer 2 around 20, BUT... the ductwork has to be leak free, and well insulated, or run inside conditioned space to be most effective.

Ductless minisplits eleminate the energy lost from poor ductwork.

The problem in a place like here is without forced air, you can't put a mini-split in every room, so the bathroom is cooking and the living room is nice. The ducted mini-splits don't have much of an advantage over just fixing your ducting and getting a nicer hvac. YMMV.

 

[Zwy]

I just turned on the 18K Senville heat pump in the kitchen. It is performance rated to -20°F and it is -10°F. It draws about 2Kw for about 20 minutes at the most, then begins turndown. Temp is set for 77°F.

 

[dreancestere]

It's currently 11℉ here and the heat pump, though sucking energy from a fire hose, is still heating the house properly. After tonight, we've got 3 more nights with temperatures in the low 20s. Then, we get back to our normal temperature ranges and I can stop worrying about the energy use. Hopefully, this will be the last such deviation from the mean (19℉ colder than it should be) of the year (for cold temperatures, anyway).

 

[wpns]

Ductless minisplits eleminate the energy lost from poor ductwork.

Indeed, the difference between our first FL summer and our second summer with a new HVAC system was night and day (same size, same function, same SEER, just 10 years older). I chock it all up to the complete replacement of the ductwork, including air returns, plus insulating the attic after it was installed (we're still finding blown-in insulation!).

Of course, the installer was "the good one" and looked at me like I was from Mars when I asked about inverter-style heat-pump systems, so we'll do that when the 'new' system hits 10 years. [Yeah, "the good one" was a bit of a nightmare when he disappeared at the end of the process and we had to find someone else to get the final inspection and signoff, but that's the FL HVAC market for you.]

 

[summit]

how does a heatpump/mini system decide when defrost is needed ? sure -20F is cold, but that doesn't mean there's ice on the coil/fins. May be by air-flow delta pressure along with temperature data ?

 

[Supervstech]

Indeed, the difference between our first FL summer and our second summer with a new HVAC system was night and day (same size, same function, same SEER, just 10 years older). I chock it all up to the complete replacement of the ductwork, including air returns, plus insulating the attic after it was installed (we're still finding blown-in insulation!).

Of course, the installer was "the good one" and looked at me like I was from Mars when I asked about inverter-style heat-pump systems, so we'll do that when the 'new' system hits 10 years. [Yeah, "the good one" was a bit of a nightmare when he disappeared at the end of the process and we had to find someone else to get the final inspection and signoff, but that's the FL HVAC market for you.]

Yikes!

Hvac-talk is a great resource for contractor assistance.

 

[dreancestere]

I have only one full year and two partial years of data, and we're strictly electric, but I wanted to get some monthly numbers similar to what @OffGridInTheCity did, above.

Heat pump is a whole house 3 ton Bryant Evolution Variable-Speed Heat Pump (288BNV - 5 stage) installed in 2023, cooling efficiency "up to" 19 SEER2, heating efficiency "up to" 9 HSPF2.

House is 1,895 - 2,000 square feet (depending on what you measure or who you believe), 2010 construction with builder installed spray foam insulation on floor of attic. Temperature set to constant 76℉ for heat and 78℉ for cooling.

Location is Wichita Falls, TX. 2023 USDA Hardiness Zone 8a. Theoretical climate (hah!) looks like this:



and, here's our heat pump energy use by month (updated for February 2025 data):

 

[dreancestere]

how does a heatpump/mini system decide when defrost is needed ? sure -20F is cold, but that doesn't mean there's ice on the coil/fins. May be by air-flow delta pressure along with temperature data ?

I'm interested in that, too. When it's cold enough, our defrost cycle kicks on just about hourly. But, at the marginal temperatures it doesn't necessarily do that. So, there must be some other deciding factor besides temperature and time.

 

[wpns]

Yikes!

Hvac-talk is a great resource for contractor assistance.

I'll probably poke around there in another 8 years when our existing system is up for renewal.

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[OffGridInTheCity]

I'm interested in that, too. When it's cold enough, our defrost cycle kicks on just about hourly. But, at the marginal temperatures it doesn't necessarily do that. So, there must be some other deciding factor besides temperature and time.

I wonder about this as well. And, I wonder about the efficiency *curve - power vs temp* as even though a heat-pump will work at lower temps, I observe the power usage can double for the same amount of heat as one descends into non-efficient part of the curve.

In my head, I imagine that the temperature difference between ambient and heat output would be a geometric curve requiring more and more power as this temp difference widens.... but there is more and more science in play these days and perhaps there multi-stage systems (or something) coming online? or maybe one can choose a better curve for the local situation? I don't understand as well as I'd like to ask the right questions to make a better decision next time.

I would like to see power vs temp difference curves but some some searching on efficiency curves doesn't really turn anything up I can understand in a practical way and real data on commercial whole house units such as my Lennox XP25 seems non-existent.

For my next 'whole house' unit, I'd like to see one that is efficient down to 20F instead of just 40F for example as I imagine this would save at least 20% of power I use now. Also, my HVAC company said that individual mini-splits 'are not approved' by the city permit process due to some air flow mumbo jumbo - which seemed to limit professional install options. Not sure I believe the technical issues they presented - something about humidity.

 

[dreancestere]

I had been hoping we wouldn't have any more cold snaps this year. That was silly of me. It's 4 in the afternoon on Tuesday, 18 February 2025 and the temperature's been falling constantly since yesterday evening. It's 16℉ and we're heading to 5℉ by tomorrow morning. The heat pump compressor is at stage 5 and the 1st auxiliary heat strip is on (for a total of 10kW near-constant power use). With where our temperatures our heading, at least I'll be able to document at what temperature that 2nd auxiliary heat strip turns on (I won't be able to do the same for the 3rd one because I had it disconnected -- but, that way I'll at least be able to see how the internal temperature holds up without it). We won't be above freezing until Friday afternoon.

 

[Zwy]

I've been running the 18K btu Senville with Arctic pump all day and it was at least -18°F this morning when I turned it on. It actually hasn't used much power all day.

 

[dreancestere]

It's 7 in the morning and it's 9℉. We might be getting a bit lucky, since they're now forecasting the low to be only 8℉ instead of 5℉. According to my heat pump, yesterday (from midnight to midnight), it used 149kWh (instead of our rough average of about 20kWh). That's about $20. Since our high today is supposed to only reach 25℉, I'm guessing today's heat pump use will be a bit worse. OTOH, there's supposed to be more sun today, so we might offset a small amount of that with the PV.

The odd thing with the heat pump's power use is that when it was 16℉ last night, we were using about 10kW in stage 1 electric heat and stage 5 compressor. When I went to bed, it was 14℉, stage 2 electric, stage 4 compressor. But, we were still using about 10kW. This morning at 9℉, it's in stage 3 electric (but the last one is disconnected), stage 4 compressor, but we're using about 12kW. The graph's a bit bumpy, so maybe it is a variable draw on the auxiliary strips instead of a straight on/off. Those 3 auxiliary heat strip stages seem mighty close together (maybe 16℉, 14℉ and 9℉? I know we weren't in Stage 3 electric at 0200 hrs when it was 10℉). But, maybe I'm not catching them when they first start. No trouble maintaining internal temperature of 76℉.

Here's yesterday's graph from the Sol-Ark:



and here's today's (so far):



EDIT 1: I'm adding this on Thursday, 20 February 2025 at about 0600 hrs. It's 7℉ (high yesterday was 23℉). To recap, the heat pump used 149kWh Tuesday (midnight to midnight). Yesterday (midnight to midnight), it used 216kWh. Our high today is supposed to be 27℉, but our low by midnight is only supposed to drop down to about 20℉. So, hopefully, today (midnight to midnight) the heat pump will use a bit less energy. After that, we're supposed to be up around 70℉ (about 10 degrees above normal instead of this ridiculous 30 degrees below normal) and this horrible energy use will stop.

EDIT 2: This is for Friday, 21 February 2025 at about 0530 hrs. It's 20℉. Yesterday (midnight to midnight), the heat pump used 176kWh. We're actually supposed to get above freezing today (though that'll still have our high running 25 degrees below normal). Our low around midnight will be 24℉ (just 11 degrees below normal). I expect our heat pump energy use to still be high, but acceptably high. So, overall, these 3 days of cold used almost an average month's worth of power.

EDIT 3: It's Saturday, 22 February 2025 at about 0600 hrs. It's 15℉ (overnight forecasted low was wrong). The heat pump yesterday (midnight to midnight) used 88 kWh (only 4 times the average). It looks like our weather will return to normal (or warmer) today. So, after the power use from this morning, we should be good. But, at our average daily net PV export of 20kWh at a 3:1 expense/revenue ratio for electricity, it's going to take darn near 3 months to make up for these 4 or 5 really cold days.

 

[Madcodger]

You keep saying,"the heat pump used" X kwh. But the fact is, it's not the heat pump, but rather the resistive heat strips "bolted on" to the heat pump, that you are forced to use because your heat pump can't handle the load, that is causing the problem.

If those cold temps happen with any sort of frequency, you need to either install auxillary heat or replace that unit with one designed for cold temps. It is not even remotely close to reasonable for less than a week of cold to erase 3-4 months of savings. Bottom line: I truly feel bad for you, but it sure sounds like someone sold you the wrong heat pump.

 

[dreancestere]

It's been brought up before in this thread, but the heat pump IS sized properly for the climate (see post #59, above for a graph of what our climate is supposed to be). The problem arises when the weather doesn't pay any attention to the climate. The heat pump works wonderfully when we're around our normal climatic temperatures. Even plus or minus 10 degrees it works fine. At about 20 degrees below our normal lows (16℉, in my case), power use gets unpleasant. But, we were 30 degrees below our normal lows during this last cold snap. Those occasional cold snaps are why those auxiliary heat strips are built into the heat pump. It's just that the heat pump (as a unit a homeowner will buy for a fairly standard, commercially built house) will use a boatload of energy when that happens.

EDIT: I've updated my Heat Pump Energy Use chart in the first post to include the February cold snap.

 

[OffGridInTheCity]

It's 7 in the morning and it's 9℉. We might be getting a bit lucky, since they're now forecasting the low to be only 8℉ instead of 5℉. According to my heat pump, yesterday (from midnight to midnight), it used 149kWh (instead of our rough average of about 20kWh). That's about $20. Since our high today is supposed to only reach 25℉, I'm guessing today's heat pump use will be a bit worse. OTOH, there's supposed to be more sun today, so we might offset a small amount of that with the PV.

The odd thing with the heat pump's power use is that when it was 16℉ last night, we were using about 10kW in stage 1 electric heat and stage 5 compressor. When I went to bed, it was 14℉, stage 2 electric, stage 4 compressor. But, we were still using about 10kW. This morning at 9℉, it's in stage 3 electric (but the last one is disconnected), stage 4 compressor, but we're using about 12kW. The graph's a bit bumpy, so maybe it is a variable draw on the auxiliary strips instead of a straight on/off. Those 3 auxiliary heat strip stages seem mighty close together (maybe 16℉, 14℉ and 9℉? I know we weren't in Stage 3 electric at 0200 hrs when it was 10℉). But, maybe I'm not catching them when they first start. No trouble maintaining internal temperature of 76℉.

Here's yesterday's graph from the Sol-Ark:

View attachment 279352

and here's today's (so far):

View attachment 279353

EDIT 1: I'm adding this on Thursday, 20 February 2025 at about 0600 hrs. It's 7℉ (high yesterday was 23℉). To recap, the heat pump used 149kWh Tuesday (midnight to midnight). Yesterday (midnight to midnight), it used 216kWh. Our high today is supposed to be 27℉, but our low by midnight is only supposed to drop down to about 20℉. So, hopefully, today (midnight to midnight) the heat pump will use a bit less energy. After that, we're supposed to be up around 70℉ (about 10 degrees above normal instead of this ridiculous 30 degrees below normal) and this horrible energy use will stop.

EDIT 2: This is for Friday, 21 February 2025 at about 0530 hrs. It's 20℉. Yesterday (midnight to midnight), the heat pump used 176kWh. We're actually supposed to get above freezing today (though that'll still have our high running 25 degrees below normal). Our low around midnight will be 24℉ (just 11 degrees below normal). I expect our heat pump energy use to still be high, but acceptably high. So, overall, these 3 days of cold used almost an average month's worth of power.

EDIT 3: It's Saturday, 22 February 2025 at about 0600 hrs. It's 15℉ (overnight forecasted low was wrong). The heat pump yesterday (midnight to midnight) used 88 kWh (only 4 times the average). It looks like our weather will return to normal (or warmer) today. So, after the power use from this morning, we should be good. But, at our average daily net PV export of 20kWh at a 3:1 expense/revenue ratio for electricity, it's going to take darn near 3 months to make up for these 4 or 5 really cold days.

We have our Rheem 50gal heat-pump water heater under our house and the temps vary from 80F hi to 40F low in winter. With 40-50F low ambient around the water heater we use ~200kwh/month. In summer (70-80F ambeent) as low as ~130kwh/month. Here's the last 2 years worth of data

Ours is situated so we could run a duct up thru the floor of the house and get house air (70F) in winter instead of raw under house cold air. The Rheem is made for this with flanges on the in/out. Maybe you can do something like that?