Analyzing and improving heat pump energy usage

[BGbreeder]

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.

I guess my view is that if you need it, you need it. I'm glad that it works for you.

I would also readily admit that our site is several thousand feet higher than the contractors who were bidding on the project, and they may have failed to account for humidity at our elevation. We rarely get fog, but 600' below us, the telephone wires are covered in moss and lichen, so I know it is a YMMV item.

All the best,

BG

 

[aesculus]

I would also readily admit that our site is several thousand feet higher than the contractors who were bidding on the project, and they may have failed to account for humidity at our elevation.

The good news is that it's an easy add I think. You either order no heat strips or pick the power you think you will need. I believe the slots and connectors are there in the heat exchangers but you would have to configure the system to know what you put it of course.

BTW this was my first winter with my new setup and setbacks. Boy it was also a doosey so it will be interesting to see what my actual power bill looks like. I unfortunately have no direct way to measure the power consumed by my heat pumps. I can get reports per month/day/hour but they are sort of like high or medium output for both the heat pump side and the strips. I would have to try to invent some sort of metric to convert that to kWh, which would be grossly inaccurate.

 

[BGbreeder]

Yes, it was a doozy. We had more snow and rain than we have ever had.

I can see that "high medium/low" is not exactly informative. Have you checked with Lennox?

Other than contacting Lennox, have you tried a clamp on ammeter to check current/voltages at various "settings"?

All the best,

BG

 

[aesculus]

I can see that "high medium/low" is not exactly informative. Have you checked with Lennox?

Other than contacting Lennox, have you tried a clamp on ammeter to check current/voltages at various "settings"?

All the best,

BG

The issue is that these variable speed heat pumps have a super wide spectrum of energy consumption. Its not like the stages that Lennox's reports are shown as. It would probably be better to take the Tesla energy reports and try to back out any non heating consumption. My base load is fairly static and some of the periodic loads are either short or very predictable.

 

[CrazyRabbit]

Does turning the air conditioning off when you're not home actually save energy? Three engineers run the numbers

Is it better to cool your house all day, or adjust the A/C setting on your way out the door? Westend61 via Getty ImagesHot summer days can mean high electricity bills. People want to stay comfortable without wasting energy and money. Maybe your household has fought over the best strategy for...

news.yahoo.com

I think it is easier on the grid to not setback the thermostat. Plus I would rather use my solar during the day.

 

[brkaus]

Does turning the air conditioning off when you're not home actually save energy? Three engineers run the numbers

Is it better to cool your house all day, or adjust the A/C setting on your way out the door? Westend61 via Getty ImagesHot summer days can mean high electricity bills. People want to stay comfortable without wasting energy and money. Maybe your household has fought over the best strategy for...

news.yahoo.com

I think it is easier on the grid to not setback the thermostat. Plus I would rather use my solar during the day.

Right, but the loss through the thermal envelope is directly related to the temperature difference between inside and outside. Even a small change there can help a large amount (depending on amount of insulation, etc). So should be considered along with many other items.

 

[sorka]

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.

That's what I did with my regular AC until we got 3 powerwalls at which point we stopped pre-gaming it except on days our PWs will discharge since we're on VPP.

 

[offandonagain]

The issue is that these variable speed heat pumps have a super wide spectrum of energy consumption. Its not like the stages that Lennox's reports are shown as. It would probably be better to take the Tesla energy reports and try to back out any non heating consumption. My base load is fairly static and some of the periodic loads are either short or very predictable.

If you're comfortable tinkering with code, you might see if there are open source packages that collect more useful data from the thermostat. For example, here's a project that seems to collect real-time inverter power for some Lennox thermostats: GitHub - PeteRager/lennoxs30: Home Assistant Lennox S30 / E30 / M30 integration

This is what I'm doing with my Bryant system. I'm now collecting daily energy usage and real-time status of all the zones. This allows me to run analysis such as: which zone is keeping the heat pump busy, or plot the energy usage against outside temperature.

I can't wait for Tesla to actually build a home heat-pump!

 

[sorka]

I'm about to add supplemental heat pumps to my my house starting with a 3 ton unit for the common living area with an HSPF of 11. This translates to 11K BTU of heating for each kWh.

My 25 year old propane furnace is about 70% efficient. Each gallon of propane at 100% is 91.5K BTU so at 70% it's about 64K BTU per gallon.

At $4 / gallon, that's 16K BTU / dollar.

The heat pump, at 11K BTU / $0.25(kWh) / would be about 44K BTU / dollar.

The calculations are going to vary dramatically based on the heating source you have now, the price of fuel, and the cost of electricity + HSPF of any heat pump you choose.

In my case, it's a no brainer and should result in a better than 60% drop in heating costs.

 

[CrazyRabbit]

Right, but the loss through the thermal envelope is directly related to the temperature difference between inside and outside. Even a small change there can help a large amount (depending on amount of insulation, etc). So should be considered along with many other items.

Only a 11% energy savings for setting back your thermostat. And will be less with better insulation.
Not worth chasing after. As solar proliferates, how we use power is changing. Now the conventional wisdom in California is to get people to charge their cars during the day.

 

[brkaus]

Only a 11% energy savings for setting back your thermostat. And will be less with better insulation.
Not worth chasing after. As solar proliferates, how we use power is changing. Now the conventional wisdom in California is to get people to charge their cars during the day.

Only 11%? I’d happily save 11% on the heating portion of my electric bill. I do agree there are a lot of other factors too.

 

[CrazyRabbit]

Only 11%? I’d happily save 11% on the heating portion of my electric bill. I do agree there are a lot of other factors too.

It buts a big evening strain on the grid. Even after the set point has been reached, the heat pump continues to run until the masses in the house come to equilibrium.

 

[dafish]

OP:
Very hard to beat a good heat pump. Solid decision. Not sure you or your HP are doing anything wrong. House seems OK. I've used air source heat pumps well below 0. They lose capacity, but where you live you made the right decision.

I assume you have something monitoring power on the HP specifically, right?

I used 1.25MWH's March and am really happy, so I suppose it's all about the climate you're in.

FWIW, insulation by "-R Rating" isn't super meaningful. Fiberglass in ceilings does a relatively poor job in real life. If you were to get 6-8" of cellulose on what area's you could, even seeing what "stuck" on the cathedrals (quite a bit should), you would see an improvement well beyond expected. Heat transfer via "lift" of hot air through fiberglass is a big deal for ceilings and is not properly presented in "R" ratings. Cellulose pretty much doesn't have that issue, and when blown on top of fiberglass makes the fiberglass start working correctly. Can make a remarkable difference. More glass = not much difference.

Glass in walls is OK, no where really for the heated air to rise to.

A few other things after I did quite a bit of testing on mine:

1) "Fan On" adds comfort and will often smooth out run times by allowing your unit to run more in low stage. In my case I get more BTU/KW in low stage (based on experience and monitoring). This is most noticable when Delta T is high. If it's really cold out the thermal envelope performs more poorly, house gets cold, and when the more central thermostats sense "cold" they run long enough to move into high (less efficient) state. When I run "fan on" the home is more comfortable and the unit doesn't run in high stage as much. Now on moderate days I run "fan off" and let the furnace kick the fan on when needed. The fan, just at idle, is running in the area of 200w, so it starts to matter, and on a milder day the house comes to temp so fast it tends to not call high stage on milder days anyway.

2) When heating I set back at night by around 5-7 degrees (depends on the zone) and let the sun help the house recover. I have the option of locking out "emergency heat" and do so, so the worst that happens is I hit high stage. I set 3 of 4 cycles to recover the house and phase it in over some hours. This lets the home recover on "low stage" sometimes too.

3) In cooling season I have a large whole house fan I use for cool nights and I'm always shocked at how long my insulation (cellulose) holds heat. It will take most of a night to get the insulation cooled down. I set the WH fan to power down at sun-rise, and shut windows as soon as awake. The house will run at least half of a spring/fall day wo/AC. Opening and closing that many windows twice a day kinds gets old though.

Hope this helps somehow.

 

[Dave EV]

50 kWh / day sounds like a lot to me - my 13 year old dual-stage 3-ton Trane heat pump maxes out around 12 kWh / day, but it helps that I live in Southern California, so daytime highs below 60F are uncommon. 2,000 sq/ft house, but insulation is marginal at most and also have cathedral ceilings. Usage has been around 150-175 kWh/month for the system this winter (I use an Emporia Vue to monitor energy consumption). Technically it's a dual-fuel system (gas backup), but I have the thermostat configured to basically never call on gas for heat.

Because my system is dual stage and not a scroll-type compressor, I ramp up my temperatures gradually in the morning for a couple reasons:

• Stage 1 should be more efficient than Stage 2 (probably close enough to not big significant, though)
• Stage 2 is louder, so keeping it in Stage 1 is more comfortable
• Gradually warming up the building just feels nicer than blasting the heat until it's warm.
• I prefer to sleep with it cooler (studies confirm this improves sleep), so prefer to have the set back.
• Waiting until the sun comes up and outdoor temps to start rising improves efficiency and also I can self-consume more solar.

So my normal schedule ramps up the temperature in a few steps:
07:00 66F (indoor temps usually in the low 60F range)
08:30 68F
10:00 70F (take advantage of peak solar and warmer temps)
16:00 65F (peak grid rates, avoid running)
21:00 55F (bed time)

Usually the system runs very little after 12PM and the house coasts until the morning. The compressor pulls around 1 kW on stage 1 and 2.7 kW on stage 2.

If you're trying to improve efficiency, the best thing to do is to run the system more when temperature differentials are larger - it's just physics.

So having the house follow outdoor temperatures can go a long way towards reducing energy consumption, but as others have noted - cold-soaked interiors also just feel colder even if the air temperature is the same.

What is crazy about at least my system (and I understand this isn't uncommon) is that there is a 100W compressor heater that runs basically all the time unless the compressor is running, or outdoor temps are over 85F. This adds up to 2.4 kWh / day or 72 kWh a month - around half of the compressors total energy usage most heating months!

 

[offandonagain]

I assume you have something monitoring power on the HP specifically, right?

My Bryant thermostat reports daily energy usage totals for the heat pump. They seem reasonable based on Tesla's home load monitoring.

1) "Fan On" adds comfort and will often smooth out run times by allowing your unit to run more in low stage.

Interesting, I'll have to give this a try. I just kept this setting at "auto" and never gave it a thought.

2) When heating I set back at night by around 5-7 degrees (depends on the zone) and let the sun help the house recover. I have the option of locking out "emergency heat" and do so, so the worst that happens is I hit high stage. I set 3 of 4 cycles to recover the house and phase it in over some hours. This lets the home recover on "low stage" sometimes too.

So my normal schedule ramps up the temperature in a few steps:
07:00 66F (indoor temps usually in the low 60F range)
08:30 68F
10:00 70F (take advantage of peak solar and warmer temps)
16:00 65F (peak grid rates, avoid running)
21:00 55F (bed time)

This seems to be the crux of it, several people in this thread suggested a setback at night. I've experimented with it in the past, but it either meant the house was too cold in the morning (family will definitely complain at 65F), or the heat pump worked in overdrive to get it back to a comfortable temperature by the time we wake up. Outdoor temperatures are still at their lowest at that point, so I can't rely on any efficiency gains.

I now just keep a fixed temperature throughout the day. Most of my heat pump's power consumption still happens during the early morning hours, but it rarely has to run at max speed. And we wake up to a comfortable house.

 

[CWhite]

The last two days here really highlights the difference outdoor temp makes with a heat pump. Yesterday it was 35-37° and we used 21 kWh between midnight and 8am. Today it was 48-52° and we used 10 kWh. Nothing else changed. We just had Tesla add more panels so I changed the lockout from 35° to 25° to use more electricity and less natural gas(dual-fuel hybrid HVAC). If your low temps are 50 and above I’d consider a heat pump close to a 100% necessity.

 

[Dave EV]

This seems to be the crux of it, several people in this thread suggested a setback at night. I've experimented with it in the past, but it either meant the house was too cold in the morning (family will definitely complain at 65F), or the heat pump worked in overdrive to get it back to a comfortable temperature by the time we wake up. Outdoor temperatures are still at their lowest at that point, so I can't rely on any efficiency gains.

I now just keep a fixed temperature throughout the day. Most of my heat pump's power consumption still happens during the early morning hours, but it rarely has to run at max speed. And we wake up to a comfortable house.

There's probably some room in there to set the temperature back a little bit. Due to your colder outdoor temps, you can't get away with the same setback I can, but you probably can get away with a couple degrees.

Keep in mind that heat losses from the house are bigger when the temperature differentials are larger - so you will lose more energy from inside to outside with a constant temperature than with a setback. So the optimization equation becomes a question of figuring out how the heat pump COP changes with temperature as well.

 

[dafish]

Some folks over estimate the efficiency loss of heat pumps in cooler weather. I encourage you to refer to your CoP charts for your units. Most anything decent remains very efficient in colder ambients. Certainly 30f for most HP’s

I also suggest some should re-read Dave’s last paragraph above, all quite true, and is also a big factor in why folks seem higher heating costs than expected when it gets colder out. It’s not just the HP losing efficneiemch, so is your house!

 

[charlesj]

Now I wish I'd kept the Canadian research on fiberglass insulation and how cold attic temperatures affect its R value when I researched how to build an energy efficient house back in 1989.
I do remember a 50% R value drop in very cold temperatures, in the minus range just don't remember how minus for that 50% loss.
It had figures for above zero temperatures as well.
Their conclusion was convective losses and recommended bats in plastic bags and layered.
This is what I did by using 6" bats, in bags, covered with a sheet of Tyvek then another 6" bats and another layer of Tyvek.

 

[dafish]

Thing to remember is the "Fan on" is helpful at extreme's sufficient to force high stage to recover. For my home if I'm below say 20-30f (depends on wind) the home perimeter gets cold enough that an "auto" call will require more than 2 (or maybe it's 5) minutes to satisfy the whole house. Whatever that value is, if the home isn't satisfied within that time the unit kicks on high stage. All HP's (I'm not air source, I'm ground-loop), run more efficently at lower stages and for longer periods. Testing suggests I'm better off running "Fan On" at such times. Hard to be certain, but every multi-day test I run shows it pretty conclusively (I use emporia, can see most of my home loads).

Anytime home recovery does not require that kick to high stage I'm wasting energy running the fan around the clock.

My t-stat will also call for high-stage if difference is over 2 degree's, and has some learning capability. It will step back down at 1 degree difference.
I can also tell it what time to recover to a certain temperature at and it will anticipate what it needs to do that. I stop heating the home at 8:00pm, begin first recovery step around 6am, tell it to be to 69 by 7:00, and step in 2 degress at a time over each hour. I don't spend much time at high stage, and all the "off" hours appear to add up.

Envelope recovery takes a lot longer than it appears of course, as I'm reheating not just the home air but all its mass and several inches of insulation air too. Thats fine, so long as the home air is comfortable enough I let low stage and sunshine help the home recover.

I suppose it's best to view this as if I'm using the home mass and insulation like a heat battery and recharging via solar heat and low-stage heating.

It's all very hard to differentiate (wind is such a factor), but I seem to be running around 15% lower using the two combined. Since this is by far my largest power consumption source it's worth chasing.