Efficient Use of Air-source Heat Pumps

[Ahh9oolu]

Question… based on the planned upgrade to our house, is the 7.2 kW system overkill? Would a smaller system suffice?

tl;dr 6-10 kWh per day to conservatively heat a mid-sized, well insulated house with a heat pump, estimated 15 to 20 to heat to 72.

I have a 7.2 kW system in the Sacramento region with south facing panels, but some late afternoon shading from trees. The house is recently remodeled so good insulation in the walls and attic. We went with a 5 zone heat pump, and I severely underestimated how much electricity it would take to heat the house. With the outside in the 30s overnight to 60s during the day, it is taking 6-10 kWh per day to heat the public areas to 70. And that is turning the heat off from 11pm to 9am, and mostly not heating the bedrooms. At a rough estimate, it would take 15 to 20 kWh to heat the entire house to 72 for all 24 hours.

I would say put in as much as you can fit and afford.

 

[h2ofun]

tl;dr 6-10 kWh per day to conservatively heat a mid-sized, well insulated house with a heat pump, estimated 15 to 20 to heat to 72.

I have a 7.2 kW system in the Sacramento region with south facing panels, but some late afternoon shading from trees. The house is recently remodeled so good insulation in the walls and attic. We went with a 5 zone heat pump, and I severely underestimated how much electricity it would take to heat the house. With the outside in the 30s overnight to 60s during the day, it is taking 6-10 kWh per day to heat the public areas to 70. And that is turning the heat off from 11pm to 9am, and mostly not heating the bedrooms. At a rough estimate, it would take 15 to 20 kWh to heat the entire house to 72 for all 24 hours.

I would say put in as much as you can fit and afford.

I use like 60 kwh per day to heat my house with my heat pumps

 

[STS-134]

tl;dr 6-10 kWh per day to conservatively heat a mid-sized, well insulated house with a heat pump, estimated 15 to 20 to heat to 72.

I have a 7.2 kW system in the Sacramento region with south facing panels, but some late afternoon shading from trees. The house is recently remodeled so good insulation in the walls and attic. We went with a 5 zone heat pump, and I severely underestimated how much electricity it would take to heat the house. With the outside in the 30s overnight to 60s during the day, it is taking 6-10 kWh per day to heat the public areas to 70. And that is turning the heat off from 11pm to 9am, and mostly not heating the bedrooms. At a rough estimate, it would take 15 to 20 kWh to heat the entire house to 72 for all 24 hours.

I would say put in as much as you can fit and afford.

I use like 60 kwh per day to heat my house with my heat pumps

Seems wasteful.

1. What temperature do you both set your thermostats to typically?
2. What temperature did you set your thermostats to before you got your heat pump?

I still keep my thermostat set at the same temperature I did before I got my heat pump: 64°F during the day, and 58°F at night. Setting it higher just because you can heat your home more efficiently is a good demonstration of the Jevons Paradox (or at least the rebound effect) and is contrary to the goal of helping the environment. By setting your thermostat higher than necessary in the winter, your system won't last as long, which means you'll need a replacement sooner and all of the carbon emissions associated with manufacturing a new system will have to be put into the atmosphere sooner. Also, setting your thermostat lower is of a greater benefit to a heat pump user than it is to a user of a gas furnace, because the heat pump's COP depends on the temperature difference between indoors and outdoors.

 

[Redhill_qik]

I use like 60 kwh per day to heat my house with my heat pumps

Based on your descriptions of your energy usage for heating and cooling, I often envision your home to be an old red barn with gaps in the siding letting light through. I know that it isn't, but that is what my mind sees.

 

[wwhitney]

I still keep my thermostat set at the same temperature I did before I got my heat pump: 64°F during the day, and 58°F at night.

That's contrary to conventional wisdom when it comes to heat pumps. If your system is right sized, then on cold mornings, it's not going to have a lot of extra capacity to quickly raise the temperature from 58F back to 64F. More of an issue when the outdoor design temperature is 20F or 30F than when it's 40F.

Cheers, Wayne

 

[Redhill_qik]

That's contrary to conventional wisdom when it comes to heat pumps. If your system is right sized, then on cold mornings, it's not going to have a lot of extra capacity to quickly raise the temperature from 58F back to 64F. More of an issue when the outdoor design temperature is 20F or 30F than when it's 40F.

Cheers, Wayne

Some people (most people?) like to sleep in a cooler environment. Having blankets around your body insulating is probably a contributing factor.

 

[aesculus]

Some people (most people?) like to sleep in a cooler environment. Having blankets around your body insulating is probably a contributing factor.

That is true, but I agree with @wwhitney that heat pumps don't like a large setback. I have my master bedroom zoned and I am trying to live with this:

Keep the core at 68F during the day, setback at night to 67F. Keep the Master Bedroom at 67F during the day and 65F at night. It still takes about 2 hours to pump the MB up in the AM (900 SF). Trying to avoid it going to heat strips if at all possible.

 

[RKCRLR]

tl;dr 6-10 kWh per day to conservatively heat a mid-sized, well insulated house with a heat pump, estimated 15 to 20 to heat to 72.

I have a 7.2 kW system in the Sacramento region with south facing panels, but some late afternoon shading from trees. The house is recently remodeled so good insulation in the walls and attic. We went with a 5 zone heat pump, and I severely underestimated how much electricity it would take to heat the house. With the outside in the 30s overnight to 60s during the day, it is taking 6-10 kWh per day to heat the public areas to 70. And that is turning the heat off from 11pm to 9am, and mostly not heating the bedrooms. At a rough estimate, it would take 15 to 20 kWh to heat the entire house to 72 for all 24 hours.

I would say put in as much as you can fit and afford.

Depending on how your heat pump operates, you may be better off not using much of a setback temperature. Many heat pumps use auxiliary electric heating strips when trying to recover from a temperature differential of more than 2 degrees F, and those auxiliary heating strips really suck the juice. Additionally, the early morning temperatures are the coldest. So if you use a temperature setback you are running the heat pump its hardest when it is the least efficient.

I have a duel fuel system heat pump system (uses a propane furnace instead of electric heating strips). Before I had much solar it took a lot of playing around with setback temperatures to find the sweet spot for the temperature to recover from a setback without kicking in the auxiliary heat. Now, since I am a net producer, I just use a setback and lockout the furnace until 30F. There are probably still days that I would be better off having a small setback instead of recovering from a large setback in the morning, but it is difficult to calculate and would be even more difficult to explain it to the thermostat. I'm waiting for heat pump predictive algorithms that take temperature forecasts into account to optimize efficiency.

 

[jboy210]

Seems wasteful.

1. What temperature do you both set your thermostats to typically?
2. What temperature did you set your thermostats to before you got your heat pump?

I still keep my thermostat set at the same temperature I did before I got my heat pump: 64°F during the day, and 58°F at night. Setting it higher just because you can heat your home more efficiently is a good demonstration of the Jevons Paradox (or at least the rebound effect) and is contrary to the goal of helping the environment. By setting your thermostat higher than necessary in the winter, your system won't last as long, which means you'll need a replacement sooner and all of the carbon emissions associated with manufacturing a new system will have to be put into the atmosphere sooner. Also, setting your thermostat lower is of a greater benefit to a heat pump user than it is to a user of a gas furnace, because the heat pump's COP depends on the temperature difference between indoors and outdoors.

I am sure setting the thermostat lower saves on electricity. But, if heat pumps are to be viable, the cost, and hassle, of operating them at the same temps as nat gas needs to be nearly the same.

We are lucky to hit 60 kWh/day on a good Spring day near the solstice and generate about 8 kWh/day this time of year because of shading. So until heat pumps get much more efficient, they will not be our first choice.

 

[wwhitney]

Some people (most people?) like to sleep in a cooler environment. Having blankets around your body insulating is probably a contributing factor.

That may be true, and to the extent it is an innate preference HVAC can be designed to accommodate that. But to the extent that it is a learned behavior due to grossly oversized HVAC systems, and night time setback as a fuel saving measure, there's no need to perpetuate that when right sizing HVAC equipment.

Cheers, Wayne

 

[wwhitney]

I am sure setting the thermostat lower saves on electricity. But, if heat pumps are to be viable, the cost, and hassle, of operating them at the same temps as nat gas needs to be nearly the same.

What's your outdoor design temperature (how cold does it get on the coldest 3 days of the year)? If it's 20F or more, and your house isn't super leaky and poorly insulated, that has been true already for at least a decade.

P.S. This side discussion isn't really about NEM 3.0 and could be split.

Cheers, Wayne

 

[h2ofun]

Seems wasteful.

1. What temperature do you both set your thermostats to typically?
2. What temperature did you set your thermostats to before you got your heat pump?

I still keep my thermostat set at the same temperature I did before I got my heat pump: 64°F during the day, and 58°F at night. Setting it higher just because you can heat your home more efficiently is a good demonstration of the Jevons Paradox (or at least the rebound effect) and is contrary to the goal of helping the environment. By setting your thermostat higher than necessary in the winter, your system won't last as long, which means you'll need a replacement sooner and all of the carbon emissions associated with manufacturing a new system will have to be put into the atmosphere sooner. Also, setting your thermostat lower is of a greater benefit to a heat pump user than it is to a user of a gas furnace, because the heat pump's COP depends on the temperature difference between indoors and outdoors.

Wasteful? Happy wife, happy life. Before solar, for 30 years, we froze during the winter and boiled during the summer. Now I basically set them at 70 degrees and leave them all year long. One never turns off a heat pump, one never catches up.
So for me, as one gets older!!, not being hot or cold is worth every penny, and a nice tax writeoff!

 

[h2ofun]

Based on your descriptions of your energy usage for heating and cooling, I often envision your home to be an old red barn with gaps in the siding letting light through. I know that it isn't, but that is what my mind sees.

Nope, all new windows, stuco, etc. But it is a big home, high ceilings, lots of windows since I have views to the lake.

 

[h2ofun]

Some people (most people?) like to sleep in a cooler environment. Having blankets around your body insulating is probably a contributing factor.

I have my down stairs bedroom and gym set at 66. I have my upstairs set to 72

 

[STS-134]

Depending on how your heat pump operates, you may be better off not using much of a setback temperature. Many heat pumps use auxiliary electric heating strips when trying to recover from a temperature differential of more than 2 degrees F, and those auxiliary heating strips really suck the juice.

That's a function of the thermostat, not the heat pump. The thermostat decides when to kick in the auxiliary heat. If your thermostat is kicking on the auxiliary when you don't want it to, and there's no way to reconfigure it, it's time for a new thermostat.

I have a duel fuel system heat pump system (uses a propane furnace instead of electric heating strips).

I also have a dual fuel system with a natural gas furnace as the backup. I have the changeover temperature set to 35°F (last year, if I had this same system, I would have set the changeover temperature to around 45°F but PG&E has raised the gas prices so high as of the beginning of this month that the economic crossover point is actually below freezing...at least until they raise the electric rates again). The thermostat (Emerson 1F95-1277 with optional outdoor sensor F145-1378) kept kicking into auxiliary heat mode after 25-30 minutes, either because it didn't like how long it was taking to meet the set point or because it didn't like the rate of temperature rise. In any case, I configured the thermostat's Auxiliary Off setting to 36°F, meaning never use auxiliary heat if the outdoor temperature is at or above 36°F, and now it uses the heat pump exclusively if it's >= 36°F outside (except when the heat pump goes into defrost mode and activates the furnace). If I really want to use auxiliary heat above 35°F outside, I can always put the thermostat into emergency heat mode and force it to do this manually.

 

[holeydonut]

You all MF'ers can actually plan your heating around some efficiency conversation? My wife just sets it to whatever she wants. You all actually get away with "but the COP is blah blah or the BTUs are $ per dino fart" and actually influence what the thermostat(s) is/are set to?????

 

[Southpasfan]

You all MF'ers can actually plan your heating around some efficiency conversation? My wife just sets it to whatever she wants. You all actually get away with "but the COP is blah blah or the BTUs are $ per dino fart" and actually influence what the thermostat(s) is/are set to?????

Thats one of the funniest posts I've seen here in awhile, and not only hilarious, but illustrates a big point.

You can read Borenstein over at Haas Energy blog till your are bored out of your skull, but the reality is that all these people who talk about market pricing for electricity live in some fantasy world - no one I know can use energy pricing knowledge to finely craft their use. You friggen need it, you use it.

If there was some other way, no one in Cali would bother with AC. Its this ridiculous assumption that you can use peak pricing to actually influence usage in a major way that cons policy makers into allowing the even more ridiculous rate differentials we now have.

 

[stopcrazypp]

Thats one of the funniest posts I've seen here in awhile, and not only hilarious, but illustrates a big point.

You can read Borenstein over at Haas Energy blog till your are bored out of your skull, but the reality is that all these people who talk about market pricing for electricity live in some fantasy world - no one I know can use energy pricing knowledge to finely craft their use. You friggen need it, you use it.

If there was some other way, no one in Cali would bother with AC. Its this ridiculous assumption that you can use peak pricing to actually influence usage in a major way that cons policy makers into allowing the even more ridiculous rate differentials we now have.

If there was a real time display (kind of like charging stations) that make it more clear the immediate actual costs, people might be persuaded to adjust their thermostats a few degrees. It may also be an automatic setting built into the thermostat (a lot of people use "smart" thermostats nowadays). If however it's just a number on the monthly bill, of course most people won't bother.

 

[swedge]

You all MF'ers can actually plan your heating around some efficiency conversation? My wife just sets it to whatever she wants. You all actually get away with "but the COP is blah blah or the BTUs are $ per dino fart" and actually influence what the thermostat(s) is/are set to?????

My wife was the one who wanted solar so as to help save the planet. Now we have a PowerWall, a Model Y, ventilation cooling, insulation, automatic sun shades. The future includes converting to heat pump for the HVAC, hot water and clothes dryer eventually. All of which keeps me busy weighing the concrete financial return on these projects so that we can save money while saving the planet. Endless tinkering opportunities.

To the ongoing conversation, we are still on gas heating, but central fan ventilation cooling (link) is our alternative to air-conditioning, quite effective in our marine climate. It drops the house temp overnight both for sleeping comfort and to keep the daytime inside temperature rise to below 76. Except on the few days when it doesn't get cool overnight.

SW

 

[BGbreeder]

You all MF'ers can actually plan your heating around some efficiency conversation? My wife just sets it to whatever she wants. You all actually get away with "but the COP is blah blah or the BTUs are $ per dino fart" and actually influence what the thermostat(s) is/are set to?????

Um...yes, of course.

For similar reasons, why we stripped the roof off and added as much insulation as we could before adding solar, and boy did that improve the comfort of our home.

We selectively use things like fleece, 100w spot heaters etc., to be comfortable and warm when it is cold out. Why heat any more space than you have to? And, yes, we all know how many kW the AC draws, and how many $/hr that is when it is running. (Ditto the furnace) Personally, I can't wait to replace the AC with something far more efficient, but as it keeps on running, and we use it so infrequently, it is hard to rationalize replacing it. (ROI, and all that)

Don't get me wrong, I would much prefer a home with R80 in the attic, R40+ walls and floors that could get by on passive heating and cooling, but the ROI of that in this house is never, so we make do.

@swedge what kind of automatic sun shades do you have?

All the best,

BG