What's your heat pump heating strategy?
January 7, 2025 11:59 AM Subscribe
We just got a whole-home ducted heat pump. Should we schedule temperature changes or just set and forget?
We've got a new whole-home hyperheat heat pump using existing ducts in our house. It's great so far!
I seem to remember hearing that heat pumps work best when you just set a single temperature and don't change it, but I'm used to turning down heat at night or during daytime when we're away.
What's the best strategy in terms of operating cost, efficiency, comfort, etc.?
We've got a new whole-home hyperheat heat pump using existing ducts in our house. It's great so far!
I seem to remember hearing that heat pumps work best when you just set a single temperature and don't change it, but I'm used to turning down heat at night or during daytime when we're away.
What's the best strategy in terms of operating cost, efficiency, comfort, etc.?
As you probably guessed, it depends....
Do you have an alternative or 2nd heat source, and if so, what is it? How cold does it get at night, and how much does the system need to bump the temperatures up each morning? Do you have a "communicating" thermostat that can take care of the variable speed heat pump?
In my case, I have a "dual fuel" heat pump / gas furnace setup. The heat pump can go from 20% to 100% in 1% increments and the communicating thermostat is from the same brand so it can take advantage of that capability. The temperature profile I have set is such that the house temp is 66° when I wake up, 68° when I'm home in the evening, then it drops to 60° at night. Nighttime temps typically vary but are generally in the 10°-25° range, but can drop to -10° a few times a year. The general idea is to narrow the gap between outside and inside temperatures to cut down on heat loss, especially when I don't need the house to be that warm like when I'm sleeping.
I have the system set so that if the outside temps are below 25° it will simply not touch the heat pump as it is more efficient to just use the gas system at that point. It will also use the gas furnace if there is a large demand on the system, such as trying to bump the temperature up by more than 2.5° (after which it uses the heat pump to maintain the set temp).
But all of those are dependent on just how big your system is, what the alternative heat source is (if any), cost of electricity vs the alternative in your area, and the fun factor of just how cold it gets at night where you live.
Edit to add: All temps are in °F
posted by SegFaultCoreDump at 1:09 PM on January 7
Do you have an alternative or 2nd heat source, and if so, what is it? How cold does it get at night, and how much does the system need to bump the temperatures up each morning? Do you have a "communicating" thermostat that can take care of the variable speed heat pump?
In my case, I have a "dual fuel" heat pump / gas furnace setup. The heat pump can go from 20% to 100% in 1% increments and the communicating thermostat is from the same brand so it can take advantage of that capability. The temperature profile I have set is such that the house temp is 66° when I wake up, 68° when I'm home in the evening, then it drops to 60° at night. Nighttime temps typically vary but are generally in the 10°-25° range, but can drop to -10° a few times a year. The general idea is to narrow the gap between outside and inside temperatures to cut down on heat loss, especially when I don't need the house to be that warm like when I'm sleeping.
I have the system set so that if the outside temps are below 25° it will simply not touch the heat pump as it is more efficient to just use the gas system at that point. It will also use the gas furnace if there is a large demand on the system, such as trying to bump the temperature up by more than 2.5° (after which it uses the heat pump to maintain the set temp).
But all of those are dependent on just how big your system is, what the alternative heat source is (if any), cost of electricity vs the alternative in your area, and the fun factor of just how cold it gets at night where you live.
Edit to add: All temps are in °F
posted by SegFaultCoreDump at 1:09 PM on January 7
Response by poster: Do you have an alternative or 2nd heat source, and if so, what is it?
No, our unit is at 100% efficiency down to 10 degrees F below the lowest expected temperature in our immediate area, so we're 100% heat pump without anything to supplement (i.e., the unit is 100% efficient at 5 degrees and if our area gets down to 15 degrees, it's so unusual that it's news). We do have the official thermostat so it can control the heat pump incrementally as described above.
posted by msbrauer at 1:28 PM on January 7
No, our unit is at 100% efficiency down to 10 degrees F below the lowest expected temperature in our immediate area, so we're 100% heat pump without anything to supplement (i.e., the unit is 100% efficient at 5 degrees and if our area gets down to 15 degrees, it's so unusual that it's news). We do have the official thermostat so it can control the heat pump incrementally as described above.
posted by msbrauer at 1:28 PM on January 7
Heat pumps are fantastically efficient, with an energy efficiency performance of 300% or more. (For every unit of electricity they consume, they can transfer 3 units of heat.) Newer, "inverter" based units (where the fan speed and pump rate can be adjusted in near-infinite combinations) often have a COP of 4 or more (400% or more efficiency). Ours is so much more efficient and cheaper to run than our gas system, we just "set it and forget it".
At first, we did try lowering the temp at night to about 60 and having it warm up in the morning (as we did with our old gas setup) but since it's a little sluggish to "catch up" to desired heat settings, and because we weren't really seeing any appreciable impact to our bill between this and just leaving it set at 67F all the time, we just keep it steady and let it do its thing.
posted by xedrik at 1:35 PM on January 7 [3 favorites]
At first, we did try lowering the temp at night to about 60 and having it warm up in the morning (as we did with our old gas setup) but since it's a little sluggish to "catch up" to desired heat settings, and because we weren't really seeing any appreciable impact to our bill between this and just leaving it set at 67F all the time, we just keep it steady and let it do its thing.
posted by xedrik at 1:35 PM on January 7 [3 favorites]
So the US DOE says that you shouldn't adjust the temperature for heat pumps, but that's because "when a heat pump is in its heating mode, setting back its thermostat can cause the unit to operate inefficiently." Whereas you should adjust the temperature for a source like gas, where efficiency doesn't vary, because when the thermostat is set lower, the house loses less energy to the outside. That implies that if the efficiency of your heat pump doesn't vary much, it's more like a gas furnace, and you should vary the thermostat temperature. The efficiency of even very efficient pumps like yours varies somewhat with outdoor temperature, but if it doesn't vary a lot, I think that would be an argument for varying the thermostat setting.
posted by Mr.Know-it-some at 1:46 PM on January 7
posted by Mr.Know-it-some at 1:46 PM on January 7
No, our unit is at 100% efficiency down to 10 degrees F below the lowest expected temperature in our immediate area, so we're 100% heat pump without anything to supplement.
In that case, I'd go with the set it and forget it route. Maybe use a 2° night time setback if the house is a bit too warm as that is usually not a big deal for the system to bring it back up to temp in the morning.
posted by SegFaultCoreDump at 1:47 PM on January 7
In that case, I'd go with the set it and forget it route. Maybe use a 2° night time setback if the house is a bit too warm as that is usually not a big deal for the system to bring it back up to temp in the morning.
posted by SegFaultCoreDump at 1:47 PM on January 7
Response by poster: Oh, and what about when you're gone for a trip? Weekend away just leave it, but maybe turn it down the heat for a 10-day trip? Or something else?
It looks like our thermostat has a vacation setting, which I presume would allow me to lower the temp for a week or so and then heat up the day we return.
posted by msbrauer at 1:58 PM on January 7
It looks like our thermostat has a vacation setting, which I presume would allow me to lower the temp for a week or so and then heat up the day we return.
posted by msbrauer at 1:58 PM on January 7
One thing to keep an eye on as you worth through this question: the word "efficiency" gets thrown around a lot with different meanings. I found my thought process was much clearer once I started unpacking what I meant when I said "efficiency" or "efficient" by rephrasing it.
Here are some examples from this thread:
our unit is at 100% efficiency down to 10 degrees F below the lowest expected temperature in our immediate area
This could mean "Our unit has 100% heating capacity down to 10 degrees F below the lowest expected temperature in our immediate area," meaning the BTU output doesn't begin to drop until this point. This is definitely possible; we're in Chicago and our winter design temp is around 0 F. Lots of modern heat pumps (like Mitsubishi's Hyper Heat or Gree's Sapphire models) maintain 100% BTU output capacity down to -17 F or something. But they're not maintaining their peak COP down to temperatures that low, and can get progressively more expensive to operate at subzero temperatures. But you could have actually meant "Our unit's COP doesn't drop until we're more than 10 F below our lowest expected temperature." This is absolutely something that could happen with modern heat pumps in a moderate climate, like Seattle, which has a winter design temp of 24 F.
I have the system set so that if the outside temps are below 25° it will simply not touch the heat pump as it is more efficient to just use the gas system at that point.
In this case, I suspect "more efficient" means "more cost efficient," and is saying nothing about the heat pump's COP, which is a measure of its efficiency relative to resistive heating. Resistive heating is, by definition, 100% efficient, but it can simultaneously be outrageously expensive. Because of the differential in cost per unit of heat energy between electric and gas, heat pumps tend not to be cost efficient--that is, cheaper than gas heating--until they're operating at something like 300% efficiency (COP of 3). But this varies an incredible amount depending on local gas and electric rates.
posted by pullayup at 2:34 PM on January 7 [2 favorites]
Here are some examples from this thread:
our unit is at 100% efficiency down to 10 degrees F below the lowest expected temperature in our immediate area
This could mean "Our unit has 100% heating capacity down to 10 degrees F below the lowest expected temperature in our immediate area," meaning the BTU output doesn't begin to drop until this point. This is definitely possible; we're in Chicago and our winter design temp is around 0 F. Lots of modern heat pumps (like Mitsubishi's Hyper Heat or Gree's Sapphire models) maintain 100% BTU output capacity down to -17 F or something. But they're not maintaining their peak COP down to temperatures that low, and can get progressively more expensive to operate at subzero temperatures. But you could have actually meant "Our unit's COP doesn't drop until we're more than 10 F below our lowest expected temperature." This is absolutely something that could happen with modern heat pumps in a moderate climate, like Seattle, which has a winter design temp of 24 F.
I have the system set so that if the outside temps are below 25° it will simply not touch the heat pump as it is more efficient to just use the gas system at that point.
In this case, I suspect "more efficient" means "more cost efficient," and is saying nothing about the heat pump's COP, which is a measure of its efficiency relative to resistive heating. Resistive heating is, by definition, 100% efficient, but it can simultaneously be outrageously expensive. Because of the differential in cost per unit of heat energy between electric and gas, heat pumps tend not to be cost efficient--that is, cheaper than gas heating--until they're operating at something like 300% efficiency (COP of 3). But this varies an incredible amount depending on local gas and electric rates.
posted by pullayup at 2:34 PM on January 7 [2 favorites]
I live in a warmish area (nights down to 32F, rarely) and I have multiple head units.
Last year I was turning it way down in the front of the house (2 ton unit) at night and turning it back up early afternoon. It was quite slow to warm up from a few degrees below the setpoint, and it worked quite hard in the process (the noise the outside unit makes is a clue as to the effort it's putting in).
This year I've basically just left it at 70 all the time, day and night; the house is comfortable and the electricity bill appears to have come in about the same. I think it's using more power at night and less during the day, but it never seems to run as hard.
So, on that somewhat shaky data I'd say set it and forget it.
posted by How much is that froggie in the window at 4:33 PM on January 7
Last year I was turning it way down in the front of the house (2 ton unit) at night and turning it back up early afternoon. It was quite slow to warm up from a few degrees below the setpoint, and it worked quite hard in the process (the noise the outside unit makes is a clue as to the effort it's putting in).
This year I've basically just left it at 70 all the time, day and night; the house is comfortable and the electricity bill appears to have come in about the same. I think it's using more power at night and less during the day, but it never seems to run as hard.
So, on that somewhat shaky data I'd say set it and forget it.
posted by How much is that froggie in the window at 4:33 PM on January 7
Weekend away just leave it, but maybe turn it down the heat for a 10-day trip? Or something else?
I'd turn it down a little for a weekend trip and down as far as you can for a 10-day trip (our Fujitsu vacation mode is 10C and even when the temperature outside is between -5C and 0C, the energy used while away for a week is nearly zero, something like $0.50, and not that much to heat the house back up).
posted by ssg at 5:34 PM on January 7 [1 favorite]
I'd turn it down a little for a weekend trip and down as far as you can for a 10-day trip (our Fujitsu vacation mode is 10C and even when the temperature outside is between -5C and 0C, the energy used while away for a week is nearly zero, something like $0.50, and not that much to heat the house back up).
posted by ssg at 5:34 PM on January 7 [1 favorite]
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But if you have time of day pricing for electricity, it may not make sense to decrease your setpoint at night (or it might even make sense to increase it in the early morning or decrease it for peak price periods).
You can also do your own experiment to see how different operating regimes change your overall energy consumption. Just pick a few days when the weather is relatively constant.
posted by ssg at 1:02 PM on January 7 [2 favorites]