Heat pumps and smart thermostats
bsawers
| Posted in Mechanicals on
To heat our second floor, we have a heat pump and a conventional air handler. I set the smart thermostat to heat to 68F for wake-up and bedtime. During the day, the thermostat is set to 60F while the overnight temperature is 65F.
Our local utility sends periodic emails about saving energy. The most recent told me to set the thermostat to a constant 68F if I have a heat pump.
I understand the benefit of long run times with heat pumps, but a constant temperature seems excessive.
What do people think?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
It’s a bit unclear. Conceptually, running lower for longer is more efficient for modulation compressors. However, there’s certainly a crossover point where it’s more efficient to use a set back. My utility gives hourly data and after a few months of a cooling setback, it didn’t seem like I was saving any kWh. If you’re curious, try a constant month and see if kWh/heating degree day goes up or down vs. previous months.
Some of the problems with heat pumps and setback thermostats are
1 Many thermostats will energize the second stage heat IE the strip heat whenever the room is more than a degree below the set point. The strip heat cost 4 times as much to operate as the HP. The end of every set point forces the unit to run the strips and more than eats up the small saving from the set back.
2 Most heat pumps are sized very close to the actual load and recovering from a big setback will take a long time to recover. On the coldest days it may never happen.
I think heat pumps work best if you set it and forget it.
Walta
I admit I have not been monitoring the system very closely, but I haven’t noticed any problems with the heat pump struggling to meet the set point. I’m in Maryland, right near D.C., which means 4A (I think). The last couple weeks have been about as cold as it gets here and I believe the heat pump has met the set points. The only difference I noticed was that the inside temperature fell more quickly from 68F towards 60F during the day when the set point was only 60F.
That said, it sounds like an experiment might be in order. I will set the thermostat at 68F and leave it.
If my heat pump can meet the set point of 68F, is there any reason to leave the supplemental heating on as an option? I am inclined to leave the strip heat off, even if that means waiting slightly longer to return to temperature.
Not really, maybe peace of mind? Sometimes it’s used for defrost times too. If you know the logic of when it turns on, that might help.
I’m in MD too, now use a slight setback with no resistance strips installed and everything works well.
The answer depends on your situation.
If the house and heating system has a high inertia like radiant floor or external thermal insulation, or if the outside temperature is extremely low during winter or high during summer, the reduction of the average heating or cooling will be low, as well as potential gains on the electric bill. Plus the heat pump will have to make up for the heat that has not been produced during the set-back period and, depending on the conditions and particular heat pump characteristics, the cop might be quite low during this phase.
On the other hand, if the temperature swings a lot in the house when you stop the system, and if the heat pump works below half the full capacity at the current outside temperature, some gain (10% ?) can be expected. Larger gains can be obtained on over sized systems, or those with small turn-down ratio because instead of a constantly cycling, the heat pump will work with a longer cycle and a longer off period.
My personal case is a house with low inertia in a mild climate and a multi-split system combining wall and ducted units. During the « cold » days this winter (-2°C), the heat pump worked near 45% of the nominal capacity on average. I switch off during the night and I restart early in the morning, with a power limitation to 70% of the nominal capacity. The restart time is adjusted depending on the outside temperature. Doing this sort of thing requires some hacking of the system and spending time optimizing for the particular situation. I think that I can reduce my electricity usage by 20% using this strategy, because the temperature swing in the living room is large and because the multi-split has a bad turn down ratio.
Finally, I think that one goal of the instructions from the electricity provider is to avoid power draw peaks in the morning and in the evening. Telling people to not touch their thermostat is by far the best for the grid. Without careful control of the heat pump working conditions, it is probably the safest advice also. It is probably sub-optimal for well designed heat pump systems, but not by far. And, on other systems, it can be much better than systematic use of a set-back temperature. To perform a more optimal control, heat pumps should accept a power set point from the smart thermostat instead of a temperature set point.
With fossil-fuel burning heaters, a BTU is a BTU, you always have the same efficiency. With a heat pump, it's not so simple, your COP depends upon the conditions. Since part of what a smart thermostat does is time-shift heat production, it becomes important to look at the efficiencies of the time shifted from and the time shifted into.
For cooling, this often works out. If the building is unoccupied during the day, you can time-shift cooling from mid-day to the evening, and the COP is going to be higher when it's cooler outside. For heating the time-shifting generally works against you, moving more of your heating into the coldest parts of the day.