Setting back a Heat pump
nynick
| Posted in General Questions on
We have 3 ton Mitsubishi Hyper Heat supplying our renovated home and set at 70 degrees. It keeps the house nice and toasty, but runs almost all the time.
Recently I programmed the Kumo Cloud app to setback the temperature to 62 at night and mid-day. This effectively turns the unit off for 15 hours a day unless the interior temperature falls below 62. It hasn’t (yet!) since the house is so tight and well insulated and the outside temps haven’t been too cold.
I know these units are highly efficient when running, but does setting back the thermostat help or hurt the efficiency?
TIA
Nick in CT
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Replies
Setting back hurts efficiency.
You should save a decent amount of energy by using nighttime setback temperatures.
It'd be interesting to see the data over time to see the different energy usages. If you could graph the outside air temperatures and compare that to the energy use profile over time it could help to determine just how much energy is saved.
When you set back the thermostat, the heating system runs less for two reasons:
1. As the house loses heat, instead of that heat being replaced, it comes out of the heat capacity of the house as the temperature drops.
2. As the temperature drops, the temperature difference between the inside and the outside decreases, which makes the house lose less heat.
Only the second actually saves energy, the first just time shifts the heating from the middle of the night to the early morning when the thermostat goes back up. With a heat pump, that time shifting causes the heat pump to operate at reduced efficiency, which often more than wipes out any energy savings from a lower average temperature overnight. The heat pump is going to be less efficient for three reasons:
1. Modulating heat pumps are most efficient when running at part load, when recovering from a setback they will run at full output and a lower COP.
2. The efficiency of a heat pump is determined by the difference between indoor and outdoor temperature. Typically the coldest time of the day is right before dawn, which is also typically when a setback thermostat resets.
3. Many heat pumps will activate supplemental heat when the interior temperature falls significantly below the thermostat setpoint. This is typically resistance heat with a COP of 1.0.
>"3. Many heat pumps will activate supplemental heat when the interior temperature falls significantly below the thermostat setpoint. This is typically resistance heat with a COP of 1.0."
This can also happen if the heat pump can't produce enough heat to raise the temperature in the space quickly enough, which would be another way that the setback might actually INCREASE overall energy consumption: you might be using electric resistance auxillary heat during the "recovery" period when you go from the lower to higher temperature after the setback period ends.
Bill
Heat pumps prefer to modulate, not cycle outright. This means heat pumps really work best while maintaining a constant temperature. You CAN use setback temperatures, but you run into two issues: 1, you may hurt operating efficiency enough to cancel out any savings you get from the lower setpoint, and 2, you force the heat pump to run at full power when the set back period ends and you go back to the higher temperture, which make take a long time if the heat pump has only limited available capacity to heat the space up (it takes more heat output to raise the temperature of the space than it does to maintain a constant temperature in the space).
You'd need to do some experiments to determine if you actually save any money here. You'll probably need more than just a few degrees' worth of temperature delta to actually gain any overall energy savings too. BE SURE to actually MEASURE the energy use of the system while doing these experiments, because if your goal is to save energy overall, you need to be sure you're not actually using MORE energy with the setback configured.
Remember that heat pumps do not operate the same way as conventional furnaces, so not all of the same rules apply.
Bill
"It keeps the house nice and toasty, but runs almost all the time."
Modulating heat pumps are able to adjust their output level. They are most efficient when they are running continuously at a constant output. This is not a sign that they are using excessive energy, it's actually a sign that they're operating well.
Thanks to everyone for their replies so far. Two things I failed to mention. 1) I have the supplemental electrical resistance heat switch on the air handler on "OFF". It's never been switched "ON". 2) I have an ERV that runs continuously, regardless of the HP or air handler.
Of course I know an oil or gas furnace operates differently than a HP. It just seemed counterintuitive to me that something uses LESS energy when it's running continuously than when it's essentially OFF 15 hours a day. This is why I asked you experts and I appreciate the collective knowledge!
The problem is that after that 15 hours of "OFF", you want to warm things back up again. That requires enough BTU input to both overcome the thermal losses of the structure AND enough to also RAISE the temperature up to your new post-offtime setpoint. The heat pump might not have the capacity to do that, since they tend to be tightly matched to the needs of the structure, with less excess capacity than a typical gas fired furnace would have. Heat pumps also operate more efficiently when essentially running "a little bit all the time" rather than "a lot for a short time", which is how gas furnaces operate.
So basically yes, you use less energy during the 15 hours of "off time", but you might cancel the savings out trying to warm things back up again when you're back in "on time".
Bill
We all have our gut feeling but that is not data it is just our guesses.
Could the don’t set back a heat pump be a wife’s tail that is no longer true?
I am sure it was true in the early day of heat pumps when the resistance heat kicked in after every setback because suddenly the room was more than 1.5° below the set point and that is how they programed the back up heat.
HPs and and the controls are much better today.
There is tons of data showing setting back cooling setpoints does save energy. Keeping the AC off in the coolest part of the day and have it run more at the hottest time does lower usage.
Yes, I have my heat pump set to the same temp 24-7 in both heating and cooling modes but I have zero data for this choice.
Walta
In post #4 I laid out my case for why it's not a good idea.
"There is tons of data showing setting back cooling setpoints does save energy." For a long time the various federal programs were pushing setback thermostats, but in the past few years it seems to me like they've backed away from it.
The studies I've seen found that in the real world, setbacks tend not to work that well, because most homeowners don't actually understand how a thermostat works. The most common misperception is that the setpoint is actually an output level, like the dial on a cooktop or a hair dryer, and it should be set higher in cold weather and lower in warm weather.