Optimal Thermostat Setting for Air-Source Heat Pump
Hi good folks,
We’ve got three air source heat pumps in our old house, here in the suburbs of Boston. Each of the ASHPs is matched to it’s own outdoor unit. We’ve been told to “set it and forget it” for the ASHP best efficiency. We have therefore set the thermostats to 62 degrees Fahrenheit.
However, 62 degrees is too warm for sleeping under blankets and too chilly for eating dinner, unless you’re really bundled up.
So how is everyone else dealing with the thermostat dial? Does everyone with ASHPs live with the “set it and forget it” model? What temperature do you set it and forget it at?
Note that we are trying to use less power here to heat the house. We are attempting to get as close to zero fossil fuel use as we can, with solar panels and ASHPs and a wood fireplace insert.
thanks! Kris
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Replies
Zoning could help - eg, 62F in the bedrooms and 68F in the dining room.
At the theoretical level, PV solar and "cooler at night" would benefit from thermal storage (like a large water tank) and air-to-water heat pumps.
Can you set the thermostat for the bedroom area to 60 F and the thermostat for the dining room area to 67, or whatever is comfortable there?
Other ideas to consider include using blankets that aren't as warm, including "weighted blankets" if you want the feeling of being under heavy blankets without the warmth, or using a programmable thermostat to shift the temperature setting very slightly, slowly ramping maybe 1 degree per hour. You could also look at air sealing and insulation if you have trouble with some places being consistently warmer or cooler than the setting.
>"...here in the suburbs of Boston..."
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>"Note that we are trying to use less power here to heat the house. We are attempting to get as close to zero fossil fuel use as we can, with solar panels and ASHPs and a wood fireplace insert."
If you don't have sufficient roof/yard area to cover the power use, Massachusetts has favorable terms for community solar, which should come in well under Nat'l Grid or Eversource pricing. (Read the fine print for escalator clauses- some are a bit snake-oily and can sting you later if you're not careful.)
Energy Sage now has a search function to find community solar projects available to you for a given electric utility & ZIP code:
https://communitysolar.energysage.com/
The Energy Sage pages will give the rough outlines of how the contracts are structured, but are not a substitute for a hard quote from the community solar project.
Short of that, in MA 100% green energy supplier contracts are also available via brokers, often at rates slightly lower than the standard mix offerings. The best time to sign up is usually in the fall, before the winter rates get set. The state maintains a website marketplace for comparing rates:
https://www.energyswitchma.gov/#/
The thing to watch out for from the supply contracts are termination fees and aut0-renewal. Very often the auto-renewals will be at a rate considerably HIGHER than the standard mix, and the brokers then nick you for a termination fee. The MA Attorney General has been going after the worst of the bunch in that regard, but it's still better to just keep track of it yourself to not get stuck. (I recently encountered a situation where a home heated by electric baseboards in MA had let the auto-renewals go for a couple of years, and had been paying nearly 1.5x the standard mix from National Grid, this in what is already one of the highest priced electricity markets in the lower 48!)
One relevant difference between Community Solar and 100% green power through a broker is location. The brokered green power can be sourced pretty much anywhere in the US (for 3 years I was buying wind power going onto the ERCOT grid in Texas, despite my home being in MA.) Community solar projects in MA have to be on your side of the major substations, so it's power going on to YOUR local distribution grid.
IMHO, the two main disadvantage to big temperature setbacks on heat pumps are either long recovery times (if no aux heat) or more power usage/less efficiency due to aux heat strips kicking on (but faster recovery).
If your system is a bit oversized you could try disabling or locking out the aux heat and still have okay recovery except when it's really cold out. Our 2.5 ton 14-Seer 2007 ASHP will keep the inside of our 2400 sq ft house at 70 when it's 17 degrees outside. But once it's under 30 out it takes hours to raise the temp by 1 degree (aux heat is disabled). So I might set it back by a couple degrees overnight but not if it's going to be much colder than the mid 20s.
This is really getting into the weeds but time of use billing and solar generation could be taken into account too. If electricity is cheap in the middle of the night you might be better off with less setback (use more electricity when it's cheap). But since you have solar generation maybe you are better off with more heat pump use when the sun is out and a bigger setback.
Definitely be careful not to use aux heat if you have it. If you have it, lock it out, or don't use setback at all. But even without it, the heat pumps will be more efficient running at a lower compressor speed, so going to full power recover from a setback can cost you even when aux heat is locked out or isn't present, which is why I recommend slow steps in the thermostat setting on the way back up. And only programming a varying thermostat setting for improved comfort, not for energy savings, which are unlikely to actually be realized.
We keep our bedroom door closed, and the heat register in that room closed. We use a 61F night time setback, then a slow scheduled warm up to 64F. If it's going to be a cold, cloudy day, I'll give another bump to 65. It it's sunny I leave it and it will get well past that on it's own. By limiting the temperature increase to about 1F over 45 minutes, the compressor doesn't go to full speed, and the aux heat never kicks in.