Do Heat Pumps Work in Minnesota?

Yes, heat pumps work in Minnesota . . . and Alaska, Vermont, and Ontario, too. Today I’ll tell you about one particular house in Minneapolis, Minnesota, that’s been heated with a heat pump for the past four winters. It’s the home of Gary Nelson, founder of The Energy Conservatory, which makes the Minneapolis Blower Door and the Duct Blaster for testing air leakage in homes and duct systems.

First, fix the building enclosure

As you might expect from someone who has spent the past several decades quantifying infiltration rates, Nelson’s house is pretty darn airtight. He’s lived in the same house for a long time, and has done work on it more than once—most recently with a substantial renovation in 2017. They moved back into the house in November 2017, and have been using the heat pump ever since.

Here’s the current status of the enclosure:

  • Infiltration rate: 1 air change per hour at 50 Pascals (ACH50), 500 cubic feet per minute at 50 Pascals (cfm50)
  • Walls: R-30 to R-40
  • Ceiling: R-50
  • Floor: R-0 in the old part, R-20 with foam under the slab of the addition
  • Windows: Triple-pane, argon-filled, three low-e coatings

The result of improving the enclosure so much is that the heating load is very low. He didn’t do a formal load calculation, but he did know how much heat he was using before the renovation. His heating system then consisted of a Polaris water heater (fossil gas fired) and an air handler to distribute the heat. He found that the system ran almost continuously when the outdoor temperature was -10°F, the 99% design temperature, and the amount of heat he got from the water heater was 17,000 BTU/hr. Then he calculated that the load reduction from his enclosure improvements would be offset for the additional load from an addition that was part of the renovation, so he figured he needed a heat pump with a capacity of 18,000 BTU/hr.

Gary Nelson standing next to his 18k Fujitsu ducted mini-split heat pump
Gary Nelson standing next to his 18 kBTU/hr Fujitsu ducted minisplit heat pump, with tubes and wires for a lot of monitoring

So he put in a Fujitsu ducted minisplit heat pump with a capacity of 18,000 BTU/hr. And because he understands heat transfer and Minneapolis weather, he put this heat pump in with no auxiliary heat. Really!

Heating performance

Winter 2017-18: This was the first winter after the renovation, and the low in Minneapolis was -15°F. The system performed very well. Even though the outdoor temperature went 5°F below their design temperature, the 18k heat pump held the house at the 72°F setpoint.

Winter 2018-19: The outdoor temperature got down to -27°F. The house temperature got down to 62°F but they were away in Australia at the time. Nelson told me that if they had been home, they probably could have gotten the house up close to the setpoint with their body heat and by baking some cookies.

Of course, Nelson measures and logs everything, so he also knows how much heat the heat pump was pumping and how efficient it was. During that -27°F cold snap, he calculated that the heat pump capacity was 8597 BTU/hr (2.52 kilowatts) and the power consumption was 1834 watts (W). The coefficient of performance was 2.52 ÷ 1.834 = 1.37. For comparison, electric resistance heat has a coefficient of performance of 1.

When the temperature rose to -17°F, the heat pump output rose to 13,000 BTU/hr and the power consumed to 1959 W. The resulting coefficient of performance was nearly 2, or double what electric resistance would have provided. (And to think that some HVAC techs tell people to switch to emergency heat when the outdoor temperature drops into the 30s°F!)

Winter 2019-20: He had nothing remarkable to report. They didn’t have any weather cold enough to call for any kind of auxiliary heat.

Winter 2020-21: This was another winter that tested his decision to skip the auxiliary heat. Here’s what he wrote to me:

This February we had a few days in a row when it didn’t get above 0°F and was down around -17°F at night. There was very little sun, which I think is unusual when it’s this cold. I think it ran flat out for at least three or four days, and didn’t quite meet the setpoint. The third morning, I turned on the [electric] oven for an hour or so with the door open and then set it to 350°F with the door closed for much of the day. I’d guess we probably used 20 to 40 kWh of resistance heat.

Cooling performance

Nelson’s heat pump is sized just about perfectly for heating. In a place like Minneapolis, that means it’s oversized for cooling. Minneapolis does get humid, too, and the result is a house that can stay at the setpoint temperature easily but doesn’t get dehumidified enough. After two summers of dealing with muggy indoor air, he installed an Ultra-Aire dehumidifier in the summer of 2020. As a result, he “enjoyed much better humidity control” during the cooling season.

Yes, heat pumps can carry the load in Minnesota

Gary Nelson is a smart guy and knows how to calculate heat transfer. He understands the heating needs for his house. He can read and apply the specifications for the performance of a heat pump. And he’s proved in his home over the past four winters that heat pumps work just fine in cold climates. He sized his heat pump close to the heating load and even installed the system without any kind of backup heat. Yes, he needed a bit of supplemental heat for an unusual few days of cold, cloudy weather, but the 40 kWh of supplemental electric resistance heat he used might have added about $5 to his electric bill.

There’s no reason to be afraid of installing a heat pump in a cold climate if you’ve done your homework. You don’t need to be as conservative with the sizing as Nelson has been, and you can get auxiliary heat installed to cover those rare weather events that make it difficult for the heat pump to supply all the heat you need.

Going with a heat pump is a great idea, especially if you’re replacing a gas heating system. Unlike fossil gas, electricity is getting cleaner all the time.

_________________________________________________________________________

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.

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