In case you’re dying to know the answer, I’ll go ahead and blurt it out right here: Yes, heat pumps work in Minnesota and in other really cold places, 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 Pa (cfm50)
- Walls: R-30 to R-40
- Ceiling: R-50
- Floor: R-0 in the old part, R-20 foam under the slab of the addition
- Windows: Triple pane, argon-filled, 3 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, their 99% design temperature, and the amount of heat he got from the water heater was 17,000 BTU per hour. 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.
So he put in a Fujitsu ducted mini-split heat pump with a capacity of 18,000 BTU per hour. And because he understands heat transfer and Minneapolis weather, he put this heat pump in with no auxiliary heat. Really! (For more on the topic of auxiliary heat, see my followup article.)
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 their 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 their 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 8,597 BTU/hr (2.52 kilowatts) and the power consumption was 1,834 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 1,959 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 Fahrenheit!)
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:
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 kilowatt-hours 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.
Followup article: Heat Pumps, Auxiliary Heat, and Resilience
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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