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Building Science

Do Heat Pumps Work in Minnesota?

Yes! Gary Nelson does it without auxiliary heat

Do heat pumps work in Minnesota? (House shown is not Nelson's.) [Image credit: Energy Vanguard]

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.

17 Comments

  1. William Hullsiek | | #1

    Good article, looking forward to the new book so I can place it next to the Sigenthaler books and the Manual J. Curious to know what is used for the DHW. The new air to water heat pumps can cool, heat and produce DHW. Water for heat only needs to be 83 to 103f, so DHW at 120F would require more BTU.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #3

      Bill, Gary has a GE GeoSpring heat pump water heater, which you can see behind him in the photo above. GE has since stopped producing it.

      Thanks for the kind words!

  2. Jon R | | #2

    > Yes, heat pumps work in Minnesota
    Some do, some simply shut off when it gets too cold.

    > he also knows how much heat the heat pump was pumping

    How does he know? It's not like measuring the heat output accurately is easy.

    > -17°F, ...The resulting coefficient of performance was nearly 2

    There are various models, but looks to me like Fujitsu measured a COP of 2.0 at +5F (NEEP). So COP = 2 at -17F sounds wildly optimistic.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #4

      Jon, Gary has his heat pump connected to a lot of wires and tubes to monitor temperature, pressure, and relative humidity. And he's got an Energy Conservatory TrueFlow permanently installed in the system. When you know the air flow rate, temperature change, and humidity change, calculating the amount of heat produced is straightforward.

    2. Expert Member
      Dana Dorsett | | #7

      >" ...COP = 2 at -17F sounds wildly optimistic."

      Agreed, but "...nearly 2... " is what it being alleged, not "... COP=3..." .

      The "...nearly..." part is subjective- a COP of 1.5 would qualify in some peoples' minds, 1.8 (still a credible number for best in class air source heat pumps) in others'.

      1. GBA Editor
        Allison A. Bailes III, PhD | | #8

        Jon & Dana, I should have put in the actual number from the beginning, I guess, but I did include the numbers you could use to calculate the COP at -17° F.

        Output = 13,000 BTU/hr x (1 kW / 3,412 BTU/hr) = 3.81 kW
        Input = 1,959 W = 1.959 kW
        COP = 3.81 / 1.959 = 1.94

        So, nearly 2 means very close to 2.

        Now, whether that's correct or not is another matter. Gary did tell me, "This seems pretty impressive.  I believe Manufacturer’s data only go down to -5 and my data agree pretty well with theirs at -5 and above."

        1. Jon R | | #9

          Would be nice to see much more of the collected data. Including the exact model #, so we can find the manufacturer’s data. Can the few COP points that manufacturers list be accurately extrapolated for lower temps?

  3. Doug McEvers | | #5

    The new age heat pumps might be just what we need to highlight very efficient building envelopes. This really makes economic sense for homes off the natural gas grid for which there are many in MN. So much wind power is being added in MN, ND and SD electric rates may be stable for the foreseeable future.

  4. Stephen Sheehy | | #6

    My 99% design temperature is a comparatively balmy 0°F. We've never had an issue over six winters with our Fujitsu heat pumps keeping up with cold weather. I'm surprised that the COP is as high as reported at very low temperatures, but it sounds like he's pretty organized about data collection. And even if some supplemental heat is needed once in a while, it's no big deal.
    Obviously, the key is a robust, efficient envelope because it allows you to cruise through the occasional cold snap without interior temperatures dropping much.

  5. Steve Grinwis | | #10

    If he's running the oven to generate heat for the house, because his heating system ran for 4 days flat out without hitting setpoint, it really feels like he should have installed aux heat.

    1. Jason S. | | #12

      The only downside to the electric oven backup is its manual. If they're not home, who cares if it can't reach setpoint as long as things don't freeze? The heat pump would have to completely fail for that to happen, and every other home in MN runs the same risk if a gas furnace fails when owners are away.

    2. Jon R | | #13

      > he should have installed aux heat

      Or possibly a larger Quaternity heat pump, so that Winter load could be fully met and a dehumidifier wouldn't be needed in Summer.

  6. Keith Gustafson | | #11

    >>After two summers of dealing with muggy indoor air, he installed an Ultra-Aire dehumidifier in the summer of 2020<<

    I know I am wildly oversimplifying, but a dehumidifier is basically an AC with a tiny fan. Having AC oversized for the conditions is not an unusual situation. I would think turning down the setpoint on the AC and then manually setting the fan speed very low[taking away its ability to reach the setpoint] ought to work.

    I say this many times in the questions, that people kind of expect the minisplits to just 'work' but they are just manufactured items, they do not understand the conditions around them. Using them for AC I tend to leave the setpoint and manage comfort with the fan speed. Sure it would be nice if the auto setting was perfect, but until they get 50:1 turndown ratio and wifi integration with the National Weather Service, we may have to deal with occasionally hitting the button on the remote.

    If I leave my 22k Mitsubishi to its own devices, I will sometimes wake up to a muggy living room, where it finally gives up and shuts the compressor off in the middle of the night. If I set the fan to low, I have to remember to turn it up in the morning or by mid afternoon it cannot keep up.

    1. Jon R | | #14

      Agreed, the popular and wrong opinion is that over-sized AC provides inferior humidity control (as compared to right and under-sized heat pumps). But it's almost completely about CFM/ton. Get that right and even 2X over-sized inverter heat pumps work fine.

      1. Maximilian T | | #16

        Jon R - just wanted to add that I frequently see your comments on threads related to AC equipment and dehumidification and I appreciate your perspective on the matter. The field research you have linked to (some outfit in Florida?) is helpful but beyond my ken, and you do a good job of simplifying it.

        I live in 4c now where I don’t have to think about dehumidification, but when my wife moves me back south I will likely be pinging you for advice!

        1. Expert Member
          Malcolm Taylor | | #17

          Max,

          I agree. I've learned a lot useful stuff from Jon about both humidity and pressure differentials.

    2. Trevor Lambert | | #15

      A dehumidifier is different in that it doesn't cool the space. It's not just about fan speed. If the load is 100% latent, then A/C alone probably can't manage the humidity. My heat pump is matched to both peak heating and cooling loads. I still can't keep the humidity below 60% a lot of the time, no matter what settings I use on the heat pump.

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