Can a Heat Pump Work in Minnesota?
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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This Post Has 27 Comments
I’m thinking Nelson is a fairly optimistic person. I commend him for relying on a heat pump through Minnesota winters, it sounds like he was able to overcome some frustrations with a little ingenuity. Clever to use the stove for auxiliary heat. Could this work for folks with little ones or pets running around?
Maybe a packaged unit that combines heat pumps with resistive heaters would make more sense for colder climates. An added bonus – the resistive heat could run in the summer to get the air conditioning to run more often until the desired humidity and temperature are achieved!
A small resistance backup heat strip could solve this, which are standard with unitary (central) heat pumps. That also allows for reheat dehumidification which runs the strip heat and the air conditioner to make dry room temperature air. Sounds crazy but it’s not that energy intensive, most of our clients use 500 kwh/year or less for it, we’ve removed 10 gas meters in Cleveland Ohio.
Nate. Just curious how you would set up controls for dehumidification with reheat since Fujitsu doesn’t speak Americanized control language.. Mitsubishi doesn’t even use the thermostat to energize auxiliary heat. So unless someone is good with a soldering iron and knows how to hijack DDC code, I’m not seeing how it could fly. Don’t see it being not plug and play like Carrier, Trane et al. Have you cracked such a case?
He could not use a stove, but an oven would do just fine. The door could be cracked at no real risk to pet and children, or even closed. The heat is still staying in the house envelope. That said, buy a few space heaters to place around the house, in case of extreme cold.
For my backup heat, I use sealed gas fireplaces. I know they create CO2 and such, but when the power goes out, we will not freeze. It’s a tradeoff I am comfortable with and a lot less expensive than a natural gas generator.
Jake, no, I wouldn’t recommend using an open electric oven for auxiliary heat with kids around. But that heat can be replaced easily with small electric space heaters, which are safer. Alternatively, he probably could install strip heat in his supply plenum.
Folks with small children and pets have solved the problem of access to a hot woodstove years ago.
Most heat pumps installed in Minnesota include a backup source of heat — I have an 8 kW resistive heater in my air handler that can be automatically triggered by the thermostat if the heat pump can’t keep up. My unit is rated down to -5 F and this is our first fall/winter with it, but it should be a huge upgrade from my old electric heater that used up to 1.5 megawatts a month during winter.
The bottom line as shown in this article is that if you want a heat pump to work cost-effectively in northern climates, you need to size it for heating, not cooling. That usually means that it will be oversized for cooling. If that is the case, you would do well to go with a system with modulated capacity so that it can run at lower capacity in cooling and with appropriate airflow rates to provide the necessary dehumidification.
Apparently, Gary didn’t have any electrical power outages during these cold conditions. We weren’t that lucky in Texas and Oklahoma.
That’s true, Roy. The Manual S guidance to size the heat pump according to the cooling load and then add auxiliary heat to make up the difference isn’t always the best solution.
No, they didn’t lose power in their cold snaps. If they had, I think they still would have fared better than many Texans because of their robust building enclosure. But how long they could survive that would depend on how long the power stayed out. Then again, those Minnesotans were surviving cold winters before they had electric power or fossil gas delivered right to their homes, a feat this Southerner shivers to imagine.
Electrical outages will be more common and NO house can provide heat without electricity. Most (all??) furnaces and boilers need electricity to operate. We are installing solar batteries which will provide backup power during outages. What happened in Texas is a travesty and will happen in other areas too.
I am in Minnesota, too, and I just installed a mini-split heat pump in my 1994 home. It was built to be very efficient at the time. With the walkout basement it is about 3,000 square foot of conditioned floor area. It is well-insulated, 350 cfm@50Pa, triple-pane low-e windows, and Manual J heat loss just under 30 kBtu/hr.
Just like Gary, I have been heating it with a high-efficiency sealed combustion modulating water heater and a centrally ducted fan-coil unit. This has worked remarkably well and is very comfortable. But when my AC went out I saw an opportunity to replace it with a centrally ducted mini-split unit (quite similar to Gary’s). I really wanted it for cooling, so I sized it accordingly and went with a 2 ton unit. I wasn’t as brave as Gary, so I simply put a hot water coil on top of it as my back-up heat. I don’t track things as well as Gary, but I can share this. At 10 degrees the heat pump could carry the house at night (with low internal and solar gains). At -10 degrees it fell behind at night, but recover the next morning as internal and solar gains picked up. On a similar night (-15 degrees) I turned off the heat pump and used the back-up hot water coil (with fan on) and it held the temperature just fine. I certainly could have gone to a smaller coil for back-up heat only. But the cost savings would have been minimal and now I have an independent heat source, if needed. Also, I didn’t have to oversize the heat pump to meet the design heating loads and I can ensure a very comfortable delivered air temperature in colder weather.
For me, this system is a win, win, win. Total winner over conventional AC in summer, outstanding performance in moderate weather, and excellent comfort for cold temperatures! And I get super high-efficiency domestic water heating, too! [Note: In high performance homes water heating can be more than 1/4 of your whole house energy consumption.] This system isn’t carbon emission free, but it could be in the future.
How do you get super efficient hot water from a mini-split heating system? Do you mean to say you “got” a high efficiency hot water heater as well?
Designing a new house in Minnesota – plan on a air-to-water heat-pump tied to Solar Cells. Now can a Zehnder ERV with a hydronic chiller – de-humidify and cool a house in Minnesota? Interesting question for Energy Vanguard.
Bill, you may be able to make something like that work, but I doubt it would dehumidify as well as a regular dehumidifier. Worse, it may mess up your air flow in the ERV. Zehnder makes a finely tuned machine, so why would you want to mess with that?
Heat pumps are actually a no-brainer in cold climates, especially in areas without natural gas service. Heat pumps are now being successfully used in climates like Maine and Alaska!
Here’s the thing most folks don’t realize: the colder the climate, the more heating hours in the range that the heat pump can fully handle the load (above thermal balance point). Below the balance point, the heat pump still kicks ass in terms of cost per therm. It’s just a matter of providing for supplemental heat.
As Gary’s project demonstrates, the supplemental fraction (% of annual load supplied by aux heat) can be quite small in super tight homes. This affects cost-benefit considerations when deciding among the various supplemental heat options. In older homes, especially those with higher leakage rates, a high supplemental fraction can justify a more expensive supplemental heat solution. For example, the considerable additional cost of a gas-fired wet coil (for supplemental heat) may be justified in a less efficient home that has access to natural gas. It depends on the supplemental fraction, relative cost per BTU of electricity vs natural gas, and whether a single water heater will supply both supplemental heat and domestic hot water.
Lastly, coordinating control of non-ducted supplemental heat sources (e.g., space heaters, wood stove, etc) is tricky… in particular, it’s impossible to limit the supplemental heat source so that it doesn’t reduce the heat pump’s contribution, at least not without some sacrifice in comfort.
BTW, a conventional dual-fuel furnace setup (fossil fuel + heat pump) is never a good option when using a propane or oil furnace. In that case, forced air electric supplemental heat will yield a lower overall operating cost, even if electricity costs more than propane or fuel oil per BTU of heat output. That’s because electric elements must only fill in the heat pump’s shortfall, whereas in a dual fuel setup, the furnace handles 100% of the heat load once it kicks in. Here’s why: a heat pump coil is necessarily mounted downstream from the furnace. That means the heat pump must shut down whenever the furnace operates, otherwise the refrigerant wouldn’t condense.
OTOH, electric heat elements are mounted after the coil, which means they can operate simultaneously with the heat pump — thus the term ‘supplemental ‘ heat. Keep in mind that even at 0F, a heat pump will have a COP in the 2.0 range with output close to 50% of rated (even higher for ‘high-heat’ mini-splits), so it makes no sense to not take full advantage of the heat pump throughout its operating range.
Allison, did you size your mini-split for your heating load or your cooling load?
Tim, I sized them for cooling load, but not the current cooling load. If you look at the article I wrote about my Mitsubishi heat pump (link below), you’ll see that my heat pump has a cooling capacity about 12% lower than the cooling load. It’s 35% lower on the heating side. But I know that load calculations always come out on the high side, and I’m going to be making significant reductions to the load as I improve the house.
Great article. I have done much reading on cold-climate heat pumps, and would love to replace the aging gas furnace in my Twin Cities house with one, but the $17,000 estimate I received from a well-known St. Paul company recently put this out of the realm of possibility, as it would for many other middle-income people I imagine. The estimate was for a Mitsubishi P-Series 2.5 ton with some ductwork modifications in my 1955, 1,100 square-foot house. There would have been an extra $2,500 panel upgrade from 100A to 200A, too. Who can afford this stuff? Does anyone see prices for these things coming down? I already spent $2,000 extra to replace a gas water heater with a Rheem heat pump model, which I like, but an extra $13,000 to upgrade from a new gas furnace to a heat pump puts this way out of reach, unfortunately.
@Ben, you may want to consider a conventional heat pump from a manufacturer such as Carrier, Trane, Goodman, etc. Whole-house ducted mini-splits generally cost more than conventional splits, especially the P Series, which is a commercial system. Mitsubishi’s M Series for residential is more reasonably priced (though still more expensive than a conventional split). Also, with a conventional split, you can save even more by going with non-variable technology. Mini-splits all feature variable speed compressors, a cost that may not be justified, depending on local energy costs.
Moreover, you’re likely to see wide variation in pricing from reputable dealers so it’s always a good idea to get multiple quotes on a project like this.
Lastly, it sounds like your furnace replacement quote was $4,000, which suggests it doesn’t include a new air conditioner. If your A/C is not approaching its end of life, or you don’t have or need A/C, then it may not make sense to install a heat pump at this time, especially if you have natural gas service, as opposed to propane (see my previous comment). Heat pumps generally add less than $1k to the cost of an otherwise identical cooling-only system.
Thanks, David. Good info to consider.
Hi Ben, thanks for adding in the nugget of installing a heat pump water heater in MN. I am considering installing one to replace an older gas model in the unfinished basement of my 1920s home – I’m about an hour south of the Cities. It gets chilly down there – mid 40s, and I am concerned about taking some of that heat out. I don’t want to freeze any pipes. What has your experience been?
Heat pumps are great, I have 2 in my house, I use them to avoid using my oil heating system in mild weather. However, as an HVAC contractor, I also know how sophisticated heat pumps can be, and how unskilled most HVAC techs are. This is particularly true when it comes to “ductless” type heat pumps as they are the most complex to troubleshoot. Imagine a failure of your super efficient HP at -27 degrees, you think an HVAC tech is going stand in the cold, wind, snow, dark, and figure it out? Maybe they will… I would not expect my techs to be exposed to extreme outdoor temps, especially in the dark. Take a look inside a multi-zone ductless HP on a nice day, its as complex as any computer and you wouldn’t dream of opening that up to service at -27 outside. Anyone that installs a ductless HP without auxiliary heat of some kind is taking a big chance in my opinion. One more thing about my “unskilled tech” comment, its going to get worse as boomers retire and less people enter the trades.
South Central Alaska, 80 air miles S of anchorage. 70 degrees is hot, 80 almost unknown. Winter, these days, might reach minus 10, but 12 to 8 years ago we had regular periods of minus 20-30-at-night. Can’t imagine needing cooling. No access to natural gas . Present heat: EPA approved woodstove with fans, & 2 Panisonic Whisper Greens for air distribution for 1200 square foot 1-story house. The older heat is a newer oil-fired Toyo, Laser 73, as backup & for being gone more hours than a load in the woodstove, but it can heat the house as long as power doesn’t go out. It’s 110 V,s might get a minimum job-sight portable generator. 17 cents kW price on electric, then fees. Does Mini-split makes sense, especially since firewood is getting harder to get? It’s been free for 12 years, so imagine how many gallons of heating oil I didn’t burn. I’m no longer fully able-bodied, either.
@leif, you didn’t mention how much you typically pay for fuel oil, but if your marginal winter electric rate is 17 cents per kWh (i.e., excluding all fixed costs in your electric bill), a mini-split would cost roughly $1.50 to $2.00 for each therm (100,000 BTU) of heat produced (0.17 x 29.3 divided by COP, which varies with outdoor temperature). Seasonally, we can assume an average of roughly $1.75 per therm. Keep in mind you’d still need to use the Toyostove to supplement the heat pump during the coldest weather.
A gallon of heating oil produces approximately 1.39 therms of heat (139,000 BTU). Your Toyostove is rated at 92%, which works out to about 1.27 therms delivered to the house. So the break-even point for the Toyostove versus a mini-split would be about $2.22 per gallon ($1.75 x 1.27 therms per gallon). I’m curious how much you paid for fuel oil last winter…?
Alison, can you tell us the sf of Gary’s house is? And is he CZ 6? Thanks
Cindi, I believe it’s about 2,500 sf. Yes, it’s in climate zone 6.
I’m reading this from the toasty California Delta, land of year-round fire season, where sub-zero almost never happens, but we do go sub-freezing occasionally. I happened to go to my panel to adjust my Trane communicating zone-controlled ducted system, and noticed the compressor power gauge was showing the graphic bar in green color. It’s usually red for heating and blue for cooling. What is this green, I asked myself. The answer was that it was in defrost cycle. So that’s my question for Minnesota: during these sub-zero times, do systems there go into defrost cycle often, or is the outside air humidity so low at that point that sublimation coil icing isn’t even an issue?
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