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Building Science

Heat Pumps, Auxiliary Heat, and Resilience

Do you have a plan for what to do when the heat pump stops working or the power goes out?

An open electric stove may not be the best way to heat your home but it can serve as an emergency heat source if a heat pump stops working.

What happened in Texas this winter has a lot of people thinking about our dependence on electricity and what happens when the power goes out in extreme weather. Recently I wrote about Gary Nelson’s house in Minneapolis that’s heated by a heat pump with no auxiliary heat. That spurred this comment from reader Rich Lovoie:

Heat pumps are great. I have two 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 heat pump at -27℉. You think an HVAC tech is going stand in the cold, wind, snow, and dark to figure it out?

Should we be afraid of relying on heat pumps in our homes? Let’s take a look at the considerations.

Resilience begins with the building enclosure

If you live in a place that can get perilously cold—and we know now that includes places like Texas—a house that holds onto its heat should be your first defense. That means airtightness, insulation, good windows, and little to no duct losses. Gary Nelson’s home is extremely airtight at 1ACH50. His walls are insulated to between R-30 and R-40, the ceiling is R-50, and the windows are triple-pane, argon-filled, with low-e coatings on three surfaces.

Gary told me: “In an extended power outage the high-performance envelope would probably make it pretty easy to live in the house for at least a couple of days, and I think it would prevent the pipes from ever freezing.” He also found that the basement stayed warmer than the upstairs, so he could always close the basement door and stay down there longer. And the warmer and sunnier it is outdoors during the power outage, the longer he can stay in the house. That’s resilience!

Lots of air sealing and insulation are the foundation of resilient buildings
Lots of air sealing and insulation are the foundation of resilient buildings

Lloyd Alter at Treehugger wrote an excellent article on this topic after the Texas disaster titled Why Every Home Should Be a Thermal Battery. One important point he mentioned was Alex Wilson’s writing on resilience 10 years ago. Wilson made the connection between resilience and the energy efficiency we’ve been promoting for decades. According to Wilson, the most important thing we can do is transform our builds so they can “maintain livable conditions in the event of extended power outages or interruptions in heating fuel.” And the best way to do that is with robust building enclosures that are airtight and have high insulation levels—you know, like Passive House.

Alter also covered an interesting phenomenon that occurred during the Texas power outage. Mark Siddall, an architect in the UK, started what became hashtagged on Twitter as the #PassivhausChallenge, a period during which Passive House occupants voluntarily turned off their heat pumps and monitored the temperature inside their homes to see just how resilient they were. Alter’s article was written in the middle of the challenge, and most of the examples he included were for homes where the outdoor temperatures were above freezing. Here’s a nice below-freezing result I found in the #PassivhausChallenge search results:

#PassivhausChallenge The heat has been off since Feb 11th, and the temp has been no lower than 62℉ inside at 10℉ outside—David Hill (@DavidHi12070892) February 20, 2021

Resilience starts with the building enclosure and makes living with heat pumps in cold climates much easier. But that doesn’t mean they can’t be used in anything but a super-airtight, super-insulated house.

What happens when the heat pump stops pumping heat?

Lovoie, an HVAC contractor, naturally worries about having to make a service call in extreme weather. In Nelson’s case, he wouldn’t need someone to service the system immediately. As previously mentioned, his house would remain livable for at least a couple of days without any heat. If he still had electricity, the four 1500-watt space heaters would cover more than his design heating load, if he wanted to heat the whole house. (And he’s always got the electric oven,)

I live in an all-electric, not super-airtight, not super-insulated home. We had the gas meter removed in November 2019 after I installed a Mitsubishi mini-split heat pump with two ducted air handlers. With a winter design temperature of 23°F, Atlanta does get cold sometimes, although we were spared the storm that tanked Texas in February. Like Gary Nelson, I also have no auxiliary heat in my system. What would I do if the heat pump stopped working during a cold spell?

After installing the heat pump that heats most of our main floor, I had a ductless unit installed in the sunroom. If the main unit stopped working, we could always close the doors and hangout in the sunroom to stay warm until the other heat pump got repaired. This kind of redundancy serves as a good backup. And it’s another reason to install one-to-one minisplit heat pumps instead of multiple indoor units running on one outdoor unit.

The outdoor unit for our Mitsubishi inverter-driven mini-split heat pump system
The outdoor unit for our Mitsubishi inverter-driven minisplit heat pump system that heats and cools most of the main floor of my house

Speaking of one-to-one minisplits . . . at the time I got our main heat pump, Mitsubishi didn’t yet have outdoor units with Hyper-Heat for one-to-one setups. Now they do, so maybe someday when I have some extra money, I’ll replace the one outdoor unit with two. That would help with the view from the windows in that basement bedroom, too.

The answer: resilience and redundancy

Ideally, every home would be resilient enough to get through extreme weather events, survive equipment failures, and manage during power outages. That starts with the building enclosure. Seal it up. Insulate it as much as you can. And don’t forget ventilation!

Beyond the building enclosure, think through your heating and cooling sources. Auxiliary heat doesn’t have to be built into the heating system. Gary Nelson proved that. Multiple heat pumps can help with the mechanical failure of one. And one-to-one minisplits rather than multi-splits provide a safety factor, too.

We really do have the know-how and the technology to make all-electric homes work, even in cold climates. We can get fossil fuels out of homes now.


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.


  1. CollieGuy | | #1

    Almost all of our space heating needs are met by two, thirteen year old, 12,000 BTU/hr ductless mini-splits (2,700 sq. ft., 53-year old Cape Code, CZ5). They've been pretty reliable overall, but one was temporarily sidelined by a failed control board and the other a leaking evaporator coil. And, occasionally, they can be knocked off-line by snow drifts that can reach upwards of two to three meters in height.

    In terms of an alternate or supplemental heat source, we have a couple options. There's in-floor electric radiant heat in my home office, front foyer and bathrooms, and portable electric heaters that can be brought out of storage if needed. We can also revert back to using our oil-fired boiler which we can transfer over to our back-up generator in the event of an extended power outage. Our last line of defense, should all else fail, are four propane fireplaces that can operate without electricity. Having lived through the great ice storm of '98, I was determined never to be without heat ever again.

    We've tried as best we can to reduce our home's space heating requirements. In the year prior to our purchase, the previous owners had consumed 5,700 litres of fuel oil and 12,300 kWh of electricity. Although the home was in otherwise good shape, there were serious heat loss deficiencies that had to be addressed.

    With various improvements to the thermal envelope, and by replacing the original boiler and oil-fired water heater with a new Slant/Fin unit, SuperStor Ultra indirect water heater, and a Tekmar Control system, we got that down to 2,000 litres a year. This winter, with our two ductless heat pumps carrying virtually all of the load, we used just 31.3 litres (8.3 US gallons) for supplemental heat and to periodically exercise the boiler to help keep it in proper working order. Our total electrical usage is now averaging 6,500 kWh a year, and we consume approximately 50 litres of propane to operate our tumble dryer. We purchase 9,000 kWh of 100 per cent renewable energy each year from Bullfrog Power, which effectively makes our home carbon negative.

    The attached PDF provides a more detailed breakout of our energy usage.

  2. user-723121 | | #2

    I had a conversation with a friend (building scientist) shortly after completing an air sealing and insulation upgrade on my home in 2006 in Minneapolis. I had been setting back the indoor temperature nightly for sleeping comfort and noticed after the upgrade the indoor temperature after the setback fell more slowly. My scientist friend said I was on to something. He worked with NYSERDA for a number of years doing residential energy retrofits and they would conduct a simple test before and after the work. Before work began on a house they would turn off the heat source and record the indoor temperature fall in degrees per hour taking into account the outdoor temperature. After the retrofit they would conduct the same exercise to see how much the energy improvements slowed the flow of heat through the building envelope. This gave them an instant snapshot of the improved thermal performance.

    1. Jon_R | | #3

      You would have to account for differences in solar gain and wind speed.

  3. john_campbell | | #4

    heat is not the only issue when the power fails. active ventilation and heat recovery systems also fail. now what? has there been a study on humidity build up in passive buildings overtime with no electricity running?

    I live in northern vermont. this issue has held me off from leaving wood heat source out of my house plans. if power fails and i have another heat source i can at least crack the windows in winter to ventilate.

  4. clint229 | | #5

    Cannot some minimal solar panel/battery be a heating back-up to a mini-split?

    1. GBA Editor
      Martin Holladay | | #6

      No. For an example of costs: my $4,600 lead-acid battery (price does not include any solar panels or controls) has a useful storage capacity of 19 kWh. If I wanted to run two small 1,500-watt space heaters, I could only power the heaters for 6 hours before the battery was drained.

      1. clint229 | | #7

        Thanks Martin. I live in Brattleboro VT and bought land in Townshend, with plans to build a 28X30 aging-in-place 1 BR, 1 1/2 Bath, 1 floor, insul slab, very well insulated, pretty well sealed house. I am thinking all-electric (am 73, will be older later: safety), like a mini-split. But what about when the power is out for a week, like Bennington not so long ago? A small generator?

        1. GBA Editor
          Martin Holladay | | #8

          If your house is well sealed against air leaks and well insulated, it will never freeze, and your plumbing will be fine. Buy a small generator if you don't want to use candles and kerosene lamps.

        2. hudson_valley_gregg | | #15

          Clint: Curious why not install a wood stove system as backup, particularly considering you're still in the planning phase. Yer in the midst of the Green Mountains...

          1. clint229 | | #18

            Love wood stoves. Was my only source of heat for years. Am thinking of other ways because I want an aging-in-place home, for myself at 90, and for others down the line.

  5. sarahgrant | | #9

    I find it interesting that people fixate on back-up heat for a heat pump and yet most homes in Southern Ontario (where I am) that heat with gas (boiler or furnace) it's rare to have a back-up form of heating. So the furnace or boiler breaks and you get it replaced. Also when the power goes out the air handler wouldn't work for a gas furnace anyway so same problem as someone with a heat pump. Why is it so much more worrisome for a heat pump?

    1. Jon_R | | #10

      In many cases, heat pumps turn off when it gets really cold. And such cold can continue for longer than the typical power outage.

    2. BirchwoodBill | | #14

      Worried about lack of qualified people to work on them.

  6. hudson_valley_gregg | | #11

    When "grid" electricity is down for an extended period and your house is powered mostly or completely by it, wood is the obvious fuel of choice as a hybrid or secondary system for us 99%ers who don't live in a #PassivHaus. Wood stoves including a wood cooking stove along with an auxiliary battery system for low power draws... golly, it all seems pretty solid for a primary system for some of us.

    1. Expert Member
      MALCOLM TAYLOR | | #12


      Or propane, which gives you heating, cooking and the ability to use all your electrical appliances and devices.

      1. hudson_valley_gregg | | #13

        Malcol: a blessing to have access to such a surfeit of options. On "Green Building Advisor" I'd lean towards wood... but then the definition of "green" is one of serious debate in this forum, so I'll leave it at that.

  7. JonathanBeers | | #16

    Of course, a well-insulated and air-sealed home is the best defense. And, a heating system that doesn't require electricity, (such as a properly installed wood stove), provides resilience. Realistically, how soon will most city-dwellers have either of these defenses, though?

    Previous winter power outages might have some lessons? A 1998 ice storm knocked out electrical power (and thus heat) for many, and some for weeks:

    If I recall correctly, the basement temperatures in Montreal homes remained in the 50+F range after many days without heat, about the same as the deep soil temperature below the frost line. I don't know whether frozen pipes/toilets/etc. were a significant problem during that storm. Do any GBA readers have stories from that ice storm? I think most Montrealers sought shelter away from home rather than stay in their basements?

    Natural gas and propane, for all their fossil fuel evils, can provide valuable backup, and not just via adding a generator. (For the record, I think we have to green the grid as fast as we can and electrify as much as we can, including transportation.)

    Most gas fireplaces will work without power from the grid. The blower won't work without electricity, but the burner usually will. That big x-shaped pilot light heats a thermocouple that opens the gas valve to allow ignition. I only turn on our pilot light when we're using the fireplace because otherwise it uses 8 therms per month - the same as our gas water heater. We moved into our house on a warm day, and I noticed that the fireplace made the living room uncomfortably hot. Turning off the pilot light solved that problem.

    Gas water heaters that aren't power-vented work without electricity. A customer told me that during a power outage he filled the bathtub, kitchen and bath sinks, laundry tub, and several 5-gallon buckets with hot water. During an extended outage I wonder if this approach might create humidity problems? However, he was very proud of his short-term solution.

    Any updates on the idea of using inverters connected to vehicles to power just the critical loads in homes? Several years ago some Prius owners did it during outages, and Nissan sold kits for their Leaf EVs in Japan, but not in N. America. Furnaces with ECM blowers require much less electricity to start and run, so they make this option more realistic in cold weather.

    A search turned up this 2020 article on the topic:

    1. hudson_valley_gregg | | #17

      "I think we have to green the grid as fast as we can and electrify as much as we can, including transportation." Keep on smoking the good green stuff, my friend - I do sometimes, as well! That noted, some of us are working sans pipe dreams to expand hybrid live-electric power generated by muscle, wood, water, wind, and sun and NOT by petrol, "natural" gas, and coal; as well, the hardscape skips the aforementioned toxic three phenomena. More and more of us, not to mention the soil and organic matter of our home planet, would benefit by leaving behind fossils for anything but museums. Also time to act beyond the Federal Reserve Note as a decision-making fulcrum. Appreciate the insights, to be sure.

    2. henryrose | | #19

      The upcoming Hyundai Ioniq 5 and Kia EV6 come with a special piece that plugs into the car’s charge port. Into the other end of this device (about 12” long) you plug a 120v extension cord. It will supply up to 3.6Kw (3 times that mentioned in your quoted article’s method). The upcoming EV pickup trucks will similarly provide job site power. Looks we’ll be seeing more and more of this functionality.

  8. SierraWayfarer | | #20

    Get my sleeping bag out...go to a friends, a motel... That actually happened to me for three days during an ice-storm. It was miserable! I spent extra time at work, where it was heated, you bet...

    But, is it really green to buy all of that extra stuff for a rare event? Isn't it really consumption that drives global warming?

    Take care of your health, take a few modest precautions like a sleeping bag for winter and tarp for summer shade and you'll make it and the world will be greener.

  9. Knod | | #21

    Hi, not sure if this is monitored. However I had some questions re: PTAC, AC-heat pump.
    Live in pgh, 2-300 sq ft addition. House has forced air but addition is on concrete with no ducts from hvac unit. A ptac was the ac and heat source, it failed after 5 yrs. I now need to replace and wonder if ac with heat pump and 5.0kw back up strip will be the way to go in our climate. Any thoughts?
    Thanks, keith
    [email protected]

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