Heat Pumps

Heat Pumps and Subzero Temperatures: Do you need backup?

Clay Dusel
February 8, 2023

December's record-breaking cold snap put our heat pumps to the test. They passed the test with flying colors. No Backup, No Problem

High performance heat pumps are an efficient and cost-effective way to heat homes and buildings in cold climates, but their performance in subzero temperatures has often been a point of concern. Helio has installed over 100 systems in the last few years and over 90% of them don't have any form of backup heat.  All of them performed very well in the recent Denver cold snap. We'll take a look at two case studies:  One without any backup, and one with backup that was used minimally, even in the recent extreme temperatures.


First, let's talk about how a heat pump works. In the winter, a heat pump takes heat from the outdoor air and transfers it into the indoor air. Because the heat pump is simply moving heat that already exists it is much more efficient than burning fossil fuels to create heat.  Though it's hard to imagine, heat pumps can still extract heat when the temperatures plunge to -17 in F or colder.

Of course, it's harder to move that heat when it's so cold, meaning the heat pump is less efficient.  At the same time, there is a decrease in the heat pump's capacity (i.e. it can move less heat).  Fortunately, advances over the past decade have greatly improved the heat pump's ability to operate at subzero temperatures, to the point that backup heat is no longer necessary.   We Coloradoans experience these extreme temperatures just a handful of hours every year, and with the latest systems and proper system sizing, we can keep your home comfortable.

Recent extreme weather in the Denver metro provides an excellent test of heat pump performance, with NOAA reporting temperatures of -16 F in South Denver and -18F in South Boulder.  This marked the second coldest day ever recorded in Denver.

One Helio client had their Denver home updated in 2022.  The work included air sealing, additional attic and crawl space insulation, and the replacement of two existing furnaces with heat pump air handlers.  The system was put to the test with the plummeting temperatures. While the interior temperature did go a few degrees below the thermostat setpoint for a few hours, the heat pump performed well and  the client reported that overall comfort was much better than before the work was done: specifically, a previously cold main bathroom was much more comfortable due to the air sealing and insulation.

Another client in South Boulder had his 1960s townhome furnace replaced with a heat pump air handler (plans for insulation and air sealing are still in the works).  This system did include an electric resistance backup heater.  He recorded performance data from his heat pump (see his tweet thread, or the energy.gov mention of his house) during the cold snap, and found that the backup heat ran for just a few hours in the early morning hours.  Without backup, the temperature  would have been a couple of degrees lower than setpoint, similar to our first example.  It's important to emphasize that this mostly occurs when most folks are sleeping under their nice warm comforters.

Cold climate heat pumps really have made impressive strides in performance over the last few years.  As our clients' examples have shown, heat pumps continue to keep their houses comfortable, even  during record setting lows, even without backup heat.