Heat pumps as path to electrify home heating: Lessons from the polar vortex
So it got a little cold in the upper Midwest this week, giving Minnesotans plenty to complain about.
But as energy enthusiasts at CEE, our biggest (& nerdiest) complaint was that we weren’t actively monitoring cold climate air source heat pumps during the frigid snap, having ended our field test of Mitsubishi’s latest model at the end of the last heating season.
To heat a home, air source heat pumps employ a refrigerant compressor and condenser to absorb heat from outside air and release it inside — and as the technology has been refined for colder climates in recent years, it's emerging as a legitimate residential option in Minnesota. But with temperatures approaching -30°F, it’s unlikely that even heat pumps designed for our cold climates could keep up, and would have instead triggered auxiliary electric heat as backup.
How we heat our homes offers a big opportunity for electrification. But opportunity or not, winters as cold as ours inevitably raise questions about the viability of electrifying space heating. So if 30 below is too low, how low can modern air source heat pumps go?
As thankful as we are that strings of days below zero aren’t our norm, they represent an extreme we need to plan for in our part of the world, the same as planning for August heat in southern Florida. To effectively electrify space heating as part of a regional decarbonization strategy, we need to get realistic about that transition pathway.
Thanks to fantastic advances, modern heat pumps can squeeze heat out of a -10° January day and deliver it to a home. But our field tests verify that performance drops with outside temperatures below a point, especially at the coldest extremes. When things get really frosty, a heat pump is able to move less heat with the same amount of electricity — partly because, in addition to supplying heat to the home, the pump also needs to keep itself warm.
At some tipping point between about 5°F and -20°F (depending on the model), heat pump performance is no longer beneficial compared to conventional heating systems. On the coldest days, the pump simply can’t pull enough heat out of the air to heat the home, no matter how much electricity it uses. And when auxiliary heating takes over, you’re likely facing higher source energy and emissions than a gas furnace.
But don’t give up on heat pumps yet, because “extremes” are called that for a reason.
Even in Minnesota, we don’t spend that much time below zero. Over the course of a typical winter, heat pumps in our region compare very favorably to other heating options. In fact, figuring average emissions factors for electricity over a typical weather-year, a cold climate heat pump in Minnesota fares better than a natural gas furnace in terms of both energy efficiency and carbon emissions.
And heat pumps compare even better when you take into account their energy savings from cooling, or if the electricity powering your pump (via your electric utility) has better than average emissions.
So even with extremes, it’s definitely time to increasingly electrify space heating. But to really move the dial to attract average consumers and keep them happy during our next polar vortex, the energy field needs to tackle some pretty big issues:
Heat loss. Heating your home efficiently is one thing, but keeping that heat inside is a separate challenge. Insulation and air sealing are critical. As heating units struggle to keep up in extremes, things get worse when a home doesn’t have enough insulation or isn’t well sealed against air leaks. Fewer leaks and more insulation give heat pumps a better shot at keeping up on the coldest days.
Sizing and controls. Pairing the right amount of heat with the space you’re heating hinges on appropriate system sizing, as well as solid controls interaction between the pump and the auxiliary unit. When we compare field performance to rated performance, sizing and controls are usually behind any mismatch between what we expect and what we actually find.
Training. To electrify homes, we need contractor training and quality control on all of the above, so Minnesota’s heat pump pioneers get enough good experiences to compel their friends and neighbors to give heat pumps a try too. In our state, one fantastic option is to distribute training via utility energy efficiency programs — they’ve been delivering similar training for years, so it’s a natural fit.
And as we lay out this pathway to electrify home heating, it’s worth identifying some low hanging fruit that can offer big wins in the right markets. One potential opportunity is to target households primarily looking for a summer cooling solution. While not necessarily seeking cold climate models for heating, they’d still benefit from standard heat pumps that heat well in spring and fall “shoulder” seasons.
No surprise, CEE's employees are enthusiasts and early adopters. Below is an email excerpt from one who installed a heat pump in her 3-unit Victorian last year, illustrating both the challenges (for example, make sure you have a functioning backup system...) and the vision needed for success:
This whole heating situation is probably one of my biggest failures (also included: having to run out at 8pm last night to spend a crazy amount on space heaters), but I guess someone had to try this out on a Victorian behemoth.
Still, I'm of the mind that if we're going to solve climate change, we're going to have to start experimenting. There are many areas I wish I could have had better information and made better decisions, but such is life. We've learned a heckuva lot, and at some point we'll write a case study on this mess of a house.
Lastly, in order to see something positive in the energy of the house, I shoveled the snow off of our garage solar panels at around 11:15 and saw our power production grow! Gotta love cause and effect.
We’ll save those snow-covered solar panels for another post. For now, let’s concentrate on her “cause and effect” emphasis: Thanks in part to what CEE's learned through research and the experiences of our own staff — knowledge that we share regularly with the State of Minnesota, utilities, and field leaders — this market is evolving, and evolving fast. And as our colleague wished above, you actually do
have better information now to make better decisions.
Even as we smooth out bumps in the extremes, air source heat pumps are emerging as the future in Minnesota, especially when combined with smart sizing and controls, sealing air leaks, and contractor training. And among the current hurdles to cross, it's also worth noting that although this post focuses on comfort and performance, heat pumps are still not cost-competitive with other traditional home heating options — but prices should continue to improve as the market matures.
Keep an eye on CEE's field research to help ensure you're a bit more prepared when the next vortex strikes.
Note: CEE colleagues Katie Jones, Rebecca Olson, Audrey Partridge, Ben Schoenbauer, and Josh Quinnell contributed to this post.
Cold Climate Air Source Heat Pump Field Assessment (CEE project page)
Field Assessment of cold climate air source heat pumps (presentation)
Study Update: Cold Climate Air Source Heat Pumps (presentation)