UPDATE: I stupidly didn’t notice, until a commenter pointed it out, that this article talks only about air-to-water heat pumps, which do the harder job of heating up water to be used in home heating, rather than the more common air-to-air system. These conclusions should NOT be used for air-to-air heat pumps.
When I moved to New Hampshire in the late 1980s our apartment had an air-sourced heat pump and it really sucked – didn’t do a good job for many weeks in the winter. But the technology of removing heat from the air through compression and expansion of liquids has improved by leaps and bounds since then and heat pump fans say they are now a viable alternative here.
Sustainableheating.org has a small article looking at the issue (read it here) and says that air-to-water systems can carry the load when the air temperature is above 15 degrees F. Below that, things get iffy; below 0 F, they’re useless.
The above chart is from a firm called Appropriate Solutions.
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Brooks earned a bachelor’s degree in mathematics but got lost on the way to the Ivory Tower and ended up in a newsroom. He has reported for newspapers from Tennessee to New England. Rummage through his bag of awards you’ll find oddities like three Best Blog prizes from the New Hampshire Press Association and a Writer of the Year award from the N.H. Farm and Forest Bureau, of all places. He joined the Concord Monitor in 2015.
My only source of heat is two Fujitsu heat pumps. I live in Maine. When it gets well below zero, they still put out plenty of heat. All over northern New England, tight, well insulated houses are being comfortably heated with air source heat pumps. The idea that they don’t work at below zero temperature is just wrong.
The article from sustainable heating talks about air to water heat pumps. They’re pretty rare. Air to air heat pumps, which are becoming common in Northern New England, work fine at temperatures way below zero F. Mine are our only source of heat here in Maine.
As a homeowner on a budget, I’m not just interested in whether a heat pump “works” (keeps up with heat loss) as with whether it is cost-effective to run (Is the true net COP, including defrost cycles, greater than 1.0?)
Looking at overnight lows this week, I’d have needed to keep a fossil fuel burner to affordably avoid freezing.
I own a home with three air to air heat pumps (Mitsubishi units new in winter 2016/17). I would agree that in general, they do not do the job at below 10 degrees F temp. Our two units that heat relatively small spaces for short periods and work even when the temperature is low. However the unit that heats most of the first floor area really struggles in the low temps, it runs constantly and can’t keep the indoor temp over 55. I would suggest that the placement of the units and the configuration of the space, are critical factors in whether the units can produce adequate heat at low temperatures.
I have solar panels for electricity, and a backup LPG furnace, so I’m really motivated to use the electric heaters, but there are some days every winter when I am forced to turn on the backup furnace just to stay warm. Putting in another heat pump might help, but it’s just not worth it for the relatively few days it gets this cold. Open plan houses are great for wood stoves, but not so much for heat pumps IMHO.