Heat Pumps and Outdoor Design Temperature
What happens in cold climates during the 88 hrs of year when temps fall below outdoor design temp? Do heat pumps keep working but indoor temp fails to reach desired level? We like indoor temp 75 F 100% of time. When outdoor temp falls below our community’s design temp of one degree F, how warm will we be able to keep inside of house using mitsubishi hyper heat pumps alone? Will we need space heaters (eg, radiant ceiling panels, cadet heaters) in addition to pumps? I know 88 hrs are a small fraction of the year, but who wants to shiver for 3.5 days?! What do you GBA readers do to keep warm?
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In a typical climate, about half of the 88 hours are 1 degree below the design point, half of the remainder are 2 degrees below, half of the remainder are 3 degrees below. So you get something like:
44 hours 1 degree below
22 hours 2 degrees below
11 hours 3 degrees below
6 hours 4 degrees below
5 hours 5 degrees below or more
The short answer is that you get what people in the biz call "excursions," where the heat is on 100% and not able to keep up. For the most part these excursions aren't noticeable. First, it's not like they happen all at once, they're spread out over several months of winter. One hour of a one-degree excursion isn't going to be noticeable. Also, your house has some heat capacity, so even if the house is losing more heat than the heating system is producing it's going to take some time for it to cool off.
So for the most part excursions are ignored.
Thank you, DC, for this helpful and reassuring info
Allison Bailes addresses your concern in this article: Three Types of Heat Pump Heat. As he puts it: As long as the outdoors isn’t at absolute zero—and it’s never absolute zero anywhere—there’s still heat available. It just gets harder and harder to move it indoors. Eventually, the amount of compressor heat available drops below the amount of heat needed inside the home. That’s when you need supplemental heat.
Several years ago, our power here in Maine went off for six days at the end of October/start of November. We've got a tight, well insulated pretty good house. The interior stayed at 68° or above with no heat, no cooking. Night temperatures were well below freezing.
Our design temperature is 0° F. It gets colder than that on occasion. Our two minisplits have no trouble keeping up with the 72° set temperature. On occasion the power goes out in winter. If it happens in the middle of a cold night, I wait until morning to fire up the generator. Several hours or even a full day without heat is unnoticeable.
Build a low load house and you don't need to worry about the 88 hours below design temperature.
Thank you, Stephen, for sharing your experience. Very reassuring!
This is assuming you have heat pumps with exactly the same capacity as your design day heat load. That's not likely to happen because your heat load calculation will be imperfect and heating appliances don't come in infinite sizes. Even if you have only 20% extra capacity, you'll be covered for all but a few hours per decade.
Thank you, Paul
Where are you located? I would think about how close your design temperature is to the lowest rated ambient temperature of the unit. If your 99% design temperature is -10 F a cold climate heat pump may shut off (depending on model) the 1% of the time it gets colder. In this case backup heat becomes much more important. However if your design temperature is +10 F your heat pump will continue to put out heat well blow your design temperature.
Also, look at the extended temperature charts when sizing a unit. Published rated capacities are typically at an indoor temperature of 70 F. Capacity will be reduced with an indoor design temperature of 75 F.
We are at the tristate intersection NH-VT-MA. 99% outdoor design temp is one degree F. I believe the mitsubishi hyper heat units are rated down to -13 Fahrenheit but I will need to look at their literature again to be sure
On occasion, temperatures will go well below design temp - for extended periods.
> heat pump may shut off
This is the most important issue. It's also not clear what happens you when you operate below the listed minimum operating temperature. Increased wear?
Most heat pumps incorporate a multi stage heater, so you have COP of 1.0 on occasion. You can always add a buffer tank with an electric element as necessary. For DHW there are several companies that make boosters to bring your DHW up to 107F which is warm enough to take a shower.