I run a heat pump for my swimming pool in Calgary. It's good until it hits about 10 or 11 Celsius, then it falls off the cliff for effectiveness and eventually turns into a little frozen ice cube that takes a day to thaw out. So I always have to remember to turn it off after supper, even on nights like tonight when it's hot out.
Now keep in mind, these new heat pumps are a lot more efficient and effective. But when its minus 30 out, I don't care how futuristic it is, it's not going to be effective OR efficient. Even at -10 that efficiency is going to be flying off a cliff. This is why they tell you to have a back up gas powered furnace on the prairies, which sort of defeats the purpose in my opinion.
The other thing that kills them out here, is the altitude. When I was buying my pool heat pump, there was a whole bunch they didn't recommend using over 2,000 feet, which wouldn't work out here. It always makes me wonder when everyone is throwing that -30 number around, just how accurate that is.
Don't get me wrong, I'm the biggest fan of heat pumps, and I don't even notice my pool heat pump on my electrical bill, even though it runs all day. That thing is the cat's meow, and the best (by far) way to heat a swimming pool. But you won't find me hooking one up to the house, not just yet anyways.
I believe I've heard that heat pumps are good all the way down to -30 or so, though the efficiency at that point is pretty bad. That said, geological heat pumps can circumvent that as long as the ground doesn't freeze too deep where you live. Dig far enough and heat pumps work anywhere that's not permafrost, and I'm talking about 20-30 meters deep, not something insane like tapping underground magma.
I thought ones that shallow are only good if the ground doesn't freeze much? I know there are horizontal ones about that deep or so, and vertical ones that are designed for places where the ground freezes, or if you have little space for a horizontal installation.
The ground does not freeze that deep until you get to very northern latitudes but yea you do need the space for horizontal loops which not everyone is going to have.