[robotpony]

Around Vancouver our heat pump has performed adequately even down to -15C/5F. This may be related to it being a deep ground loop geothermal setup (vertical instead of horizontal), and the type of building (concrete). We do have a gas fireplace as backup, but have not needed it at the temperatures we see in this corner of the world.

[timbit42]

It is definitely related to it being a deep ground loop geothermal setup. An air-to-air heat pump is trying to get heat out of air that has less heat as it gets colder. A ground loop heat pump is trying to get heat out of water that is not affected by the temperature of the outside air.

With an air-to-air heat pump, you need a backup system for when it gets too cold out for the heat pump to pull enough heat out of the air, and for power outages. With a ground loop heat pump, you only need a backup system for when the power goes out because the ground water will never be too cold for the heat pump to pull heat out of.

[gwbas1c]

I'm air based.

When I built my house, I called around and it didn't sound like geothermal works very well where I live. I suspect the reason why I was talked out of it is because my yard is very rocky.