Heat pumps do not do well when it's -40 outside. You can say "fine, but how often does it get that cold?" but consumers are not going to be happy with a heat pump if their pipes freeze during an extended cold snap.

I live in Southern Ontario and I have a heat pump with an auxiliary natural gas furnace for emergency heating. The heat pump shoulders most of the heating load but the thermostat does kick on the furnace when the heat pump starts falling behind.

It should also be noted that although heat pumps are very efficient, even when it's below freezing outside, they cannot raise the temperature of the house very quickly. Consumers are generally quite unhappy when it takes 8 hours to raise the temperature of the house by 1 degree, so the thermostat usually calls for the furnace to start up before things get that bad.

air source heat pumps, though they're improving won't do well in extreme cold; even if they can operate they'll still be running at much lower efficiency.

For temperatures significantly into negative territory a ground source heat pump would perform far better, where it can draw on a source of heat that will always be at least above freezing.

A hybrid system doesn't seem like a bad trade-off though..

Canada's climate is really different when it comes to the extreme cold.

Heat pumps are getting better at lower temperatures, but in an environment like Canada you still want auxiliary heat to be safe.

> It should also be noted that although heat pumps are very efficient, even when it's below freezing outside, they cannot raise the temperature of the house very quickly. Consumers are generally quite unhappy when it takes 8 hours to raise the temperature of the house by 1 degree

That would be an undersized heat pump in any regard. The installer would be at fault for screwing up that badly.

You're right that efficiency falls off at lower temperatures, 8 hours to move 1 degree would be from the installer sizing the unit wrong.