Just a little info on my whole house ASHP. My backup heat is entirely controlled by my thermostat which is set to auto. Emergency backup heat kicks on at about -5C or if I turn the thermostat up "a lot" like 3 degrees in one shot. The thermostat will then kick on the backup automatically to rapidly increase the house temperature.
The thermostat also seems to be smart enough to start ramping the temperature up early so if I say I want it 19 in the house at 6 am the heatpump fires up around 5:30 to get it there.
Having said all of that I leave the thermostat at 18 and let the sun during the day and the woodstove at night heat the house most of the time. I just set the furnace fan to circulate to keep that air moving and the bedrooms warm.
I have recently been thinking about letting the heatpump handle heating on warmer (5-10C) days and save on wood. I'm just not sure how much more efficient that actually is with my older HP.
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