I don't understand why the outside temperature makes any difference. Our system is a three stage. Stage 1 and 2 are geothermal (loops in our pond) and stage 3 is the auxiliary resistance electric.
We have seen -30F without out our thermostat calling for auxiliary heat. Even if it did call for it, I have the breaker off on the heating elements so they can't turn on and burn up all that electricity. I figured it was smart to have the auxiliary installed and available in case the compressors go down. In that event, at least we wouldn't freeze to death though it would be pretty chilly.
I may be missing something on the outside temperature, after all, I am no expert.
