I think @aphyr has a cleverly designed climate control system that balances a heat pump for primary climate control duty with at least one supplementary system that kicks in when needed, but I also think he's at a conference right now and very tired so he might not see this tag for a couple of days.

@gnomon @casseagull heat pump here plus resistive aux heat in the air handler. Same deal, kicks over to aux heat around 0 degrees F. I should have insisted on cold climate heat pumps.