My heat pump is primary. I home-brewed a gadget based on an Arduino board that runs my boiler for a minute every hour when the outside temp is below 35, to keep the pipes from freezing, and to provide some heat to some closed-off rooms that don't get good air circulation from the heat pump.

One year in, and the system works well, and I'm saving real money.

Of note, I ran the boiler non-stop for that one 15-below day in early February. Gave the heat pump a break.