Yeah, that's pretty much the direction we're heading: Cold weather heat pumps are primary heat, need backup heat when it's too cold for heat pumps, need some source of hot water (same fuel/system as backup boiler, or something entirely different?)... Sounds like we may be on the right track...

The HVAC guys are telling us a combi unit wouldn't work well for our usage, so they're looking at tankless water heater as one unit, backup boiler as a separate unit... but we did ask about the combi units.