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Heat pump + furnace: dual fuel, explained.
How a heat pump and a gas furnace share the work through a Tri-Cities winter — in houses and in commercial rooftop units — and how the switchover should be set.
Each does the part it's good at.
A heat pump moves heat rather than making it, which is efficient for most of the year. As the outdoor air gets colder it has less heat to give, and the building needs more. A gas furnace makes heat from fuel and doesn't care how cold it is outside.
In a dual-fuel system the heat pump runs whenever it can keep up, and the furnace takes over when it can't. The thermostat decides when — at the switchover.
Three things decide where it should be.
- The balance point
- Below it, the heat pump can't keep up on its own. It depends on the building's heat loss and the heat pump's capacity — which is why no single number is right for everyone.
- Running cost
- What electricity and gas cost you, and how efficient each piece of equipment is. When the heat pump is still able to keep up but the furnace is cheaper to run, some owners switch over earlier. Rates change, so this is worth revisiting.
- Comfort
- Heat pump air feels cooler at the register than furnace air. Some people prefer the furnace on the coldest days even when the numbers don't require it.
What we check on a dual-fuel system.
- That the thermostat is actually set up for dual fuel — not treating the furnace as emergency heat only, or never using it.
- Where the switchover sits, and whether it still makes sense for your rates.
- Defrost operation, refrigerant charge and airflow on the heat pump.
- Burner, igniter, venting and safety controls on the furnace.
- That the outdoor unit sits high enough to clear snow and drain its defrost water.
Rooftop units work the same way.
Many rooftop units heat with gas; newer heat-pump rooftop units heat with the refrigeration circuit and use gas or electric heat as backup. The same questions apply: where the switchover sits, whether the controls use it properly, and whether the economizer is bringing in cold outside air when it shouldn't.
We'll look at every unit on the roof and write down how each one is set to heat.
No switchover temperature on this page — on purpose
Websites often quote a "best" switchover temperature. There isn't one: it depends on the building, the equipment and your utility rates, and rates change. We'll work out a sensible setting for yours and explain why.
Is your dual-fuel system set up right?
Houses and buildings across the Tri-Cities.