Pairing a heat pump with a furnace: how dual fuel works
A hybrid or dual-fuel heat pump system combines an air-source heat pump with a gas furnace and uses whichever is cheaper or more capable at the moment. It makes most sense in cold climates where electricity costs much more than gas, or where the heat pump alone would struggle on the coldest days.
What is a dual-fuel heat pump system?
Both heaters share one set of ducts. The heat pump handles the bulk of the heating, down to a chosen outdoor temperature. Below that point, the control hands over to the furnace, which runs alone. In a ducted set-up the indoor coil sits above or beside the furnace, so the two do not run at the same time.
This is different from a heat pump with electric backup strips, where the second source is resistance heat. A gas furnace as the backup usually costs less to run when the weather is very cold.
How does the switchover temperature work?
The simplest control uses a fixed outdoor temperature, say -10 C, below which the furnace takes over. Smarter controls compare the actual cost of heat from each source, using the entered gas and electricity prices and the heat pump's efficiency at the current temperature. The best switchover point depends on rates, not on a universal number.
The idea is related to the balance point described in heat pump sizing for cold climates, where the unit's output equals the house's heat loss.
When does a hybrid system make sense?
- You already have a good furnace and ducts and want to add cooling and mild-weather heating cheaply.
- Your electricity price per kWh is high compared with gas, so the furnace wins on the coldest days.
- The house has a high heat loss that would require a very large heat pump to cover alone.
- You want a second heat source for equipment faults, with the understanding that the furnace fan still needs power in an outage.
It makes less sense where electricity is cheap and a cold-climate unit can carry the full load, or where gas is not available. A direct comparison with your own bills is in the heat pump versus furnace guide.
What should you ask an installer?
Ask who sets the switchover and how it is programmed. Check the thermostat or controller is compatible with two stages of heating from different fuels. Ask for the heat loss figure and the capacity of the heat pump at the design temperature. Request itemized pricing, since a new indoor coil, a controller and extra electrical work can all appear. Compare the typical cost drivers with the quote you receive.
Is there a link to Drake Landing?
Drake Landing was a form of hybrid in its own right. Each home kept a conventional backup, and the district loop supplied most of the heat. The loop's low-temperature water was delivered to an air handler in each house, and the earlier years of the monitoring record show how much of the load the solar side carried. The lesson carries over: a primary source that does most of the work, with a backup for the extremes, can be a sensible design.
What are the downsides of a dual-fuel set-up?
Two systems mean two things that can fail and two service visits. The controller is another point of failure, and a badly set switchover can leave the furnace running in mild weather or the heat pump struggling in deep cold. You also keep paying the fixed gas connection charges, which can be a significant part of the gas bill in a low-use home.
If you plan to remove gas entirely in the long run, a dual-fuel system may be a stepping stone rather than the end point. In that case, ask the installer to size the heat pump so it could carry most of the load later, which ties back to the cold climate heat pump ratings you looked at earlier.
Quick answers
What is the difference between hybrid and dual-fuel heat pumps?
In practice the terms are used interchangeably for a heat pump paired with a fuel-burning furnace, usually gas. Some people use hybrid for any heat pump with backup heat, including electric strips.
What temperature should a dual-fuel system switch to the furnace?
There is no single best number. It depends on your electricity and gas prices and on how efficiently the heat pump performs at that temperature, so the installer should model it for your rates.
Will a dual-fuel system work in a power outage?
Not on its own. A gas furnace still needs electricity for its fan and controls, so a backup power plan is needed if you want heat during an outage.