Key Considerations
Cost Comparison at Sample Temperatures
| Outdoor Temp | Heat Pump Cost / GJ | Furnace Cost / GJ | Cheaper Option |
|---|
How to Use the Dual Fuel Calculator
Check a recent bill for accurate electricity and gas or propane rates. These drive the entire cost comparison.
Choose natural gas or propane to match your furnace and enter the corresponding rate.
Enter your heat pump's coefficient of performance at a moderate outdoor temperature and your furnace's AFUE rating. Check equipment spec sheets or the sales documentation for these numbers.
Click Find Switchover Point to see the outdoor temperature where the furnace becomes the cheaper option to run, then use that value as the outdoor temperature lockout on a dual fuel capable thermostat.
Understanding the Dual Fuel Switchover Point
A dual fuel system pairs a heat pump with a furnace and automatically switches between them based on outdoor temperature. The key number that makes this work well is the switchover point, the outdoor temperature where running the furnace becomes cheaper than running the heat pump.
Why Heat Pump Cost Changes With Temperature
A heat pump's efficiency, measured as coefficient of performance or COP, is not constant. It's highest in mild weather and drops as outdoor temperature falls, because the heat pump has to work harder to extract heat from colder air, a relationship you can explore further with a COP calculator. A heat pump with a COP of 3.5 at 8 degrees Celsius might drop to a COP of 2.0 or lower at -15 degrees Celsius, meaning it needs significantly more electricity to deliver the same amount of heat.
Why Furnace Cost Stays Roughly Flat
A furnace's efficiency, measured as AFUE, stays relatively constant regardless of outdoor temperature since it burns fuel to generate heat directly rather than moving heat from outdoor air. Run your own furnace's rating through an AFUE calculator to see its cost per unit of heat. This means the furnace's cost per unit of heat delivered barely changes with the weather, while the heat pump's cost climbs as it gets colder.
Where the Lines Cross
Because heat pump cost rises with falling temperature while furnace cost stays flat, there's a specific outdoor temperature where the two lines cross. Above that point, the heat pump is the cheaper option. Below it, the furnace becomes cheaper. This calculator uses your specific utility rates and equipment efficiency to find that crossover point for your home.
Setting Your Thermostat
Most dual fuel capable thermostats let you set an outdoor temperature lockout, below which the system switches from heat pump to furnace automatically. Setting this at or near the calculated switchover point optimizes for cost. Some homeowners choose a lower lockout temperature than the cost-optimal point to run the heat pump more often and reduce emissions, accepting a small cost tradeoff in exchange, an approach Natural Resources Canada's heat pump guidance also discusses for Canadian homes.
Getting the Inputs Right
The accuracy of this calculation depends heavily on using real efficiency numbers for your specific equipment rather than generic estimates. Check your heat pump's performance data sheet for COP at a specific outdoor temperature, and use your furnace's rated AFUE. ENERGY STAR's buying guidance for air-source heat pumps covers dual fuel setups and cold-climate sizing considerations in more detail. If you haven't chosen equipment yet, use the heat pump selection calculator and furnace selection calculator first to narrow down options.
Frequently Asked Questions
A dual fuel system pairs a heat pump with a furnace, using the heat pump as the primary heat source in mild to moderately cold weather and automatically switching to the furnace when outdoor temperatures drop below a set point. This combines the efficiency of a heat pump for most of the heating season with the strong output of a furnace during the coldest weather.
The switchover point is the outdoor temperature where the cost of heating with the furnace becomes lower than the cost of heating with the heat pump. As outdoor temperature drops, a heat pump's efficiency, measured as coefficient of performance, declines, which raises its electricity cost per unit of heat delivered. Comparing that changing cost against the furnace's relatively stable fuel cost identifies the crossover point.
No, these are different numbers. The switchover point in a dual fuel system is set by cost, not capability. A cold-climate heat pump might still work efficiently well below the calculated switchover temperature, but if the furnace is now the cheaper option to run, dual fuel systems switch to it anyway to save money. Some homeowners set a lower switchover point than the cost-optimal one to run the heat pump more and reduce carbon emissions instead of purely minimizing cost.
Yes, this is one of the most common ways to add a dual fuel setup. A cold-climate heat pump can be installed alongside an existing furnace, with a compatible thermostat controlling the switchover automatically based on outdoor temperature. This avoids replacing a still-functional furnace while gaining the efficiency benefit of a heat pump for most of the heating season.
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