A dual-fuel system uses the heat pump in milder weather and the gas furnace when it is colder (or when the heat pump cannot keep up). The economic switch point is the outdoor temperature at which the heat pump’s dollars per MMBtu, at that temperature’s COP, exceed the furnace’s dollars per MMBtu. Owners in Midwest forum threads often report a default around the mid-30s °F, sometimes because of duct or equipment limits rather than a calculated breakeven.
You can sketch the breakeven COP without inventing a temperature: it is the COP at which heat-pump unit cost equals furnace unit cost, or COP = (electricity $/kWh × 293.1) / (furnace $/MMBtu), since 1 MMBtu is about 293.1 kWh of heat (1,000,000 / 3,412). At U.S. average 2025 prices and 95% AFUE, the furnace unit cost is about 15.59/MMBtu, so the breakeven COP is about 3.24. If the heat pump’s COP at a given temperature is below that, the furnace is cheaper to run at that moment.
This is not a control setting you should copy from a website. An HVAC contractor should commission the changeover using design temperature, the actual COP curve, and your rates — or your solar and time-of-use schedule. The calculator compares seasonal ratings; it does not pick a thermostat changeover.
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