How Does the Supplemental Heat Feature of a Heat Pump Work?

Why heat pump capacity drops in the cold

A heat pump moves heat from the outdoor air into your home, so it is affected by the temperature around it. As the outdoor temperature drops, the heating capacity of any heat pump diminishes. The unit measures the demand you place on it, meaning your indoor thermostat setting, and compares that to the outdoor temperature. When the spread between the two is high, it automatically energizes the supplemental heating element to maintain the setting you have selected.

Is there a temperature where a heat pump stops working?

There is no single temperature at which a heat pump stops producing heat. The heating capacity of all heat pumps is rated at a standard 47 degrees. As the outdoor temperature drops, the ability to produce heat decreases in a linear fashion rather than cutting off at one point.

When supplemental heat takes over

Using a 3 ton heat pump rated at 35,000 Btu as an example, output falls steadily as it gets colder. By the time the outdoor temperature reaches about 17 degrees, the supplemental heating element has energized automatically to restore the system's ability to heat your home. Because the decline is gradual and relative to the outdoor temperature, there is no exact point where a heat pump becomes useless for heating. In very cold climates, however, a heat pump may not be the best choice on its own.