How Do I Size a Heat Pump and Its Supplemental Heating Element?

Step one: size the heat pump itself

A heat pump is sized the same way as an air conditioner. Start by determining the required cooling capacity for your home, then pick the matching tonnage. Heat pumps come in the same tonnage increments you expect from standard air conditioning. The heating capacity of a heat pump is normally very close to its cooling capacity, so the tonnage you choose for cooling sets your starting heating output too.

Why output drops when it gets cold

A heat pump extracts heat from outside air and moves it indoors. Its heating capacity is rated at a standard outdoor temperature of 47 degrees Fahrenheit so different brands can be compared fairly. As the outdoor temperature falls, the air holds less heat, so capacity declines in a roughly linear fashion. A unit rated near 46,000 BTU at 47 degrees may only produce around 30,000 BTU at 17 degrees, a drop of about 35 percent. The colder it gets, the more the heat pump alone falls short of what your home needs.

Step two: size the supplemental heating element

The shortfall is covered by a second stage of heating called a supplemental electric heating element, sometimes called a heat strip. To size it, find your home's heat loss, then find the heat pump's output at your local winter design temperature, and subtract.

Worked example

Suppose a home needs 58,000 BTU to stay comfortable and sits in an area with a 27 degree design temperature. A 4 ton heat pump might produce about 32,000 BTU at 27 degrees. Subtract 32,000 from 58,000 and the element must supply roughly 26,000 BTU. An 8 Kw element is the closest match, though many people step up to a 10 Kw for a small margin. A larger element draws more power to run, so size it to your real need.

Heating element capacity

Colder climate zones place a larger load on the heating system, so they need both adequate heat pump tonnage and a correctly sized element. Use your average winter low, not the rare extreme low you might see once every several years.