Heat Pump Balance Point

The balance point is the outdoor temperature where the home's heat loss exactly equals the heat pump's heating capacity. Above it, the heat pump carries the whole load by itself. Below it, capacity falls faster than the load grows, and something has to make up the difference — electric resistance, a gas furnace on a dual-fuel system, or a changeover to backup.

Where you set the changeover temperature matters. Too high and you're running expensive backup heat when the heat pump could have handled it. Too low and the house drifts off setpoint on the coldest nights.

• The load line — the home's design heat loss from a Manual J, assumed to fall linearly as outdoor temperature rises toward the indoor setpoint.
• The capacity line — the heat pump's rated heating capacity at 47°F and 17°F from the manufacturer's submittal or NEEP data, interpolated between the two points.

Where they cross is the balance point.

A standard heat pump can lose 40% or more of its rated capacity between 47°F and 17°F. A cold-climate unit holds capacity much further down — often close to rated to 5°F — which pushes the balance point lower and shrinks the backup heat requirement. If you're working a cold market, size from the extended performance data rather than from the two ratings alone.

The gap between capacity at design temperature and the design load is what the auxiliary heat has to cover. Oversizing it costs money every time strip heat runs; undersizing it means the house can't hold setpoint on the coldest nights. Match it to the actual design load, not to the heat pump's nominal tonnage — and remember that the balance point shifts as outdoor temperature and load change, so this is a design estimate rather than a control setting.