Heat Pump Thermal & Economic Balance Point Calculator
Calculate the outdoor temperature (°F) where heat pump capacity intersects building heat loss (thermal balance point), size auxiliary electric resistance strip heat, and determine the economic dual-fuel switchover point vs natural gas.
Building Heat Loss & Design Conditions
Manual J total heat loss at outdoor design temp
99% ASHRAE winter design temperature
AHRI 47°F heating capacity (e.g. 3 Ton = 36,000 BTU)
Thermal Balance Point
Aux Heat Required Below
22.5 °F (-5.3 °C)
Heat pump carries 100% of building heat load above this temperature
Required Aux Heat Strip Size
10 kW (34,120 BTU/hr)
Covers 19,200 BTU/hr deficit @ 5°F
Heat Pump Capacity @ Design Temp (5°F)
28,800 BTU/hr
60.0% of total building load covered by heat pump
Auxiliary Heat Electrical Sizing
Auxiliary Electric Current (240V):
41.7 Amps (Requires 60A Dedicated Breaker)
Hourly Aux Heat Run Cost:
$1.80 / hour (COP = 1.0 direct electric)
Economic Dual-Fuel Switchover:
~28°F to 32°F vs $1.30/therm natural gas
Frequently Asked Questions
Why does my thermostat say "Auxiliary Heat" on cold mornings?
When outdoor temperature drops below your thermal balance point, or when you raise the thermostat by more than 2°F, the heat pump engages electric resistance strips (aux heat) to satisfy the temperature deficit.
What size electric heat strip should I install?
The heat strip must cover the deficit between building heat loss at your winter design temperature and the heat pump capacity at that same temperature. Standard sizes are 5 kW (17k BTU), 8 kW (27k BTU), 10 kW (34k BTU), and 15 kW (51k BTU).