Heat Pump Balance Point & Backup Heat Calculator
Find the thermal balance point where a heat pump's output equals the house heating load as outdoor temperature falls, using the design heat loss and the heat pump's capacity at 47 °F and 17 °F, plus the auxiliary heat needed at design temperature, the economic balance point versus gas or resistance heat, and hours below the balance point.
How it works
UA = design load ÷ (T_in − T_design); load(T) = UA × (T_in − T); capacity(T) linear between the 47 °F and 17 °F ratings; balance point where they are equal; aux = load − capacity at design; economic balance where COP × gas cost per BTU = electricity cost per BTU
A heat pump's output falls as it gets colder outside while the house's heat loss rises, so the two lines cross at the thermal balance point — typically 25–35 °F for a conventional unit sized for the cooling load — below which supplementary heat must make up the gap. Plotting the two lines from the design heat loss and the AHRI 47 °F and 17 °F capacity ratings shows how much auxiliary heat is needed at the design temperature and how many kilowatts of electric strip heat or what size furnace to specify. A separate economic balance point compares the cost per BTU of the heat pump, which worsens as its COP drops in the cold, with gas or another backup: in dual-fuel systems the thermostat switches to the furnace below that temperature even though the heat pump could still run.
Worked example
A house losing 48,000 BTU/h at 5 °F (UA 738 BTU/h·°F) with a 3-ton heat pump rated 36,000 BTU/h at 47 °F and 24,000 at 17 °F balances at about 30 °F; at the 5 °F design point it delivers 19,200 BTU/h against a 48,000 load, so 28,800 BTU/h (8.4 kW → 10 kW of strips) of backup is needed. At $0.16/kWh and $1.40/therm through a 92% furnace, gas becomes cheaper below roughly 35 °F.
Frequently asked questions
Is a lower balance point always better?
For heating, yes — less backup. But a heat pump sized to hit a very low balance point may be oversized for cooling and cycle inefficiently, unless it is a variable-capacity unit. Cold-climate models solve this by holding capacity to −5 °F or lower.
Why is my thermostat's balance point different?
Many thermostats use a fixed "aux lockout" or "compressor lockout" temperature set by the installer rather than a calculated balance point; setting the aux lockout near the calculated thermal balance point and the compressor lockout near the economic one is the usual dual-fuel strategy.
Related calculators
Standard HVAC field formulas (sensible heat equation, fan laws, ASHRAE 62.2 and blower-door conventions) at sea-level air density unless noted. Use manufacturer data, a Manual J/D/S load calculation and a licensed HVAC technician for design and commissioning decisions.