Dual-Fuel Economic Switchover (Heat Pump vs Gas Balance Point)
The setpoint a dual-fuel (hybrid) thermostat should hand off to gas at.
Example
You enter
- Electric rate ($/kWh) 0.15
- Gas rate ($/therm) 1.5
- Furnace AFUE (0-1) 0.95
- Heat-pump COP at current temp 2.5
You get
- Gas delivered cost $15.79/MMBtu
- Heat-pump delivered cost $17.58/MMBtu
- Economic switchover COP 2.78
- Run (hp) No
- Verdict Switch to gas - below the economic switchover COP the furnace is cheaper
Checked against published worked examples from: Ameren Illinois (Ameren Illinois Energy Efficiency program).
Details, formula, and sources
Scopeas the outdoor temperature falls the heat pump's COP drops until the gas furnace becomes cheaper per delivered Btu. Heat-pump $/MMBtu = 293.07 / COP x rate_kwh against gas $/MMBtu = 10 / AFUE x rate_therm gives the crossover COP; the outdoor temperature where the unit falls to it is the economic switchover. Set it too warm and the customer burns gas the heat pump could deliver cheaper; too cold and they run the heat pump into its expensive low-COP range. An economic setpoint aid, not a controls-commissioning procedure.
Formula or tablegas_per_mmbtu = 10 / afue x rate_therm; hp_per_mmbtu = 293.07 / cop_now x rate_kwh; cop_switch = 293.07 x rate_kwh / gas_per_mmbtu; run_hp = hp_per_mmbtu <= gas_per_mmbtu.
Source and editionThe delivered-Btu fuel-cost comparison (heat-pump $/MMBtu = 293.07 / COP x rate_kwh, gas $/MMBtu = 10 / AFUE x rate_therm, switchover COP where the two are equal), by name; 1 MMBtu = 293.07 kWh and 10 therms per MMBtu are named constants.
Free accessThe delivered-Btu cost comparison is public arithmetic; dual-fuel setpoint worksheets are widely published.
What governsEstimate. AHJ and licensed professional govern.
API field namesrate_kwh, rate_therm, afue, cop_now, gas_per_mmbtu, hp_per_mmbtu, cop_switch, run_hp, verdict
- Delivered cost heat-pump $/MMBtu = 293.07 / COP x rate_kwh; gas $/MMBtu = 10 / AFUE x rate_thermdelivered-Btu comparison
- Switchover COP the COP where heat-pump delivered cost equals gas delivered costfirst principles
- Mapping to temperature the switchover COP maps to an outdoor temperature only through the unit's COP-vs-temperature curvemanufacturer data / AHRI