Dual-Fuel Economic Switchover (Heat Pump vs Gas Balance Point)
The setpoint a dual-fuel (hybrid) thermostat should hand off to gas at: as 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 and source
gas_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.
The 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.
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