Heating Equipment and Heat Pump Systems
Furnace sequence and heat pump balance
Gas furnace sequence of operation: thermostat calls for heat, the inducer energizes and proves at the pressure switch, ignition (hot surface or spark) lights the burners, the flame is proved by flame rectification, the blower comes on after a delay, and it runs past shutdown on blower-off delay. Safety devices include the high-limit switch, rollout switch, flame sensor, and on condensing units the blocked-vent and condensate switches. AFUE rates seasonal efficiency; steady-state combustion efficiency is a different number.
Temperature rise is the best field airflow check: rise = output Btu/h divided by (1.08 x CFM). A rise above the nameplate range means low airflow; below means too much air.
On a heat pump the reversing valve swaps the roles of the two coils. Capacity and COP fall as outdoor temperature drops. The balance point is where heat pump capacity equals building load; below it, auxiliary resistance heat makes up the difference. Defrost termination uses time, temperature, or pressure and energizes auxiliary heat so the space does not blow cold. Emergency heat and auxiliary heat are not the same mode.
Worked example: 100,000 Btu/h input at 80 percent AFUE is 80,000 Btu/h output. At 1,200 CFM, rise = 80,000 / (1.08 x 1,200) = about 62 F. If the nameplate range is 35 to 65 F, that passes; a measured 72 F points to restricted airflow, not a gas problem.