Heating Systems
Gas furnace operation
Know the sequence: thermostat calls for heat, the induced-draft blower starts, the pressure switch proves draft, the igniter energizes, the gas valve opens, the flame sensor proves flame, and the blower starts on a time or temperature delay. Questions love the failure that stops the sequence - a blocked flue or bad pressure switch means no ignition at all, while a dirty flame sensor means the burner lights and drops out after a few seconds.
Temperature rise and combustion
Temperature rise is supply-air temperature minus return-air temperature, measured in the plenum a few feet from the unit, and compared with the nameplate range (commonly 35–65°F). A rise above the range means low airflow: dirty filter, restricted coil, or wrong blower speed. Below range means too much air. Combustion questions cover manifold pressure, flue draft, CO versus CO2, and derating or orifice changes for LP conversion.
Heat pumps and electric heat
Heat pumps add defrost (time/temperature or demand), auxiliary and emergency heat, and the balance point where capacity equals load. Know that defrost failure modes produce ice on the outdoor coil in heating and that aux heat is staged by outdoor temperature, not by the thermostat alone. Electric furnaces use sequencers or contactors and limit switches.
Worked example: clocking the meter
Input Btu/h = (cubic feet metered × heating value × 3600) ÷ seconds. If 2 ft³ pass in 60 seconds with gas at 1,000 Btu/ft³: (2 × 1,000 × 3600) ÷ 60 = 120,000 Btu/h input. Compare that with the rating plate input, not output.