Heating Systems
What this domain tests
Heating Systems covers gas and electric heat, heat pumps, combustion safety, venting, airflow, and efficiency. Expect questions on furnace sequence of operation, temperature rise, ignition and flame safety, and heat-pump defrost and auxiliary heat.
Core rules and concepts
- Gas furnace sequence: thermostat calls for heat, inducer starts, pressure switch proves, igniter heats, gas valve opens, flame proves, blower starts after a delay.
- Combustion air and venting must follow the installation manual and code; a blocked flue or heat exchanger can spill carbon monoxide.
- Temperature rise = supply-air temperature minus return-air temperature. Compare it with the nameplate range after steady operation.
- AFUE is the annual fuel utilization efficiency; higher AFUE means less fuel is wasted.
- Electric resistance heat is about 3.41 Btu/h per watt.
- A heat pump moves heat; defrost clears outdoor-coil frost. Auxiliary heat may stage during defrost or cold weather.
- Never jumper a pressure switch, flame rollout switch, or limit control to force operation.
Worked example
A gas furnace has a return-air temperature of 68°F and a supply-air temperature of 120°F. Temperature rise is 120 - 68 = 52°F. The nameplate range is 35°F to 65°F, so 52°F is acceptable. If the measured rise were 75°F, the first check would be airflow: a dirty filter, restricted duct, or wrong blower speed can cause a high rise and overheated heat exchanger. The reasoning is that the furnace adds a fixed amount of heat, so lower airflow produces a larger temperature rise.
Traps
Do not measure temperature rise during the first minute of operation. Do not adjust gas pressure without a manometer and manufacturer's specification. Do not assume a heat pump is broken when auxiliary heat runs during defrost.