Parts glossary
Gas furnace parts, explained
What each part inside a furnace does, how it fails, and which fault codes point at it. This site does not sell parts, and nothing here is a do-it-yourself repair instruction: work inside a gas appliance belongs to licensed technicians.
Parts on this site
- PartHot-surface ignitorThe hot-surface ignitor is the ceramic heating element that glows to light the gas on modern furnaces, replacing the standing pilot. It heats on every heating cycle and is a wear item: cracks, electrical failure and contamination end its life, usually showing up as an ignition code.
- PartFlame sensorThe flame sensor is a small metal rod that proves the burner is actually burning before the gas valve is allowed to stay open. It conducts a tiny rectified current through the flame; when it fouls or fails, the furnace lights and then shuts off seconds later, repeatedly.
- PartPressure switchThe pressure switch proves the inducer is moving combustion air before the control allows gas to flow. Two-stage furnaces have two switches, one for each firing stage. A pressure-switch code usually points at venting, condensate or the inducer rather than at the switch itself.
- PartInducer motorThe inducer is the small blower that starts before ignition to pull combustion air through the burner and heat exchanger and push flue gas out the vent. When it fails or slows, the pressure switch cannot close and the furnace will not light.
- PartBlower motor (ECM)The blower motor moves air across the heat exchanger and through the ducts in heating, cooling and continuous-fan modes. Modern furnaces use electronically commutated (ECM) motors; failures show as no airflow, weak airflow, noise, or RPM codes on the control.
- PartControl boardThe control board sequences every furnace operation: it runs the inducer, ignitor, gas valve and blower, monitors the safety switches, stores fault history and shows status codes. Board failures are real but are usually diagnosed after the parts it controls are ruled out.
- PartLimit switchThe limit switch stops the burner when the heat exchanger area gets too hot, protecting the furnace from airflow failures. A limit that keeps opening means the furnace is overheating — an airflow problem, not a switch problem.
- PartGas valveThe gas valve admits gas to the burners only when the control commands it after ignition conditions are met. Gas-valve faults are serious by nature: diagnosis checks that the valve opens when commanded, closes when not, and delivers the right pressure.
Planned parts
Rollout switch · Transformer · Capacitor · Thermostat