Furnace glossary

Furnace & heating glossary

The furnace-specific parts and controls, defined in plain English. For cross-system terms like AFUE, BTU, and load calculation, see the master glossary.

BTU input vs. output

Input is the fuel a furnace consumes; output is the heat it actually delivers, which is input times AFUE. An 80,000 BTU input furnace at 95% AFUE outputs about 76,000 BTU.

Flame sensor

A metal rod that confirms the burner actually lit; if it can't sense flame, it shuts the gas off for safety within seconds. A dirty flame sensor makes a furnace light then quit.

Heat exchanger

The metal chamber that separates the burning gas from your breathing air while passing heat across. A cracked heat exchanger can leak carbon monoxide, which is why it's a serious safety item.

Hot surface igniter

The electronic element that glows hot to light the burner in most modern furnaces — the replacement for a standing pilot light. A cracked igniter is a common no-heat cause.

Inducer motor

The small fan that pulls combustion gases through the heat exchanger and out the flue before the burner fires. Its job is proven by the pressure switch.

Limit switch

A safety that shuts the burner off if the furnace overheats, usually from restricted airflow like a clogged filter. Repeated trips point to an airflow problem, not a bad switch.

Thermocouple

On older standing-pilot furnaces, the sensor that proves the pilot flame is lit and keeps the gas valve open. When it fails, the pilot won't stay lit — see the pilot light guide.

Two-stage & modulating

Furnaces that can run at reduced output instead of only full blast — two-stage has a high and low setting, modulating varies continuously. Both give steadier heat and better efficiency than single-stage.