Parts of a Home Furnace – Labeled Gas Furnace Diagram

All 18 parts on the diagram

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Air side 6

Return air duct Separate ducts collect cool air from the rooms. They return it to the furnace to be heated again.
Filter A dirty, plugged filter holds the air back. The limit switch may then shut the furnace off.
Blower It draws air in through the filter. It blows the air over the heat exchanger and out to the rooms.
Blower motor Modern furnaces are direct drive: the motor turns the wheel itself. Older ones used a belt.
Heat exchanger The flames and burned gases are inside it. The house air is outside it, so the air is heated without the two ever mixing.
Supply plenum The supply ducts to the rooms branch off from it.

Gas side 7

Vent pipe (flue) On a high-efficiency furnace it is often plastic (PVC) pipe, not metal.
Condensate drain A condensing furnace cools its burned gases until water condenses out. The water has to be drained.
Gas valve It opens only after the igniter is hot. It closes if no flame is sensed.
Manifold It runs from the gas valve along under the burners. It has one outlet for each.
Burners Each burner fires into one passage of the heat exchanger.
Igniter A hot surface igniter glows hot for a few seconds. Then the gas valve opens.
Draft inducer It starts first, before the burners light.

Controls and safety 5

Control board It starts each part in turn when the thermostat calls for heat. It also watches the safety switches.
Flame sensor If no flame is detected, the gas is shut off.
Rollout switch A flame rollout switch opens at a set temperature. It has to be reset by hand. One maker calls it the rollout limit.
Limit switch A blocked filter or a blower that has stopped will trip it. Also called the primary limit or high limit.
Pressure switch Proves the inducer is running before the burners are allowed to light.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two independent sources use it or describe the part: Wikipedia, Furnace (central heating) (text only); the US Department of Energy's Energy Saver pages on furnaces and boilers and on electric resistance heating (read from Internet Archive copies); and four furnace makers' manuals read as full text on archive.org (https://archive.org/details/manualsbase-id-647847, https://archive.org/details/manualsbase-id-619857, https://archive.org/details/manualzilla-id-7409870, https://archive.org/details/manualsbase-id-660800). No maker is named on the page. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; a schematic of no maker's product, nothing traced. Licence: own work

How it works

A home furnace burns gas inside a heat exchanger and blows the air of the house over the outside of it. This parts of a gas furnace diagram has two views: the whole furnace, and its burner compartment.

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Two paths that never mix

Air: cool air comes back through the return duct and the filter, the blower pushes it over the heat exchanger, and it leaves warm through the supply plenum. Gas: the draft inducer starts, the pressure switch proves it, the igniter heats, the gas valve opens and the burners light; the flame sensor proves the flame, and the burned gases go out through the vent pipe. Numbers 1 to 9 are on the whole furnace, 10 to 18 in the burner compartment.

Three switches can stop the burners: the pressure switch, the limit switch and the rollout switch.
Only a high-efficiency (condensing) furnace has a condensate drain; it takes so much heat from the burned gases that water condenses out of them.

Questions and answers

What are the main parts of a gas furnace?

Burners fed by a gas valve, a heat exchanger that the flames heat from inside, a blower that pushes house air over it, a draft inducer and vent pipe for the burned gases, a filter, and a control board with its safety switches.

What does the heat exchanger do in a furnace?

It passes the heat of the flame to the house air while keeping the two apart. The burned gases stay inside its passages and go to the vent; the air you breathe is blown over the outside.

What does the flame sensor do on a furnace?

It proves that the burners have lit. It is a thin rod that sits in the flame. If no flame is detected after the gas valve opens, the gas is shut off and the furnace tries again or locks out.

What is the draft inducer on a furnace?

A small blower on the burned-gas side. It starts before the burners light, pulls the combustion gases through the heat exchanger and pushes them out through the vent pipe. A pressure switch checks that it is running.