Anatomy of a Forced Air Furnace

When furnace problems arise, and it’s time for a heater repair on your forced air furnace, it helps to know what parts you are looking at. The major components involve the plenum and the furnace itself. When diving in to your home furnace, you find the many components that your HVAC guy is going to talk about. If you are interested in how to diagnose a furnace yourself, check out my Furnace Problems – Diagnosing a Furnace article for a more in-depth how-to. Moving on to the components, Let’s have a look at my diagram:


Heat Exchanger

The component which return air converts to heated supply air over specially shaped baffles. This turns cold air into hot air.

Burner Assembly

The burner assy is responsible for the combustion of natural gas or propane. This is where the flame sensing rod and igniter are housed.

Gas Valve

This is the component which regulates whether gas enters the burner assy. This is either an on or off component.

Inducer Motor

This is responsible for purging air from the combustion which includes carbon monoxide. This component also creates a slight vacuum which is present through the vacuum hose to the pressure switch.


This ignites the natural gas creating the flame. This can either be a spark igniter or a silicon carbide wafer.

Flame Sensing Rod

This is the component which generates millivolt voltage to return back to the control board to give a flame proving signal. This is a safety mechanism built in designed to cut of gas when there is no flame.

Pressure Switch

The pressure switch closes the limit chain where any open limit switch or pressure switch turns off the gas valve. The pressure to activate this switch is from the vacuum created from the inducer motor. This proves that the inducer is purging the combustion air out the vent flue.

Limit Switch

Limit switches are heat responsive, meaning if the temperature in the vent flue, the fire box, or the burner assy is higher than its design temperature, the gas valve is cut and the blower kicks on. The Limit switch is a safety device which prevents the unit from getting out of control.

Furnace Blower Motor

The furnace blower motor turns the blower wheel. This is powered by line voltage and is usually accompanied by a fan capacitor. This can either be a multispeed motor or a high-efficiency ECM motor. The construction of this motor can make a huge difference in efficiency.

Furnace Blower Wheel

The furnace blower wheel, or blower, takes the return air and blows it across the heat exchanger and or refrigeration coils to produce either a cool or hot air effect. In this application, it is hot air.

IFC – Control Board


Also, if you need replacement parts, you can message me on the website messaging board with your model number and serial number. I can ship parts anywhere in the country. I do mark up my parts, but you get a whole better deal with me than with your local HVAC contractor. I also do facetime to help you install the component and return the unit back to working order for a small charge. Message me if you need help, that’s what I’m here for.