Understanding the Different Parts of a Furnace

A time may come when the majority of U.S. homeowners use heat pumps to heat and cool their homes, but we’re not there yet.

According to Elements, in 2020 more than half of U.S. homes had forced-air furnaces. Of the more than half a million homes built that year, 83% were equipped with furnaces. So chances are good your home probably has a furnace. If so, it’s a vital piece of equipment you need to understand.

First off, you can’t expect something as complex as a furnace to last forever. Says Tim David, owner of Airlucent, an HVAC consultation service: “If your system is nearing 15 years old or older, and you have had to call out an HVAC tech at least once a year for three years in a row, it’s time to replace your system.”

You can extend your furnace’s lifespan with regular maintenance. Our gas furnace is in its 20th year and just got a clean bill of health from our service pro. “Yearly furnace tune-ups are so important to ensure you have a warm, safe and energy-efficient home during the winter,” says John Gabrielli, owner of Delaware-based Air Temp Solutions.

To diagnose problems between tune-ups, you need to know what you’re looking at when you open the furnace door.

What Are the Components of a Furnace, and What Do They Do?

In a furnace, all the heating and air circulation equipment is housed in one structure. Here’s what you’ll find:

Heating unit

This should be one of the first things you see. Electric heating elements resemble those in space heaters, but they’re much larger and often exposed because they release no emissions.

Burners for gas and oil furnaces are enclosed in sealed combustion chambers vented to the outdoors. An oil-burning furnace usually has a single burner, while a gas-powered furnace features two, three or four arranged in a line.

Fuel delivery system

Oil furnaces feature a nozzle that sprays oil into the burner. Gas furnaces come with a gas valve that opens and closes when it receives the appropriate electronic signals.

Heat exchanger

According to David, the heat exchanger is usually a steel or cast iron box that transfers heat from fuel to the air circulating through the space you’re heating. Think of it as the heart of your heating system. Depending on the style of your furnace, David says the heat exchanger could take on one of a number of shapes, “transferring heat in its own unique way.”

Plenum

This is where the ducts connect to distribute warm air to all the heat outlets (aka registers) in the house. You’ll find it above or to the side of the heat exchanger. Dampers inside the plenum can be adjusted to regulate airflow via handles on the outside of the housing.

Blower

Often situated just below the heat exchanger, the blower is a finned cylinder that rotates on a horizontal axis. It circulates the air that collects in the plenum. It also draws fresh air into the furnace through a filter on the side of the housing.

Draft inducer

This is exclusive to gas furnaces, particularly high-efficiency ones. It’s a fan that blows air into the combustion chamber to force exhaust gas out through the vents.

Igniter

Furnaces that burn fuel need an igniter. The spark igniter on oil furnaces and some gas ones typically generates an electronic spark; you can usually hear it clicking when the furnace starts up.

Newer gas furnaces usually have a hot surface igniter instead of a spark one. It glows red-hot rather than generating a spark, and it doesn’t click. Really old gas furnaces may have a standing pilot, a small flame that constantly burns. These waste fuel and are largely obsolete.

Safety sensors and switches

Every furnace has a high-limit cutoff switch which turns off the unit if the internal temperature gets too high. In addition, gas furnaces feature a flame sensor that also shuts everything down if the burners don’t ignite when the gas comes on.

Control board

This is usually at the bottom of the furnace housing, near the blower. It’s a sensitive piece of electronic equipment that sends signals to the various furnace components to turn them on or off. It’s connected to the thermostat in the living space that allows users to regulate the temperature.

Exhaust vent and drainage pipe

Oil and gas furnaces need a vent to dissipate combustion gases outdoors. It’s typically made of galvanized steel and extends through the roof or a wall.

High-efficiency furnaces (aka condenser furnaces) feature a secondary heat exchanger that reduces exhaust gases to acidic water. They still need an exhaust vent, but it’s made from PVC, which acidic gases won’t corrode. These furnaces also have a drainage pipe that extends through the wall, so water (aka condensate) can be transferred outdoors.

What Parts Most Frequently Go Out on a Furnace?

Thermostat

You can trace many furnace problems to a thermostat that isn’t set properly. “A poorly calibrated thermostat can lead to inefficient heating,” Gabrielli says. “Our team checks and recalibrates the thermostat if necessary, so it communicates with the furnace correctly.”

Air filters

If the air filters are dirty, air can’t circulate efficiently inside the furnace housing or the ductwork. This affects system performance and can even cause breakdowns. Filters are easy to replace. Do it monthly to keep your furnace in top form.

Control board

The control board is full of sensitive switches that can stick or fail due to faulty connections or power surges. This is often the cause of electrical problems. Always remember to check the breaker in the main electrical service panel to see if it tripped before suspecting internal furnace problems.

Flame sensor and igniter

The flame sensor and hot surface igniter on a gas furnace can become coated with black carbon deposits and fail to work properly. These are relatively easy parts to clean or, if necessary, replace.

About the Experts

• Tim David owns Alabama-based Airlucent. He has more than 30 years experience in the HVAC industry.
• John Gabrielli owns Delaware-based Air Temp Solutions. He has been involved in the HVAC trades since he was a boy.