How Does a Furnace Work? Components, Functions, and More

Ben Kissam
Written by Ben Kissam
Updated February 2, 2022
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Photo: FreshSplash / Getty Images


  • Furnaces use natural gas, electricity, or propane to heat your home.

  • The thermostat controls a furnace’s heating cycle and can become less efficient over time.

  • Furnace repairs can be expensive, and sometimes it’s simpler just to replace an item.

  • Changing filters and other regular maintenance can preserve the life of your furnace.

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Your home’s heating system accounts for around 29% of your energy bill, according to The Department of Energy. But how exactly does a furnace work? What has to happen inside the unit to heat your home? And which parts make up your furnace to circulate warm air in your home?

Learn everything you need to know about how home furnaces work in this informational guide.

How Does a Home Furnace Work?

A home furnace uses natural gas or propane to create heat that flows through your ductwork to warm your home. Furnaces are typically located in a central area like the basement, attic, or utility closet. (Although some homeowners choose to build a separate furnace room.)

Today's furnaces can also run off electricity. Depending on the type of fuel source a home furnace uses, you may pay more or less in energy costs. Gas is most popular, thanks to its quiet heating abilities and cost-effectiveness. With gas furnaces, heat is channeled through a flue pipe.

How Does the Heating Cycle In a Furnace Work?

The sequence of operations used to create heat from a natural fuel source is known as the “heating cycle,” controlled by your thermostat. A heating cycle lasts a predetermined time to generate heat efficiently and then shuts off.

Basically, gas is ignited in the burner. The heat exchanger warms and transfers heat into the incoming air. Once warmed, the air is blown into your ductwork and circulated to rooms in your home.

Anatomy of a Furnace

A home furnace has many parts, all of which are vital to a functioning furnace.


The thermostat connects to the furnace’s damper, which controls the amount of hot generated then channeled into your home during a cycle. Many models today can be controlled through an app on your phone.

Air Filter

As is the case with many appliances, your furnace’s air filter is the first point of contact for cool air destined to be warmed. The filter helps take pollutants out of the air, which keeps your air quality high and also prevents your system from being damaged by debris.

Gas Valve

The gas valve controls how much fuel pumps into your furnace. Most units have two valves—a primary valve on the outside and a main valve on the inside—that control gas flow to the burners. In most cases, due to cost, it’s simply cheaper to replace an entire valve than have it repaired.

Nuances like these are why only a trusted HVAC technician should service and install your furnace if there’s an issue.


Furnace burners mix fuel with air to burn it, which creates heat. The more efficient your burners are, the lower (potentially) your energy costs. Newer furnaces tend to be more efficient in this regard.

Heat Exchanger

The heat exchanger sits between the blower and the combustion chamber, which helps separate flue gasses from clean, warm air channeled to your home. 

Blower Motor

A blower motor is a fan that pumps hot air from your furnace through your home’s ductwork and out vents. This is the engine, so to speak, that ensures heat gets dirtiest throughout your house or to the specific rooms or areas you've selected.


The ducts in your walls and floor help circulate air through your home. They’re generally made of metal or plastic, and attached to vents at the base or towards the ceiling in a room. Homeowners can control the amount of heat flowing into a room by opening or shutting the vents.

How to Improve Furnace Efficiency (And Preserve Your Furnace Lifespan)

Here are five tips to help keep your furnace functioning efficiently to help save you money on heating costs and avoid costly repairs.

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Photo: Zinkevych / Getty Images

1. Set Your Thermostat at One Temperature

The more you fiddle with your thermostat, the harder a furnace must work to meet your demands. Frequent tampering with the thermostat can lead to wear and tear issues, as well as higher energy bills.

2. Change Air Filters Regularly

Clean filters help keep debris and pollutants out of your machine. They also prevent obstructions, making the heating process more difficult or time-consuming.

3. Have Your Furnace Serviced Regularly

A furnace repair technician can help fix issues when they crop up. But it’s also recommended for most models to have their furnace serviced annually before cold weather crops up. Small tune-ups can help prevent big issues from forming over time.

4. Clean Your Ductwork

Similar to filters, making sure you regularly clean your ducts prevents dust and debris from making your furnace work harder than it has to. Vacuuming around the edges and having your ducts cleaned by a pro every year or two is definitely recommended.

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