How Does a Furnace Work?

Ever wondered how a furnace works? We cover it all, from the heating cycle to furnace safety.
Written by Kathryn Mae Kurlychek
Reviewed by Melanie Reiff
No matter what type of furnace you own, the appliance is pretty simple. A burner ignites and warms the air in the appliance, which is then pushed out through a venter motor to circulate and heat your home.
When we’re hot or cold, we may add or shed a layer or two to adjust—or we may turn to our thermostats and change the temperature of our home. In the winter months, you’ll probably set yours around 70 degrees, but what happens after you’ve pressed the buttons and walked away?
A key component of your HVAC system, a furnace works to circulate heat throughout your home. Exactly how it’s done depends on a variety of factors, including the type of furnace you have.
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How does a furnace work?

Understanding the heating cycle of your furnace can do more than simply help you choose the best one for your needs—you’ll also stay better prepared to handle problems as they arise over the years, keeping your home toasty warm in the winter months for seasons to come. 
Let’s dive in with an overview of how a furnace works:
  1. The heating process always begins with your thermostat. When you adjust the thermostat, there’s a disparity between your desired temperature and the current temperature of your home, so your thermostat will signal to your furnace that it’s time to start producing more heat. 
  2. Once your furnace receives the signal, it will ignite and begin to heat up. Exactly how heat is exchanged here depends on the type of furnace you have. Older models typically have a pilot light, which is the initial flame that sparks its main burner. More modern models usually feature either hot surface ignitors or heat coils
  3. As the air is heated up, a venter motor will begin to turn, pushing the hot air through the vents and ductwork to circulate throughout your home. 
  4. As the rooms in your home begin to heat up, dense cold air will sink into the return ducts, circulating back to your furnace—and so the process repeats. 
Once your home reaches the preset temperature of your thermostat, the furnace will receive a signal to shut back off until the next time it calls for heat. 
Of course, variation occurs depending on the type of furnace you own—particularly when it comes to how the initial burner is lit and the fuel your furnace needs to heat up. Furnaces may also differ in their energy output and efficiency, which we’ll discuss below. 

How furnace energy efficiency is measured (AFUE) 

When it comes to heating your home, no furnace is 100 percent effective. Even the newest state-of-the-art models are only ever about 98% effective—and that’s the very top of the line. What happens to the remaining 2%?
Inevitably (and unfortunately), some energy winds up escaping from vents and cracks in your home. 
You can estimate exactly how efficient your furnace will be using its Annual Fuel Utilization Efficiency, or AFUE. An AFUE calculation helps determine how much of the heat produced by your furnace actually goes into heating your home—and what percentage slips through the cracks (literally). 
The average AFUE rating for standard-model furnaces is around 80%—meaning 20 cents of every dollar you spend on your average energy bill go toward wasted heat. 

The anatomy of a furnace

Your furnace is a pre-assembled unit of parts—and how a furnace works depends on the individual functions of each part. Let’s take a closer look at some of the most important parts:
  • Thermostat: Your thermostat helps regulate the temperature inside your home year-round. Turning it up signals to your furnace that it’s time to warm up. 
  • Ignitor: Once the signal from your thermostat has been received, the ignitor creates the initial spark needed to heat the air inside your furnace. 
  • Combustion chamber: This part of your furnace is where your air and fuel mixture ignites to create the heat. 
  • Heat exchanger: Heat is pushed from the combustion chamber into a series of metal coils known as the heat exchanger, which do the primary work of heating the air to be circulated throughout your home.
  • Venter motor: Also called a blower motor or circulating fan, this component forces the warm air from the heat exchangers up into your home’s vents and ductwork, distributing heat throughout your home.
  • Filter: This piece helps to protect your venter motor from any pollutants in the air (dirt, dust, pet hair, etc.) that come back into your furnace through the return ducts. Regular filter maintenance is key to keeping your furnace healthy!

Types of furnaces

Now you know the basic parts of your furnace and how they work together to heat your home—but do you know what type of furnace you have? 
The most popular furnace models tend to be gas furnaces, which use natural gas for ignition. Other furnaces may use oil, propane, or electric fuel sources to run. Here’s a brief overview of each:
Fuel type
Ignition process
Average cost
Average efficiency
Average lifespan
Natural gas
Gas is piped in from storage tank or line
89% to 98% AFUE rating
15 to 20 years if properly maintained
Oil is drawn in from the fuel pump through a filter to the burner, where it’s administered as a fine mist or spray
80% to 90% AFUE rating
Up to 25 years if properly maintained
Liquid propane is converted to gas and mixes with air inside the combustion chamber
55% to 65% AFUE rating
15 to 20 years if properly maintained
Air is heated by electric heating elements in the heat exchanger
100% AFUE rating
10 to 15 years (although they’re predicted to last 20 to 30 years if well maintained)
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Single-stage, two-stage, or variable-speed furnaces 

Furnaces may also have different settings to help better control the heating process in your home, including when and how much heat is distributed. 
A single-stage furnace will have essentially one setting—and that’s on. If it’s not giving 100%, it’s not giving anything at all. 
On the other hand, two-stage and variable-speed furnaces give homeowners greater control over the energy output of their furnaces, which can help reduce energy usage and maintain a more comfortable home temperature. 

Furnace safety

There are tons of measures you can take as a homeowner to help maintain your furnace’s health—but unless you’ve got some serious experience working with HVAC systems, when problems arise with your furnace it’s best to defer to a professional. 
That’s because furnaces can be dangerous to work with, especially if you’re unfamiliar with them! To be sure, modern systems include built-in safety features like rollout switches and heat sensors that act as fail-safes, but here are some extra measures you can take to improve your furnace safety at home:
  • Vacuum around your furnace to keep dust, lint, and flammable hazards to a minimum
  • Change your filter regularly and schedule yearly inspections to keep your furnace in top shape
  • Keep vents uncovered to improve airflow. This reduces the amount of work your furnace has to do to pull air in from the ducts
  • Keep flammable items far away from your furnace at all times
  • Install smoke alarms and/or carbon monoxide filters in your basement as an early-warning system for furnace problems 

Does homeowners insurance cover furnaces?

If you’re facing the cost of a new furnace, you’re probably asking yourself: will homeowners insurance cover any of this? The answer depends on the type of coverage you have and what caused the damage to your old furnace. 
Fires, explosions, or damage caused by a natural disaster are typically among the covered perils listed on your policy. These are all of the events that your insurance policy will cover. 
On the other hand, you probably won’t be covered if your furnace has broken down due to plain aging or wear and tear. 

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8-10 years is the average furnace lifespan. Technically speaking, your furnace should last at least 15 years or more, but to get it to do so you’ll need to stay proactive with maintenance right from the beginning (and that includes hiring a company that follows best installation practices).
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