For decades, gas furnaces have reigned supreme as the most popular type of heating system in America. While they’re not as common here in Jacksonville as elsewhere, some homes here do have them. If you’re used to electric furnaces and heat pumps, which are more common here, you may know very little about how a gas furnace functions. That could leave you at a disadvantage if your gas furnace malfunctioned. To help, here’s a step-by-step guide to gas furnace operation. We’ll cover the combustion process, your gas furnace’s major components, and some basic safety tips.

Inside Your Gas Furnace’s Combustion Cycle

A gas furnace produces heat via a controlled burn of incoming natural gas. The process begins when your home’s thermostat signals the furnace to turn on. After receiving the signal, the furnace will turn on a small fan, called a draft inducer fan, to begin pulling air into the furnace’s combustion chamber. This provides the oxygen required for the combustion process and creates the airflow necessary to carry the resulting combustion gasses out of your home via your furnace’s flue pipe.

With the air flowing into the combustion chamber, your furnace will activate its igniter. Older furnaces used a pilot light for this purpose, which is a tiny, always-burning flame produced using a small amount of incoming natural gas. Newer furnaces, by contrast, use an electric heating element that uses electric resistance to get hot enough to cause the incoming natural gas to ignite. Once the system’s igniter signals that it’s ready, the furnace will open its gas supply valve, allowing natural gas to enter the combustion chamber and ignite.

Your furnace’s limit switch monitors the temperature inside the combustion chamber. Once the temperature reaches the appropriate operating level, the switch will signal your furnace’s blower motor to turn on. The blower motor draws cold air from your home through the system, where a heat exchanger warms it using the heat coming from the combustion chamber. After passing through the heat exchanger, the fan continues pushing the now-warm air to the vents throughout your home. This cycle continues until your thermostat tells the furnace that your home has reached the desired temperature and instructs it to turn off.

Your Gas Furnace’s Major Components

Although the parts that support the combustion cycle are the most important components of a gas furnace, they’re certainly not alone. Here’s a rundown of all of the major parts of a gas furnace.


Your home’s thermostat continuously measures the air temperature in your home. When it senses that the air temperature drops below the desired level, the thermostat signals the furnace to turn on. Most thermostats have a temperature differential threshold of about .5 degrees Fahrenheit. So, they will instruct your furnace to turn on at one-half degree below your set temperature and keep your furnace running until your home is half a degree above your desired temperature. This helps to prevent unnecessary short-cycling of your furnace.

Draft Inducer Fan

Supplies oxygen to the combustion chamber and moves heat and combustion gasses through the heat exchanger and out of your furnace’s flue.


Your furnace’s burners are a combination of gas-carrying tubes, valves, and a flame sensor. They carry a precise amount of natural gas into the combustion chamber to support the combustion process. The flame sensor’s job is to turn off the gas supply if the incoming natural gas doesn’t ignite. This protects your home from gas leaks and the potential for gas combustion outside of your furnace.

Heat Exchanger

A heat exchanger is like a radiator that absorbs heat from the combustion process and transfers it to the air passing through the furnace. Some high-efficiency furnaces feature multiple heat exchangers to support maximum heat transfer to your home’s air.

Blower Motor

The blower motor is a large fan that circulates air through your home. It draws cold air in through your return air vent, pushes it through your furnace’s heat exchanger, and returns it through your home’s vents.

Air Filter

An air filter removes dust, dirt, and other particulate matter from your home’s air as it passes through the furnace. Most furnaces feature disposable air filters that you must replace approximately every three months.


Your furnace’s flue is a galvanized steel pipe that carries combustion gasses out of your home. Some high-efficiency systems may use a polypropylene flue instead, because of the far lower gas exit temperatures produced.

Basic Gas Furnace Safety

Gas furnaces rely on mature technology and have an excellent safety record. However, they do require regular maintenance to remain safe, and you’ll need to take some basic safety steps when using them. For example, you should always keep the area around your furnace clean and clear of any debris or flammable materials. Although modern furnaces have safety mechanisms to keep any gas from escaping, they also operate hot enough to pose a risk of igniting objects around them.

For the same reason, you should also keep your home’s vents open and unobstructed at all times. If you need to close any vents in your home, try to make sure at least two-thirds of them remain open while your furnace runs. This prevents excessive heat buildup inside your furnace. It also reduces air resistance inside your ducts, which forces the furnace’s blower motor to work harder, shortening its lifespan.

Be sure to replace your gas furnace’s air filter at an appropriate interval. You should schedule a maintenance visit from one of our NATE-certified HVAC technicians at least once per year. They’ll inspect your furnace’s major components, clean and adjust them, and let you know if there are any areas of concern. That way, you’ll have time to take appropriate corrective action before a small furnace problem turns into a safety hazard.

Additionally, you should install a carbon monoxide (CO) detector in your home and test it regularly. This is essential because gas furnaces can develop small leaks that allow carbon monoxide to build up in your home. Carbon monoxide is a colorless, odorless gas that can be fatal to humans in sufficient concentrations.

Your Local Gas Furnace Experts

If your Jacksonville home has a gas furnace or you’d like one installed, J&W Heating and Air + Plumbing is the place to turn. Our team of experienced HVAC technicians works on all makes and models of gas furnaces. We offer complete HVAC installation, maintenance, and repair services, too, regardless of the kind of HVAC you have or want. We also offer duct services, indoor air quality solutions, zone control systems, thermostats, and home energy audits. As a family-owned and operated mainstay in the Jacksonville area since 1966, you can always count on us to provide you with quality workmanship, affordable prices, and 100% satisfaction in everything we do.

For gas furnace installation, maintenance, and repair, call the team at J&W Heating and Air + Plumbing today!

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