How does a gas furnace work?

How does a gas furnace work

How does a gas furnace work and why you should know

Although there are several different types of household heating systems available on the market, gas furnaces tend to be the most popular of them all. Many homeowners rely on these appliances to stay comfortable when the weather gets chilly, but only a small percentage of them would be able to tell you how gas furnaces work. While understanding the ins and outs of a heating system isn’t required for owning one, it certainly has its benefits.

The more you know about the way your appliances operate, the more equipped you’ll be to properly care for them. Understanding your home’s gas furnace could allow you to troubleshoot problems more effectively and get better heating results. Here, we’ll cover the basics of how a gas furnace works and examine the various gas furnace components.

Written by Jeremy James

Jeremy James is a veteran of the U.S. Navy who has grown his family’s HVAC business into one of the most successful home service businesses in Knoxville, TN. More than 20 years of experience in the HVAC and Engineering industries and passion for what we do has translated into our customer service, professionalism, and quality. LinkedIn

How Gas Furnaces Create Heat

As you probably know, the primary function of a gas furnace is to create and distribute heat. When one of these systems is operating properly, it can evenly circulate warm air throughout an entire property. So, how does the heating cycle work?

The Heating Cycle

To put it simply, gas furnaces work by creating heat energy through the process of combustion. Using natural gas as its fuel, the furnace creates combustion in a sealed chamber known as the heat exchanger. As a result, the temperature of the metal heat exchanger rises dramatically. At that point, the exhaust fumes from the combustion process are vented out of your home. Meanwhile, the gas furnace pulls in cold air from around the home and blows it over the heat exchanger, which is still very hot from the combustion of natural gas. That causes the air to become much warmer, and it’s then ready to be circulated to the different rooms of the house through the duct system.

Essentially, that’s the basic heating cycle that your gas furnace uses whenever it’s running. And that process continues to repeat itself until the climate of your home reaches the comfortable temperature that you’ve set on your household thermostat.

Gas Furnace Components

Gas furnaces are relatively complex systems, and it takes quite a few components working together to keep the heating process running smoothly. Listed below are the most significant parts that allow your forced air furnace to do its job.


The thermostat is the electronic device that lets the gas furnace know when it needs to produce warm air. Located remotely, the thermostat measures the temperature of the home and uses that info to regulate the furnace’s heating cycle. You can set the thermostat to your preferred temperature, and it will continue telling the gas furnace to generate heat until that set temperature is reached.

New Honeywell Home Thermostat

Gas Burners

Just like gas-powered cooking grills, gas furnaces utilize burners to ignite their fuel. These burners consist of small tubes through which natural gas is directed and burned. When the furnace requires heat, the valves on the burners open, and the fuel is ignited.

Ignition Switch

Today’s forced air furnaces do not have a pilot light. The ignition switch of a gas furnace is the device responsible for igniting the gas that the burners emit. The hot surface ignitor works either by creating a spark or producing an extremely hot surface to ignite the fuel.

Gas Valve

Another important part of the furnace’s combustion process is the gas valve. The gas valve has the important duty of regulating the pressure of the gas as it enters the furnace. Typically, this gas will be coming directly from your home’s natural gas line. Gas furnaces may also use liquid propane gas as a fuel source. In this case, a special kit will be needed to lower the pressure of the incoming gas. Liquid propane gas is stored in an outdoor storage tank somewhere near the home.

Heat Exchanger

The aforementioned heat exchanger is a series of metal tubes that are integral to the heating process. To create warm air for your home, the heat exchanger is heated up by the burning fuel of the gas burners. Cold air passes through the furnace the heat exchanger transfers heat to the air. As for the exhaust fumes, they’re vented from the combustion chamber and out of the house. If the heat exchanger ever becomes damaged, these dangerous exhaust fumes can leak into your home. For this reason, you should never let your gas furnace continue running if it has a cracked heat exchanger.

Blower Fan

When the cold air from your home is pulled through the return ducts, the blower fan is responsible for directing that air where it needs to go. First, it pushes the cool air over the hot heat exchanger so that the heat energy can be transferred. Then, once the cold air has been heated, the blower fan directs that air through the air ducts so that it can reach the different parts of your home. Many high-efficiency furnaces will come equipped with a variable speed blower fan, which can self-adjust its fan speed as necessary during operation. These variable speed blowers tend to be more efficient, quieter, and more effective at providing consistent warmth.


When a gas furnace is burning fuel, the exhaust gases from the combustion process have to go somewhere. The flue is an exhaust pipe that’s tasked with collecting those combustion fumes and removing them from the home. With standard furnaces, the flue is typically made from galvanized steel, whereas high-efficiency models tend to use flues made from polypropylene.

The flue pipe on a furnace

Flame Detector

On a furnace, the flame detector is a device primarily intended as a safety measure. Specifically, it’s a component designed to prevent excessive amounts of gas from accumulating should the igniter fail. If the flame detector doesn’t detect heat when it should, it will stop the gas flow before it can pose a danger to the residents.

Control Board

Every gas furnace has a control board inside of it designed to interpret electrical signals coming from both inside and outside the system. In response to these messages, it will send signals of its own, which cause various processes to take place. For example, when the thermostat sends the signal to produce heated air, the control board will instruct the gas valves to open and the burners to ignite.

Manometer checking pressure in the furnace

Air Filter

All gas furnaces require air filters for trapping airborne contaminants. If too much dust, dirt, and small debris get cycled into the furnace, it can hamper its efficiency and lead to poor indoor air quality. The air filter catches the majority of these contaminant particles before they can get into the system and cause problems. Eventually, this air filter will become very dirty, and if it’s not addressed, it can end up restricting the airflow of the furnace. In some cases, clogged air filters can even lead to carbon monoxide leaks. So, it’s very important to regularly replace this filter every few months at a minimum.

What Are the Advantages of a Gas Furnace?

Just like all household appliances, natural gas furnaces offer advantages and disadvantages. When compared to electric furnaces, one benefit of using a furnace is that it can warm up your home very quickly. The combustion process takes place very quickly, and once you’ve locked in the thermostat setting, it won’t take long at all before warm air fills your living space.

Another nice thing about gas furnaces is that they aren’t affected by cold outdoor temperatures. That isn’t the case with all heating systems. For example, instead of producing their heat, heat pumps provide warmth by pulling it from the air outside. So, if it’s freezing outside, it’s much more difficult to bring in a sufficient amount of heat for consistent comfort. With a furnace, though, you’ll have access to plenty of warmed air regardless of how chilly the weather gets.

In many cases, gas furnaces are also more energy-efficient than electric furnaces. Usually, natural gas is a less expensive fuel source than electricity, which means that your monthly utility bills will typically be more affordable if you’re running a gas-powered furnace.

Finally, there’s the fact that gas furnaces are widely available. Because they’re such a popular choice for households, HVAC dealers will often have a wide variety of makes, models, and sizes available when you’re in the market for a new furnace. So, not only will you have more choices, but it will also be easier to find a unit that’s the right fit for your home.

New furnace, evaporator coil, and flue

What Are the Disadvantages of a Gas Furnace?

Ultimately, gas-powered furnaces are the most popular type of heating system for a reason. However, that doesn’t mean they don’t come with any downsides. For one, when you choose to invest in a new furnace, it will be pretty expensive to install. Typically, having a new furnace set up by professionals will cost more than the installation of an electric furnace. However, there’s a good chance you’ll be able to recoup that investment over time through energy savings.

Another disadvantage of gas furnaces is that they usually don’t last as long as electric furnaces. Granted, any furnace will stay productive for longer if you take proper care of it and keep up with maintenance, but gas-powered furnaces rarely run for longer than 15 years. Furnaces that use electricity to produce heated air, on the other hand, can often run for as long as 20 or 30 years.

It’s also worth noting that if a furnace is installed improperly or neglected, it can present a serious safety hazard to you and your family. Because of the way a gas furnace works, the release of carbon monoxide and exhaust fumes is necessary. When certain things go wrong with the equipment, there’s the potential for these harmful gases to be released into your home, which can be quite dangerous.

For this reason, you should only trust reputable HVAC professionals to install and work on your furnace. You should also make sure to always have a functioning carbon monoxide detector in your household.