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How a Gas Furnace Heats Your Home

How a Gas Furnace Heats Your Home

When you adjust your thermostat, your furnace kicks on and your house warms up. The entire process behind how and why this happens is often ignored. Ultimately, this can lead to a reduction in your unit’s effectiveness and efficiency. Discover how a gas furnace heats your home, the major components in the furnace, and the differences that exist between models.

Generating Heat

What sets furnaces apart from other heating devices is how they generate heat. In most cases, the furnace will burn fuel to create heat, usually either natural gas or propane. In some cases, you may have an electric furnace, which uses electrical resistance to create heat.

For fuel-burning furnaces, the heat that’s created comes in the form of the exhaust that’s produced when the fuel burns. The furnace then has to transfer that heat from the exhaust to the circulating air without mixing toxic fumes into your home.

Transferring Heat to the Air

Once the furnace produces heat, it flows into the heat exchanger, which transfers the heat. The hot exhaust flows inside the tubes of the heat exchanger, which absorbs the heat, warming the outside of those tubes.

The air moving through the furnace then flows over the outside of the heat exchanger, absorbing the heat. The warmed air then moves back into your home. As long as the heat exchanger isn’t cracked or damaged, the exhaust never mixes with the air circulating through the furnace.

Distributing Heat Around Your Home

Once the warm air flows out through the supply vents, it has to circulate around your home to be distributed evenly. The system creates both positive and negative air pressure to create circulation in your home. The positive pressure occurs at your supply vents, while the negative pressure occurs at the return vents. It’s this circulation that then causes the warm air to distribute instead of staying at the location of the supply vents.

Critical Parts of a Furnace

Now that you understand how a furnace works, it’s time to explore the components that make it happen. This will help you identify when you have a problem, what it could be, and the possible solution to solve it.


Most people recognize this as the device that’s mounted to the wall where you set your temperature. It acts as the brain for the furnace, signaling when to initiate and terminate heating cycles.

Control Board

While the thermostat is the brain of the furnace, the control board directs what happens and when. This takes all of the inputs as the furnace cycles from one phase of heating to another. For instance, it’s the control board that monitors the heat produced and starts the circulating fan when enough heat has built up.

Draft Inducer Fan

For the furnace to work efficiently, there has to be constant air movement through the burn chamber and out through the flue. This provides the right mixture of air for the fuel to burn efficiently, along with moving the exhaust out. While this becomes a sustainable cycle once the system heats up, it needs a little help while warming up. The draft inducer fan provides that assistance at the beginning of each cycle.


Furnace burners aren’t that different from gas grill burners. They provide a place where the gas flows out and burns safely, producing the hot exhaust, which will then flow through the rest of the system properly.


The furnace has to ignite the fuel at the beginning of each heating cycle, and so it has an igniter of some form. Older models had a standing pilot light, which was a small flame that constantly burned. However, newer models have primarily gone to an electric igniter so that there isn’t a constant use of fuel for a pilot.

Heat Exchanger

The heat exchanger is a series of tubes that conduct the hot exhaust from the burn chamber to the exhaust flue. The exhaust heats the outside of the exchanger, and the circulating air then flows over the outside of those tubes, absorbing the heat.

Circulating Fan

The circulating fan does exactly what it says it does: circulates air. It draws air in through the cool air return, then pushes it back out through the supply vents. It’s comprised of a circulating fan wheel and the circulating fan motor, also called the blower motor.


The flue is the pipe that directs the exhaust from the heat exchanger to vent outside your home. You’ll see it as a pipe that comes off the top of your furnace. From outside, you’ll see it come through the roof, usually with a crown or cover over it. If you have a high-efficiency furnace, your flue may come through the side of your home rather than through the roof.

Upper Limit Switch

Since your furnace is burning fuel to create heat, it’s important that the temperature is properly controlled. An overheated furnace can cause a cracked heat exchanger, which will allow carbon monoxide to leak out. The upper limit switch monitors the temperature in the supply plenum and shuts down the furnace if it gets too hot.

Flame Sensor

The flame sensor resides inside the burn chamber and is responsible for ensuring that there’s sufficient heat to ignite the fuel. This sensor gets dirty over time and will need routine cleaning to continue operating properly.

Air Filter

An air filter is one of the most critical components for keeping your HVAC system working effectively. It keeps airborne contaminants out of the system so that they don’t clog up the internal components. However, a neglected air filter will cause an airflow restriction that hinders the efficient operation of the system.

Blower Motor Capacitor

The blower motor requires a large electrical charge to start working at the beginning of the heating cycle. To prevent an instantaneous large draw on your home’s electrical system, the furnace has a capacitor to deliver the charge. This is a special electronic device that stores and quickly releases a large electric charge.

Differences in Furnace Models

There are several types of furnaces, each with differing efficiency ratings. Standard models have a single-stage burner, a fan, and only one heat exchanger. Higher-efficiency models have a modulated burner, which changes how much gas it’s burning. They’ll also have a variable-speed fan to adjust how much air it’s circulating. Finally, if you have a high-efficiency condensing furnace, you’ll have a second heat exchanger to draw even more heat from the exhaust.

Crystal Blue Plumbing, Heating & Air has been a trusted service provider around Loomis since 1976. Our team takes pride in providing AC and heating installation, maintenance, and repair, together with duct repair and replacement, indoor air quality solutions, and a wide range of residential plumbing services. Call us today to schedule your furnace repair or replacement consultation with one of our experienced NATE-certified technicians.