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How Does a Gas Furnace Work?

The furnace is one of the most important appliances in the home, yet many people have no idea how it works.

How Does a Furnace Work?

Using either natural gas or propane, the furnace is responsible for heating the cold air that travels through the home, making the house comfortable during the winter months.

A furnace works by utilizing these four central parts:

  • Combustion chamber: As the name implies, heat really begins to develop in the combustion chamber. This area is also commonly called the burner. It is here that the gas/propane mixes with the air and is ignited by the pilot light. There the mixture burns under control as additional air enters the chamber.

  • Heat exchanger:Sitting above the combustion chamber, the heat exchanger absorbs the heat that comes from the burner and uses it to heat the air in the exchanger. Once the air has reached the desired temperature, an electric motor powers the blower fan to disseminate the air throughout the home’s heating ducts, through the registers and into the home.

  • Ignition source:The ignition source is probably the simplest piece of the furnace to understand. When the furnace is required to generate heat, a regulator opens, allowing a small stream of gas or propane to create a flame necessary for the pilot light. Many newer furnaces have moved on from a conventional pilot light to a silicon nitride ignition source otherwise known as a glow stick. In the glow stick model, electricity is fed through the stick — instead of gas or propane.

  • Plenum:The plenum is the final central component. It is also fitting this piece is last, as its role falls at the end of the furnace’s progression. Cold air forced from the living area by the newly created warm air is captured in the plenum. The plenum's exact location in your furnace varies wildly based on make and model, but it is always placed somewhere near the heat exchanger. This close proximity ensures that the cold air captured in the plenum is warmed by activity in the heat exchanger. Once the air is sufficiently warm, it then moves on into the duct systems and out via the heat registers back into the home.

When your furnace activates, the pilot light or glow stick ignites, warming the combustion chamber as fuel and air rush in. This newfound heat warms the air found above in the heat exchanger. Once the air has reached the desired temperature, the electric motor blows the air through the ducts. The warm air flowing through the ducts pushes out the existing cold air, which is captured in the plenum, allowing the cycle to begin again and making your home nice and comfortable.

How is an electric furnace different from a gas furnace?

Many homes these days employ electric furnaces to provide their heat. While the process is very similar cold air drawn in, warm air exhausted there are also subtle differences.

As you might expect, the major difference between a gas and electric furnace is the heat source. Electric furnaces do not rely on combustion chambers and heat exchangers to warm the air. Instead, when the unit is operating, a series of metal coils are heated. Cold air is drawn into the unit via the circulating fan that pulls the air in. This air then travels up through three rows of heated metal coils at the top of the unit before being blown out to be recirculated again. The process is controlled by an electronic thermostat that controls how long the furnace will run and how hot the coils will become in order to achieve the desired temperature.

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