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Gas Furnace Components You Should Know

furnace anatomy

You know your furnace keeps you warm when outdoor temperatures drop. Do you know how your furnace operates? Most homeowners may not be familiar with furnace anatomy or how it works. There are many gas furnace components that work together to deliver heat to your living areas. Understanding furnace anatomy may not help you fix it on a day it is not working, but it will help you make an informed decision on what is best for your home when faced with repair vs replacement. In this article we discuss the different gas components to help you become more familiar with your furnace.

Return Register

A return register may look like your other air vents but is typically larger in size than the others. They are usually located in a hallway ceiling. It is the part of your HVAC system where air returns to your furnace to be heated and how air is recycled in your home via the return duct.

Air Filters

We often discuss air filters, although a small part of your HVAC system they have a huge impact. When air is brought in through the return register it passes through the air filter. The air filter is responsible for catching dust and other contaminants before they reach your HVAC system and being redistributed through your home. Check your filter often and replace as needed.


The blower is a large component that blows air through your HVAC system and into your home through the supply ductwork. They can be electric or powered by a belt, often times if you hear a loud noise coming from the area it is due to a blower malfunction.

Blower Motor

The blower and the fan in the blower are powered by the blower motor. The blower motor capacitor starts up the blower motor and keeps it running. Without the blower motor, the air would not make it far out of your system. Both the blower and blower motor are housed in the blower chamber.


The burners are where the fire comes from. The burners are responsible mixing gas and air to make the flame the furnaces heat source. They pull gas from a gas line to make fire once mixed with oxygen. They usually consist of the flame sensor, as well as pilot light or electric ignition to start the fire.

Flame Sensor

The flame sensor is a safety device that detects heat from the hot surface ignitor. It will do nothing if everything is working properly, but it will shut the system down if no heat is detected to avoid any potential gas leaks.

Pilot Light/Electric Ignition

In older furnaces the pilot light is a small flame that burns constantly ready to ignite the gas when the furnace is turned on to start the heating process. Whereas newer furnaces will have an electronic ignition. With an electronic ignition your system will send a signal to the electronic ignition and in turn the electronic ignition will send a spark to the burner.

Gas Valve

The gas valve adjusts the pressure of the natural gas coming into your home. It is also responsible for controlling the gas to the furnace.

Burner Cover

A burner cover is the component that sits over the burners, flame sensor, blower chamber and electronic ignition or pilot light. It is a safety device that keeps the flames inside the furnace.

Heat Exchanger

The heat exchanger is the component that heats the air, the core of your gas furnace. Natural gases and fire are burned together to create heat, but the mixture also generates what are called combustion gasses such as carbon monoxide. The heat exchanger is responsible for keeping these toxic fumes separate from the air distributed throughout your home. A heat exchanger should be inspected on a regular basis as a cracked heat exchanger is severe safety hazard.  

Pressure Switch

The pressure switch ensures the draft inducer motor is on and pushes the harmful gases out the flue pipe. It uses a small diaphragm that pulls in when the inducer motor is on, resulting in a vacuum. If a vacuum is present and the diaphragm can pull it, a switch is activated that allows gas to flow. If the vacuum is not sensed, no gas will be allowed to enter the furnace.

Limit Switch

The limit switch detects the furnace temperature. If the furnace is too hot, the limit switch will turn off the gas to prevent any safety issues.

Supply Plenum, Duct and Register

The supply plenum is the gas furnace component that sits above your furnace and receives heat treated air from the heat exchanger and then prepares to send it back to your home through the supply duct and supply registers.