A natural gas furnace heats a home by burning natural gas in a combustion chamber, then transferring that heat to air, which is distributed throughout the building via ductwork and vents. It’s typically located in a basement, closet, or utility room, and connected directly to the home’s natural gas line.
Components of a Natural Gas Furnace
- Thermostat: Senses room temperature and signals the furnace to turn on or off.
- Gas Valve: Controls the flow of natural gas into the burner, opening only when it’s safe to do so.
- Ignition Source: Most modern furnaces use a hot surface igniter (an electrically heated element) rather than a standing pilot light, which stays lit constantly and wastes gas.
- Flame Sensor: A safety component that confirms a flame is actually present after ignition. If it doesn’t detect one, it shuts the gas valve within seconds, preventing unburned gas from filling the combustion chamber.
- Burner: Where the gas and air mixture is ignited to produce heat.
- Heat Exchanger: Transfers heat from the combustion process to the air blown over it, without letting the two ever physically mix.
- Blower: A motor-driven fan that pushes heated air through the ductwork.
- Ductwork and Vents: Distribute warm air throughout the building.
How a Natural Gas Furnace Operates
- The thermostat senses the temperature has dropped below the setpoint and signals the furnace to start.
- The gas valve opens, and natural gas mixes with air at a controlled rate before entering the burners.
- The ignition source lights the mixture, creating a flame, combustion, inside the combustion chamber, usually located at the base of the heat exchanger. The flame sensor confirms ignition succeeded.
- Combustion gas is vented safely to the outside through the venting system, while the flame heats the heat exchanger.
- Once the heat exchanger reaches a set temperature, the blower turns on and pushes air across it, picking up heat before sending it through the ductwork and out through the vents.
- Once the room reaches the desired temperature, the thermostat signals the furnace to shut down, the gas valve closes, the burner shuts off, and the blower continues running briefly to pull the remaining heat out of the exchanger before it too shuts off.
Combustion Gas Venting System
The byproduct of combustion, called combustion gas or flue gas, exits the home through the venting system, pipe, fittings, and a vent stack (also called a flue, chimney stack, or smokestack). Standard-efficiency furnaces typically vent through a metal flue using natural draft, while high-efficiency condensing furnaces (90%+ AFUE) use PVC piping and a powered vent, since their exhaust is cool enough not to need a traditional metal chimney.
Why the Heat Exchanger Matters More Than It Looks Like It Should
Combustion gas must never mix with the air a blower sends into the home, that separation is the entire job of the heat exchanger. A cracked or corroded heat exchanger can let combustion gas, including carbon monoxide, leak into the home’s air supply, which is why furnace inspections specifically check the heat exchanger’s condition, and why a working carbon monoxide detector near any gas furnace isn’t optional.
About the Author, James R. Leichter
James is a successful entrepreneur and master mechanic widely referred to as Mr. HVAC. His accomplishments include being the CEO and founder of Aptora, the President of RA Tax and Accounting, the founder of MrHVAC.com, and a partner with Pro-American Investments. He is also the creator of Flat Rate Plus®, which is hosted on this flatratesoftware.com website.
James is well known for his burning passion to help contractors and it shows in his unique speaking style. His articles have been widely published by the industry’s most popular magazines. His videos have had over one million views. James has hosted management workshops all around the United States and has conducted onsite consulting with hundreds of contractors.