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How a Gas Forced-Air Furnace Works

Heating season will be here before you know it. To get ready, we thought we would review how a furnace works so you’ll be up to speed when our Service Technician services your HVAC system.

We’re still (at least) a few weeks away from the first real cold snap of the fall, but you’ll be glad you serviced your furnace early before the first snow flies.

A greater understanding of how your furnace works could be the difference of getting your furnace running quickly by communicating the issues with our Service Tech or having it not working on the coldest night of the upcoming winter season!

The Heating Cycle

A forced-air furnace heats your home with the following typical heating cycle:

  1. Natural gas or propane is fed to the heating unit, and ignited in the burner, using an electronic spark or on older units, a pilot light.
  2. The resulting flames in the burner cause the metal heat exchanger to heat up, and the burned gasses are exhausted out of the flue.
  3. The heat exchanger, just like its name suggests, transfers the resulting heat to the house side of the system and heats incoming hear.
  4. The blower motor in the furnace spins a fan and forces the heated air into the ductwork, distributing the air throughout the home.
  5. As the heated air is forced into each room through the ductwork, the cooler, denser air in those rooms is drawn into the furnace via the return air ducts, and the process repeats again.

Components of the Gas Furnace

Let’s review some of the major components of a gas furnace and the functions they perform.

  • Thermostat: This is the device that controls the temperature in your home when all things are working correcting. Essentially a switch, activated by temperature change,
  • Draft Hood and Fan: Higher efficiency model furnaces contain a draft-induced fan that makes combustion leaner and much more fuel efficient. This fan also moves the heat from the burners into the heat exchanger, and exhausts the combusted air safely outside the home.
  • Burners: Furnace burners are tubes in the heart of a furnace through which gas flows and burns. The flames are controlled by a gas valve, an igniter, and a flame sensor, all working together. The gas valve opens when heat is ‘requested’ by the system, and the mixture is ignited. If a flame isn’t detected by the flame sensor, this built-in failsafe shuts off the incoming flow of gas to prevent a potentially catastrophic accident.
  • Heat Exchanger: Above the burners is a matrix of metal tubes that receive the heat from the burners. The heat from the warmed up tubes gets transferred through metal baffles and transfer their radiant heat into the air passing through. Damage, in the form of a crack, to the heat exchanger could allow dangerous combustion gases to leak into the house. (This is why it’s crucial to have your furnace checked annually by a qualified HVAC technician). High-efficiency furnaces may include additional heat exchangers designed to transfer the heat more efficiently.
  • Blower Motor and Fan: near the bottom of the furnace is a blower motor that spins a squirrel-cage style fan to force heated air up and out into the ductwork of the house. Some modern furnaces have blower than runs at multiple speeds. Higher efficiency furnaces include a variable speed blower than can automatically adjust its speed while it’s running to achieve the most efficiency.
  • Flue: Just like in a fireplace, the flue collects combustion gases that are a byproduct of producing heat, and carries them outside the home. On standard efficiency furnaces, the flue and exhaust ductwork is made of galvanized steel. Newer, high-efficiency furnaces, which exhaust much cooler air, use polypropylene venting to vent the exhaust gases outside.

How Can This Information Help?

Our goal with this article is to help you better understand the fairly complex system that heats your home. This can help our service technician communicate better with homeowners when explaining a needed repair.

This information can also help homeowners decide which features are important and should be included in their replacement furnace purchase (when it’s needed).

Dor-Mar to the Rescue!

Schedule an appointment for HVAC system troubleshooting or maintenance with us today by using our online contact form, or call one of our seven neighborhood offices listed below. We pride ourselves on our customer-focused service, and our reviews show it.

NEWARK  740.345.6639 • COLUMBUS 614.238.6689 • DUBLIN 614.545.8939 • REYNOLDSBURG 614.365.1579 • WESTERVILLE 614.381.1540 • GROVE CITY 614.595.3098 • ZANESVILLE 740.454.2420

Dor-Mar…Your Climate Hero!

*Please note: neither the Centers for Disease Control(CDC), the World Health Organization (WHO), nor the manufacturer of our UV-C Devices have officially announced whether UV-C kills the current strain of Corona Virus (COVID-19). We urge you to err on the side of caution until we have a definitive answer, and take other measures to prevent the acquisition or spread of this virus.


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PLEASE NOTE: If you are a renter, please have your landlord/owner contact us directly to schedule service. We can only work with property owners.


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