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How Your Furnace Works | An Inside Look

How Your Furnace Works | An Inside Look

February 9, 2018

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The furnace is the heart of your central HVAC system. It is important to understand how your furnace works so you can recognize any issues.

How Your Furnace Works

When heat is called for, the thermostat sends the signal to your furnace to turn on. Air passes through the air filter and gets pushed to the heat exchanger by the blower. The heat exchanger heats up the air, which then gets pushed through the ductwork.

In the winter, the furnace forces warm air throughout the home; in the summer, the furnace’s blower acts as the air handler for the cooling process. When cold air is needed, the heat exchanger is off and the evaporator coils cool the air as it passes through.

Your furnace is composed of five main parts: air filter, burners, heat exchanger, blower, and ductwork.

Air Filter

In order to keep furnaces clean and efficient while also reducing the number of indoor air contaminants in your home, all furnaces and air handlers contain air filters. They have the important job of capturing airborne particulates from circulating and recirculating throughout your home.

Before the air gets conditioned, it must pass through the filter first. The cleaner the air makes it out on the other side, the healthier you, your family, and HVAC system will be. Unfortunately, air filters don’t replace themselves (yet). For now, we will have to remember to check our air filters every 30 days and wait no longer than 90 days to replace them.

One way to tell if you need to change the filter is by removing it from the cartridge and holding it up to the light. If a light has difficulty passing through the filter, that means air will also have a difficult time passing through. Change the air filter if it seems looks dirty or clogged.

Pilot and Burners

Your pilot light is the ever-present gas flame that flares up to ignite a larger gas burner when heat is called for. You can tell a lot about your furnace’s health and efficiency just by looking at the pilot light. If the flame is mostly blue, with perhaps a hint of yellow at the tips, you have a healthy, clean-burning furnace.

If your pilot flame displays any of these characteristics, your fuel is burning inefficiently and perhaps dangerously:

  • Any other color than blue
  • Noisy flame
  • Waving, blowing flame
  • Small flame

If your pilot light needs relighting, turn off the gas line and wait 5-10 minutes for any lingering gases to dissipate. Then, follow the instructions provided in your owner’s manual or your local, trusted HVAC company.

Heat Exchanger

The combustion chamber creates hot gases which flow into the heat exchanger. The passing air flows over the outside of the heat exchanger, allowing heat transfer without exposure to harmful combustion gases.

After the hot combustion gases have given off their heat, they get safely vented out of your home via the furnace flue. Sometimes there is a second heat exchanger to pick up even more leftover heat before venting the gas outside.

If there is a crack in your heat exchanger, you could be leaking dangerous flue gases into your home.

Signs of a cracked heat exchanger include:

  • Soot buildup
  • Abnormal pilot flame
  • CO detector warnings
  • Strong odors

Contact a professional HVAC technician if you suspect a problem with your heat exchanger.


Located between the return air duct and the rest of the furnace, the blower is housed in its own blower chamber. When the heat exchanger warms up, air is blown over the heat exchanger and into the supply air plenum. The air continues to flow through the supply ducts, where it finally gets released into the rooms of your home.

Without the blower, air wouldn’t be able to flow in from your return ducts or out via your supply ducts. Once the thermostat detects a specified indoor temperature, it shuts the furnace and blower off.

The only thing between your blower and the air flowing in from your return ducts is the air filter. To keep your blower clean and free from dust and dirt, check your air filter every month and wait no longer than 60 days to replace it.

With recent advances, superior efficiency is available with two-stage and variable speed blowers.


There are two main parts to your ductwork: return ducts and supply ducts.

Return registers extract cool air from your home to be warmed by your furnace. Supply ducts carry the conditioned air (by heating or cooling) back into your home. If you feel air blowing from your vent, it is a supply vent.

When you need warm air, the system turns on and air flows through the return air registers back into the furnace. It does this by creating a pressure differential, effectively creating a vacuum effect on the return side.

While the air is sent through your ductwork, it can lose energy and get lost through leaks and cracks in the air duct connections. Luckily, you can improve indoor air quality and energy efficiency by investing in duct cleaning, sealing, and insulating.

Contact Fix-It 24/7  to help insulate and air seal your ducts to cut down on the energy losses associated with ductwork systems.

While there are some things you can do as a homeowner to extend the life of your furnace, nothing is more effective than professional maintenance at the beginning of every heating season (around mid-fall).

Hopefully learning more about how your furnace works gives you the ability to recognize problems before they get serious.

Fix-It 24/7 Plumbing, Heating, Air and Electric provides reliable, on-time heating service 24 hours a day, 7 days a week.

Call us now at (303) 214-0277 or schedule service online.

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