Heat exchangers get a bad rap. We spend a lot of time talking about what can happen when they are damaged, particularly when cracks in their walls cause carbon monoxide leaks in homes. And although this is extremely important information for homeowners to know about, it can leave some people wondering what a heat exchanger does when it’s working properly. So today we’re going to answer the commonly asked question: How does a heat exchanger work in a furnace?

Step 1: Hot combustion gasses enter the heat exchanger

When your furnace kicks on, its burners send hot combustion gasses to the inside of your heat exchanger (often called its “chamber”). As this happens, the heat from the combustion gasses is transferred to the metal walls of your heat exchanger, causing them to heat up.

Step 2: Air is blown over the outside walls of the heat exchanger

Now that the walls of your heat exchanger are heated up, your furnace blows cold return air from your home over the outside of the heat exchanger. This allows the air from your home to heat up without picking up any of the dangerous combustion gasses that are housed inside the chamber of your heat exchanger. This warm air is then directed through your home’s ductwork and distributed throughout your home.

Step 3: Cooled combustion gasses are vented outside

After the heat from the combustion, gasses have been transferred to the walls of your heat exchanger (and then to your home’s air supply), those gasses need somewhere to go. In standard furnaces, these gasses are blown out of the heat exchanger and vented out of your home. In high-efficiency condensing furnaces, these gasses are sent to a second heat exchanger where even more heat is pulled from them and used to further heat your home.

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