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What is a Heat Exchanger?

In the most general sense, a heat exchanger is simply a device that transfers heat from one substance to another. In the case of your gas forced-air furnace, this transference is done from the gas-powered flame to the warm air that you distribute around your home.

What It Looks Like

Today’s heat exchangers are typically made of stainless steel or aluminized steel in either a clamshell or tubular design. Clamshell heat exchangers are made up of two concave halves that, when joined either by welding or folding, form a passageway with an inlet and an outlet.

Tubular designs, which are comparatively newer, feature long U- or S-shaped chambers that create undulating series of tubes. Many manufacturers claim to use proprietary processes that strengthen the metal at the tubes’ curves, where normally the metal would stretch and thin during the bending process. In addition, tubular heat exchangers typically feature thicker walls and shorter seams than clamshell heat exchangers, reducing the likelihood of a failure.

How It Works

Once your thermostat directs your furnace that it’s time to produce heat, the furnace’s burner produces combustion gasses. These gasses enter the heat exchanger through one opening and transfer heat onto the heat exchanger’s walls. The combustion gasses are then either vented out of the home through another opening or, in the case of some high- efficiency furnaces, passed into a secondary heat exchanger, where more heat is extracted from the gasses before they are vented outside. As this is happening, the furnace’s blower motor moves intake air from the home over the heat exchanger’s now hot chambers where the air picks up heat before being pumped back out through your ducts and into the home.


It is important to maintain your furnace’s heat exchanger not only because it is a critical component of the furnace, but because a cracked heat exchanger can be dangerous. Should a heat exchanger develop a crack, its ability to vent combustion gas is impaired and, as a result, carbon monoxide could be circulated within the home.

Cracks can occur at seams or bends, as a result of corrosion, “metal fatigue” from working with a restricted air flow, weak joints or any other number of factors. Several things can indicate a problem:

  • Yellow or leaning flame (your flame should be blue)
  • Signs of corrosion, visible cracks or other instances of exterior wear and tear
  • A smell like formaldehyde
  • The presence of soot

In addition, many HVAC technicians recommend regular heat exchanger inspections for units more than 10 years old.

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