How to Diagnose the Bad Heat Exchanger in a Furnace


A heat exchanger is a safety mechanism that separates the combustible gases from the air that the furnace heats. The heat exchanger continually expands and contracts as the furnace heats up and cools down, which gradually causes metal fatigue. Furnaces over 15 years old are more likely to have a bad heat exchanger. The risk increases if you haven't had the furnace inspected and serviced annually. A few simple home tests can help determine if the heat exchanger is faulty. If the device shows signs of damage, call a technician to inspect and replace it.

Things You'll Need

  • Dental mirror
  • Flashlight
  • Spray bottle
  • Sulfur stick
  • Lighter
  • Turn off the furnace. If it's a gas furnace, shut off the gas supply.

  • Remove the access door on the front of the furnace. Remove the screws, or, depending on the type of furnace you have, slip the access door off its hooks.

  • Remove the furnace blower. The furnace blower is the object with fan blades on it. It is located behind the furnace access door. Make a note of which colored wires go to which connections. Disconnect the wires attached to the blower and unscrew the screws with a screwdriver. Slide the blower out of the furnace.

  • Remove the burner assembly. The burners contain the pilot light that lights the furnace. This is also located behind the access door. Usually, the burner is held in place with screws. Unscrew the burner assembly and lift it out.

  • Examine the heat exchanger for any signs of cracking, angling a dental mirror around the back side to check for cracks. The heat exchanger is typically located toward the back of the furnace, behind the blower and burner assembly. It is a metal wall that has tubing running up and down it.

  • Shine a flashlight into each of the heat exchanger cells. Look for light shining through a crack in the heat exchanger. This indicates a damaged heat exchanger.

  • Spray water on the exterior of the heat exchanger. Angle a mirror and a flashlight inside the heat exchanger. Check for wet spots that indicate a crack.

  • Insert a sulfur stick into each of the heat exchanger cells and ignite them. Walk around your home, sniffing for any indication of sulfur. If you smell sulfur, you have a bad heat exchanger.

Tips & Warnings

  • Keep the blades of the blower clean of dirt to extend the longevity of the heat exchanger.

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