How to Detect a Crack in a Furnace

Remove the access panel to the furnace. The access panel is typically located on the front of the furnace and is attached with metal hooks or screws. If the panel is attached with hooks, slide the panel up and away from the furnace.

Perform a visual inspection. Look for evidence of cracks along the heat exchanger. These cracks widen as they’re heated, so it’s best to look while the burners first heat up. Use a flashlight to thoroughly inspect the exchanger. Lines of rust are usually an indication of a crack somewhere along the rust line.

Look for a change in flame color or direction. Immediately after a gas furnace turns on, the burner will send a series of small flames shooting up. You can see the flame by standing a few inches away from the furnace. If the flame appears to dance or change color suddenly, you likely have a crack in the heat exchanger.