Cracked heat exchanger: signs, danger, repair or replace

The heat exchanger is the metal chamber that keeps fire and house air apart. The burners fire into it, and the hot exhaust travels through it on the way to the flue. The blower pushes house air across the outside of the same metal, where it picks up heat. The two streams never touch.

A crack or a rusted-through hole breaks that separation. Exhaust can leak into the air stream, and blower air can push into the burner area and disturb the flame, which makes the burners produce more carbon monoxide.

• A carbon monoxide alarm sounding while the furnace runs.
• Flames that flicker, lean or roll out of the burner area when the blower starts.
• Yellow or orange flames where they used to be blue.
• Soot or scorch marks on or around the furnace.
• An exhaust smell indoors.
• Headaches or flu-like feelings that ease when people leave the house.
• Repeated trips of the rollout or limit switch.

None of these proves a crack, and a cracked heat exchanger can show none of them. Several of the same signs also come from dirty burners or a venting problem. Each one is still a reason to stop running the furnace until it is inspected.

The metal expands every time the furnace heats up and shrinks as it cools. Over many years that movement fatigues it. Overheating speeds the process, and the usual causes of overheating are a clogged filter, closed registers, an oversized furnace and short cycling left unfixed. Corrosion from condensate or a damp location does the rest. Age is the largest single factor.

A proper diagnosis is something you can see. The technician inspects the heat exchanger directly or with an inspection camera, watches the flame as the blower starts, and may run a combustion test. Ask for a photo or video of the crack and its location. A reputable technician expects that request.

When a crack is confirmed, the technician will shut the furnace down and tag it as unsafe, and may be required to do so. If you were told the heat exchanger is cracked and shown no evidence, a second opinion from another licensed contractor is reasonable. Keep the furnace off while you wait for it.

A cracked heat exchanger is not patched or welded. The fix is a new heat exchanger or a new furnace. Three things decide it:

• Warranty. Many manufacturers cover the heat exchanger for much longer than other parts. Check your paperwork and registration. The warranty usually covers the part and leaves the labor to you.
• Labor. The furnace has to be taken largely apart, so the labor is heavy even when the part is free. See the heat exchanger replacement cost guide.
• Age and condition. On an older furnace, a new heat exchanger sits among other worn parts.

Ask for both prices in writing and compare them using furnace repair or replace. A newer furnace under warranty often gets the repair. An old one usually gets replaced.