Signs Your Furnace’s Heat Exchanger Is Cracked

The heat exchanger is the part of a furnace that keeps the gases from the burners separate from the air blown through your home. A crack in it is a safety concern, not just a repair, and a furnace with a damaged heat exchanger may still run and warm the house. That is why the warning signs matter, and why the answer to most of them is to have a technician look at the furnace.

What a homeowner can notice is limited. If you notice an unusual odor, soot around the furnace, a yellow or flickering flame, or anything else that seems off when the furnace is running, turn it off and call a heating professional. A crack is confirmed by a technician’s inspection, not by looking at the furnace from the outside. If a carbon monoxide alarm sounds, get everyone outside and call 911 first.

Can You Keep Using a Furnace You Suspect Is Cracked?

No. If you suspect the furnace has a cracked heat exchanger, turn it off and have a heating technician inspect it before it runs again. You cannot tell whether a furnace is safe to run by looking at it from the outside.

Why Carbon Monoxide Is the Main Concern

Carbon monoxide is a colorless, odorless gas that interferes with the delivery of oxygen throughout the body. Combustion gases and particles come from cracked furnace heat exchangers, and from chimneys and flues that are improperly installed or maintained.

At higher concentrations, carbon monoxide exposure can cause impaired vision and coordination, headaches, dizziness, confusion, and nausea. The symptoms of carbon monoxide poisoning are sometimes confused with the flu or food poisoning.

If you experience symptoms that you think could be from CO poisoning, get fresh air immediately, go to an emergency room, and tell the physician you suspect CO poisoning. EPA suggests being ready to answer these questions for the doctor: Do your symptoms occur only in the house? Do they disappear or decrease when you leave home and reappear when you return? Is anyone else in your household complaining of similar symptoms?

Furnace Heat Exchanger Symptoms a Homeowner Can Notice

The heat exchanger is found deep inside your furnace. That means most of what a homeowner can notice is indirect. Trane lists these as warning signs that you might have a cracked heat exchanger: a furnace that short cycles, banging or rattling sounds, a strong metallic odor, yellow or flickering flames instead of a steady blue flame, soot buildup around the furnace or flue pipe, a carbon monoxide detector that keeps tripping, and corrosion or rust. Signs of a cracked heat exchanger can also be the same as other furnace problems, so it is essential to have a heating professional evaluate the situation. Checking for these signs is not a substitute for a technician’s inspection, and removing furnace panels to look inside is a job for a technician, so stick to what you can see from the outside.

Soot Around the Furnace or Flue Pipe

Soot buildup around the furnace or flue pipe is worth having a technician look at.

A Yellow or Flickering Burner Flame

Trane says you should see a steady blue flame, and that if you see yellow or orange flickering you should call an HVAC specialist to resolve the issue and help ensure safe operation. Newer models use an electric ignition system that only lights when necessary, so they do not have these flames.

Unusual Odors

An unfamiliar smell when the furnace runs is worth mentioning to a technician. A furnace that smells normal is not proof that everything is fine.

Noises and Short Cycling

Banging or rattling sounds, or a furnace that keeps short cycling, are reasons to have the furnace checked, not a diagnosis.

Heavy Rust or Water Staining

Heavy rust or water staining on or around a furnace does not prove a heat exchanger problem, but it is worth pointing out to whoever services it. EPA notes that proper maintenance is important even for new furnaces, because they can corrode and leak combustion gases, including carbon monoxide.

Why Heat Exchangers Crack

Trane gives four main reasons a heat exchanger cracks: years of wear and tear, overheating, an oversized furnace, and rust or corrosion. The heat exchanger expands and contracts each time the furnace comes on, so over the years wear and tear may cause it to crack.

A furnace that repeatedly overheats can lead to a cracked heat exchanger. A clogged air filter can cause all sorts of problems with your heating system, including furnace overheating, a major cause of a cracked heat exchanger. You can help prevent your furnace from overheating by providing unrestricted airflow. Regularly replacing the furnace filter is something you can do yourself to help. Make sure that furniture and curtains are not blocking registers or return air vents.

What to Do If a Carbon Monoxide Alarm Sounds

If a CO alarm sounds, get everyone in the home, pets included, outside into fresh air and call 911 from outside. Do not go back in until emergency responders say it is safe.

Afterward, EPA advises having a qualified technician inspect your fuel-burning appliances and chimneys to make sure they are operating correctly and that there is nothing blocking the fumes from being vented out of the house. Keep the furnace off until that inspection is done.

Treat every alarm as real. Unlike a smoke detector, where you can easily confirm the cause of the alarm, CO is invisible and odorless, so it’s harder to tell if an alarm is false or a real emergency.

The U.S. Fire Administration advises installing CO alarms in a central location outside each separate sleeping area and on every level of the home. If your furnace is the suspect, Service First offers emergency furnace repair in Bucks County.

What a Heating Professional Checks

Checking a heat exchanger means opening the furnace and inspecting parts you cannot see from outside. A routine heating check-up covers the burners and the heat exchanger, according to ENERGY STAR’s maintenance checklist. If you suspect a crack, tell the technician what you noticed, when you noticed it, and whether any alarm has sounded.

If a technician confirms a crack, the next step is a repair-or-replace decision that depends on the furnace and what the inspection finds. It is reasonable to ask for the findings in plain language and to take time to weigh repair against replacement before you decide.

Preventing Problems With Regular Inspections

Have your fuel-burning appliances, including oil and gas furnaces, inspected by a trained professional at the beginning of every heating season, as EPA advises. ENERGY STAR recommends annual pre-season check-ups for heating and cooling systems. Trane says to schedule a yearly furnace tune-up every fall before heating season starts, and that as part of this visit the HVAC technician inspects the heat exchanger for corrosion or damage.

If You Suspect a Cracked Heat Exchanger in Bucks County

Service First has served Bucks County homeowners since 2009 and is a Trane Comfort Specialist. A suspected crack should be checked before the furnace runs again.

Frequently Asked Questions