Cracked heat exchanger symptoms and what to do about them

What to know first

• A CO alarm, or CO symptoms in anyone at home (headache, dizziness, weakness, upset stomach or nausea, vomiting, chest pain, confusion), alarm or no alarm, means get everyone outside to fresh air and call 911.
• The signs people link to a cracked heat exchanger can all come from other furnace problems too. Only a technician's diagnostic testing can confirm a crack.
• If a technician does find one, the furnace should not run again until the heat exchanger or the furnace is replaced.

What is a heat exchanger?

A heat exchanger is the set of metal tubes or chambers inside a gas furnace that the hot exhaust passes through while house air blows over the outside. Trane gives it two jobs: it transfers heat for space heating, and it keeps toxic combustion gases separate from the heated air.

Why does a crack matter?

A crack is a break in the wall between the exhaust and the air you breathe. The EPA lists leaking furnaces among the household sources of carbon monoxide (CO), and ENERGY STAR says a cracked heat exchanger causes improper burner operation.

Trane and Lennox both say a cracked heat exchanger can leak combustion gases, carbon monoxide among them, into your home. Neither says every crack does, and Lennox adds that even when no CO is detected, a crack cuts efficiency and in extreme cases can lead to overheating. A quiet alarm does not clear a cracked furnace, which is why a confirmed crack means the furnace stays off.

What causes them?

Trane and Lennox give the same short list: age and metal fatigue as the metal heats and cools every cycle, overheating from restricted airflow or a dirty filter, an oversized furnace that cycles too often, and rust or corrosion from moisture. A Northeast Ohio heating season runs from fall into spring, so a furnace here goes through a lot of those cycles.

What are the symptoms of a cracked heat exchanger?

Trane and Lennox list soot, rust, yellow or flickering flames instead of steady blue, a furnace that short cycles, banging or rattling, a metallic smell, and a CO alarm that keeps going off. Every one of them can also come from something else.

The other causes come from the makers. For a limit or rollout switch that has opened, a Carrier furnace troubleshooting guide lists a dirty filter or restricted ducts, a failed blower motor and inadequate combustion air. Payne's owner's manual for its condensing furnaces says dirt, soot or rust can mean improper performance, and ties a loud noise at ignition to buildup on the burners.

What about a furnace that keeps shutting off?

Payne says its rollout switch shuts the furnace off after high temperature conditions, and Carrier's guide lists a leak in the heat exchanger among the causes of a rollout switch that trips again, along with a blocked heat exchanger and misaligned burners. Do not reset a rollout or limit switch yourself: leave the furnace off and call. A furnace that keeps shutting off is one of the reasons to call our 24/7 emergency line.

Can I check for a cracked heat exchanger myself?

No. Payne's manual leaves the heat exchanger to a trained service specialist at the annual check up, and Lennox says confirming a crack takes diagnostic testing such as combustion analysis or a visual inspection. What you can do is notice the warning signs without opening anything:

1. Switch it off first. Set the thermostat to Off and turn off the furnace power switch, which looks like a light switch on or near the unit, before you pull the filter. Do that only if you can reach the switch without stepping into water near outlets, cords or the electrical panel; otherwise stay out and call for help. Let it cool: the flue pipe gets hot, and sheet metal edges can be sharp.
2. Check the filter. Payne says to check it before each heating and cooling season; ENERGY STAR says to inspect, clean or change it once a month.
3. Look at the outside of the cabinet and the flue pipe with a flashlight for black soot, rust or water. If you see soot, rust or scale, Payne says not to operate the furnace and to call. If water is standing around the furnace, do not step into it near outlets, cords or the electrical panel; stay out and call.
4. Note the status light. Payne says its blink code shows through the view port in the blower access panel, so you can read it without opening anything. Write it down for the technician.
5. Check the vent pipes outside. Payne says an obstructed vent can let toxic fumes into the home. After a lake effect snow, look for drifts or ice over pipes on an outside wall and clear them by hand. Enbridge Gas Ohio says to clear appliance intake and exhaust vents after every storm.

Stop there. Do not take off the burner door, reset a tripped rollout or limit switch, jump a safety switch or try to see the heat exchanger. If you smell anything metallic or see any of the signs in the table, leave the furnace off and call.

What should I do if my carbon monoxide detector is going off?

Get everyone, pets included, outside to fresh air right away, then call 911 from a safe place. Do not go back in until a qualified professional, such as a firefighter, says it is safe, as Enbridge Gas Ohio advises.

You cannot smell CO. The CDC lists the most common symptoms of CO poisoning as headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, and warns that people who are asleep can die before they have symptoms. The U.S. Consumer Product Safety Commission also lists nausea and sleepiness. If anyone has those symptoms, alarm or no alarm, get everyone outside to fresh air and call 911, as the CPSC advises.

Once the fire department has cleared the house, keep the furnace off until it has been checked.

What happens if a technician finds a crack?

On our furnace repair calls, a cracked heat exchanger is not a repair conversation: we red tag the furnace, which marks it unsafe to run, show you the evidence and shut it down. Our seasonal tune ups include combustion analysis, safety checks, a heat exchanger inspection and airflow verification, with the readings documented.

Can a cracked heat exchanger be repaired, or should I replace the furnace?

Repair is rarely recommended. Trane says your best bet for safety and comfort is to replace the heat exchanger or even the whole furnace; Lennox says replacing the heat exchanger is typically the fix, and sometimes the whole furnace has to go.

When to replace the furnace instead of the part

Things that tip the decision toward a new furnace:

• Age and other damage. Lennox says the age of the system, or other damage the crack has caused, can make replacing the whole furnace necessary.
• Parts coverage. Whether the heat exchanger is covered depends on the maker and the paperwork; read your manufacturer's warranty terms.
• Size. Lennox and Trane both name an oversized furnace as a cause. A replacement sized by a Manual J load calculation, the heat loss calculation for your house, helps avoid repeating it.
• Fuel choice. It is a chance to compare a gas furnace with a dual fuel heat pump system.

How do a fall check and CO alarms protect you?

The yearly check is when a technician looks at the heat exchanger, and a CO alarm is your backstop between visits.

• Book a furnace tune up in the fall. ENERGY STAR's fall heating checklist covers gas connections, gas pressure, burner combustion and the heat exchanger. The EPA, the CDC and the U.S. Fire Administration all say to have the furnace inspected or serviced every year.
• Put CO alarms in the right places. The CPSC's CO alarm guidance says on each level of the home and outside sleeping areas, but not in attics or basements unless someone sleeps there. A finished basement bedroom counts.
• Test and maintain them. CPSC says test monthly and replace batteries yearly. The CDC says replace the detector per the maker's instructions or every 5 years.

When to call Raber HVAC

• A CO alarm or CO symptoms: everyone outside, then 911. Once the house is cleared, call us before the furnace runs again.
• Soot, rust, yellow flames, a bang at ignition, or a furnace that keeps shutting itself off: leave it off and call (216) 888-4822. The office is open every day, 8:00 AM to 10:00 PM, and the 24/7 emergency line on the same number covers a furnace that keeps shutting off, no heat in freezing weather and active water leaks.
• A red tagged furnace in a cold snap: use an electric space heater only. The U.S. Fire Administration says to plug it straight into a wall outlet, never an extension cord or power strip, keep it at least 3 feet from anything that can burn and turn it off when you leave the room or go to bed. Never heat with a gas oven or a camp stove, burn charcoal or run a generator indoors or in the garage, as the CDC warns; Enbridge adds patio heaters and barbecues.
• Anything that can wait a day, including a fall check: book online, pick a day and an arrival window, and the office calls to confirm.

Sources

• Trane, Cracked Heat Exchanger: What It Means
• Lennox, What to Do If Your Furnace Has a Cracked Heat Exchanger
• Carrier, Deluxe 2-Stage Gas-Fired Induced-Combustion Furnaces Troubleshooting Guide (Form 58-3T)
• Payne, Condensing Gas Furnace Owner's Manual (OG-9GFP-08)
• U.S. EPA, Carbon Monoxide's Impact on Indoor Air Quality
• ENERGY STAR, Maintenance Checklist
• Enbridge Gas Ohio, Carbon Monoxide Safety
• CDC, Carbon Monoxide Poisoning Basics
• U.S. Consumer Product Safety Commission, Carbon Monoxide Information Center
• U.S. Consumer Product Safety Commission, CO Alarms
• U.S. Fire Administration, Heating Fire Safety handout