Cracked heat exchanger symptoms: what to watch for
You cannot see a crack from the outside, and the signs are easy to brush off. Here is what to notice and why the furnace should be off until it is tested.
The short answer: it is a safety question first
A cracked heat exchanger is a split in the metal wall that keeps burner exhaust apart from the air you breathe. You cannot confirm one by eye, and the symptoms are easy to mistake for something minor. If you suspect it, the safe move is to stop using the furnace and have a licensed technician test it.
One rule comes before everything else. If a carbon monoxide detector is sounding, or you smell gas, leave the house now. Call your gas utility or 911 from outside. Do not look for the source.
Signs that point to a possible crack
No single sign proves it. Several together make it worth a call.
| What you notice | Why it can point to the heat exchanger | What to do |
|---|---|---|
| CO detector alarms, or headaches and nausea that ease when you leave the house | Exhaust may be reaching the living space | Leave, call 911 or your gas utility from outside |
| Burner flame that flickers or changes shape when the blower starts | Air from the blower can push through a crack into the flame | Turn the furnace off, call a technician |
| Soot or heavy rust around the burner area or the cabinet | Incomplete combustion or moisture where it should not be | Turn it off, call a technician |
| A new, sharp or chemical smell when the furnace runs | Combustion by-products in the airflow | Turn it off, ventilate, call a technician |
| Clicking or ticking as the metal heats and cools | Common and often harmless, but can come with a crack | Mention it at the next service |
Why age and overheating matter
Metal expands when it heats and shrinks when it cools. After thousands of cycles, a stressed spot can fail. Overheating speeds that up, which is why a clogged filter, blocked vents or a weak blower are worth fixing early. A furnace that has been running hot is a better suspect than one that has simply been running for years.
Age alone does not mean a crack. Plenty of older furnaces are sound. Plenty of newer ones fail early after repeated overheating.
What to do if you suspect it
- Turn the furnace off at the thermostat and the switch.
- Open a couple of windows to air the house out.
- Check that your carbon monoxide detectors have fresh batteries and are on every floor with a bedroom.
- Call a licensed HVAC technician and say you suspect a heat exchanger problem.
- Keep the furnace off until it has been tested.
What a technician actually checks
A pro inspects the exchanger with a camera or a mirror, looks at the flame pattern with the blower on, and tests the air for combustion gases. Some cracks are only visible with the right tools. That is why a glance with a flashlight is not a test.
Repair or replace?
Be honest with yourself about this one. A confirmed crack is generally not a repair worth making on an older furnace. Replacing the exchanger is a large job, and on many units the cost and the labor point to a new furnace instead. On a young furnace still under warranty, ask the technician and check the paperwork before you decide. Whatever the answer, do not run a furnace with a known crack.
What not to do
- Do not open the cabinet to look for the crack. Burners, gas valves and live wiring are in there.
- Do not tape, seal or patch the metal.
- Do not keep running the furnace "just through the cold snap".
- Do not rely on smell. Carbon monoxide has none.
Frequently asked questions
What are the main symptoms of a cracked heat exchanger?
The most serious one is a carbon monoxide alarm. Others include a flame that flickers when the blower starts, soot or heavy rust near the burners, and an unusual smell while the furnace runs. None of these proves a crack on its own.
Can I check for a cracked heat exchanger myself?
No. Cracks are often hidden, and getting to the exchanger means opening a cabinet with burners, gas and live wiring. A licensed HVAC technician has the tools to inspect and test it safely.
Is it safe to run a furnace with a cracked heat exchanger?
No. A crack can let combustion gases, including carbon monoxide, reach the air in your home. Turn the furnace off and keep it off until a technician has tested it.
Does a cracked heat exchanger always mean a new furnace?
Not always, but on an older furnace replacing the unit is often the sensible answer because replacing the exchanger is a big job. A technician can tell you which makes sense for your furnace.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Furnaces
- CDC. Carbon Monoxide Poisoning Basics