In short
A heat exchanger is the sealed steel chamber that keeps a furnace's flame and exhaust on one side and your house air on the other. A crack or hole breaks that seal and can let carbon monoxide into the ducts, so in Ontario a licensed technician must shut the furnace down. Past roughly 12 to 15 years, replace the furnace instead of the part.
The heat exchanger is the wall between the fire and the air you breathe. Burners fire into one end, the hot gases travel through steel tubes or stamped cells, and the blower pushes house air over the outside of that hot steel. The gases leave through the vent and the air leaves through the supply ducts. The two never touch.
What a heat exchanger does and the two designs you will see
A heat exchanger moves heat through metal while keeping combustion products out of the house. Tubular exchangers are bent steel tubes, one per burner, and are what most furnaces built in recent decades use. Clamshell exchangers are two stamped steel halves joined into cells, found on many older furnaces.
A condensing furnace has two. The primary heat exchanger sits over the burners and does most of the work. The flue gases then pass through a secondary heat exchanger, which pulls out enough extra heat to condense the water vapour. Mid-efficiency furnaces, still running in plenty of 1980s and 1990s subdivisions, have the primary only.
Why heat exchangers crack
Heat exchangers crack from overheating and from age. The steel expands on every cycle and shrinks on every cool-down, and after enough cycles it fatigues at the bends, welds and crimps. Anything that makes the furnace run hotter than its design speeds that up:
- A clogged filter or a dirty blower wheel that starves the furnace of air.
- An oversized furnace that heats up fast, shuts off and cycles far more often than it should.
- Undersized or closed-off ductwork, common where a bigger furnace was put on a 1960s bungalow's original ducts.
- Burners over-fired because the gas pressure was never set with a manometer.
- Condensate or humidifier water sitting on the steel and rusting it through.
What a homeowner might notice is covered in how to know if a heat exchanger is cracked. A crack is confirmed by inspection with a camera or mirror and by combustion readings, never by the age of the furnace alone.
What Ontario rules require when a heat exchanger is cracked
A cracked or perforated heat exchanger is an unacceptable condition under the gas code, CSA B149.1. A TSSA-certified technician who confirms one shuts off the gas to the furnace and tags it, because the breach can let carbon monoxide into the air stream. The furnace stays off until it is repaired or replaced. Whether that makes tonight an emergency in your house is answered in is a cracked heat exchanger an emergency.
Repair or replace: what a heat exchanger costs
Replacing a heat exchanger costs $1,500–$3,000 in parts and labour on most furnaces, because the furnace is stripped to the cabinet to get the old one out. Many manufacturers warrant the heat exchanger for much longer than the other parts, but that warranty covers the part and not the labour. Check the registration paperwork before deciding.
- Furnace under about 12 years old with a valid part warranty: replacing the heat exchanger can make sense.
- Furnace past roughly 12 to 15 years: put the money toward a new high-efficiency furnace, typically $4,500–$7,500 installed.
- Any age, with a diagnosis made without photos or combustion numbers: get a second opinion before spending anything.
A real crack can be shown to you, so ask for the camera image and the combustion readings. ZK Mechanical does combustion inspections and furnace second opinions across Burlington, Oakville, Hamilton and the west GTA. For a tagged furnace or a no-heat call, furnace repair is on the 24/7 line at (647) 801-1252, or request a visit online.
Questions people ask about Heat exchanger
Can a cracked heat exchanger be welded or patched?
No. Furnace manufacturers do not approve field repairs to a heat exchanger, and a furnace with a welded or sealed exchanger no longer matches its certification. The metal beside a weld is also the next place to crack. The two legitimate fixes are a new heat exchanger from the manufacturer or a new furnace.
How long does a furnace heat exchanger last?
Usually as long as the furnace. With clean filters, gas pressure set correctly and ducts that move enough air, the heat exchanger is rarely the first part to go. Chronic overheating changes that: an oversized furnace on undersized ducts can crack one many years early. Annual maintenance that measures temperature rise is how the problem is caught before the steel fails.
Will a carbon monoxide alarm tell me if my heat exchanger is cracked?
Not reliably. An alarm responds to carbon monoxide in the room over time, and a cracked exchanger on a clean-burning furnace may leak very little until combustion goes wrong. Ontario requires CO alarms in homes with fuel-burning appliances, and they are the last line of defence, not an inspection. If one sounds, get everyone outside and call 911.