Quick answer
Yes, treat it as one. A cracked heat exchanger lets combustion gases, carbon monoxide included, mix with the air your furnace blows through the house, so the unit has to stop now and stay off until it is repaired or replaced. A licensed technician who confirms a crack will tag the appliance out of service. Arrange temporary heat rather than a workaround.
A cracked heat exchanger is an emergency in the sense that matters most: the furnace stops running today and does not run again until it is fixed or replaced. The heat exchanger is the steel wall between the fire and the air your blower pushes into every room. Once that wall is breached the furnace is no longer separating exhaust from the air you breathe, and there is no safe partial version of that arrangement.
Why it counts even when nobody feels sick
Three things separate this from a furnace that quit. The blower sits on the pressure side of the exchanger and drives house air across it, so a breach can push and pull exhaust straight into the supply ducts. Steel also moves: a crack that is shut and invisible on a cold inspection can gape open once the metal reaches operating temperature, which is why a quick look at a cold furnace proves little. And exposure creeps — low-level carbon monoxide reads as a winter headache that mysteriously clears up at the office.
A CO alarm is a backstop, not a permission slip. It is designed to sound well below the level that harms a healthy adult, but it does not sample the air inside your ductwork and it cannot certify that the exchanger is sound. For the symptom list and the ways a crack is actually found, see how to tell if a heat exchanger is cracked.
What happens when a technician confirms one in Ontario
A TSSA-licensed technician who finds an unsafe condition has to make the appliance safe before leaving. In practice the furnace is shut down, the gas to it is closed and the unit is tagged. An immediate-hazard tag takes it out of service on the spot; a lesser tag records a defect that must be corrected within a set period. Removing a tag and running the furnace anyway puts you offside with the code and gives your insurer a reason to ask questions later.
Ask for the evidence you are entitled to: where on the exchanger the crack is, how it was identified — a camera inspection, a combustion analysis showing the readings shift when the blower engages, or visible flame disturbance at blower start — and that description written on the invoice. A condemnation is also a replacement quote, so wanting proof is reasonable, and any honest contractor will show you what they saw.
Repair or replace
The part itself is often still covered. Manufacturers commonly warrant heat exchangers for 20 years or longer to the original registered owner, but that covers the component, not the labour, and the labour is the expensive half. Tearing a furnace down to swap an exchanger runs $1,500–$3,000 in parts and labour, and since cracks usually turn up on units 15 years and older, that money goes into a machine that owes you nothing.
For most homes replacement wins. A high-efficiency furnace install runs $4,500–$7,500. A straight gas-for-gas swap earns little or no rebate, but if you use the moment to move to a heat pump or a hybrid heat-pump-plus-furnace system, current Ontario programs return up to $7,500 depending on the system — check the program page for the current rules, and we file the paperwork. The exception worth checking is a furnace under about 10 years old with a documented warranty claim, where the repair can still pay. Furnace repair vs. replacement works through the arithmetic properly.
Staying warm safely until the new furnace is in
- Do not run it for one more hour to take the chill off. That hour is the entire risk.
- Keep every fuel-burning shortcut out of the house: no gas oven or range used as a heater, no barbecue, no camp stove, no unvented propane heater, and no generator in the garage.
- Certified electric heat is the safe stopgap when it is used properly — the rules are in running a space heater overnight.
- If the house will sit cool for more than a day, spend 20 minutes on protecting the plumbing.
- Winter replacement is routine work, not a special favour. The house is usually without heat for a working day, not overnight.
Questions we get after a furnace is condemned
- Can the crack be welded or sealed? No. Patching a heat exchanger is not an approved repair, and a crack is usually a symptom of metal fatigued throughout rather than damage in one tidy spot.
- How long should an exchanger last? Cracks generally appear past the 15-year mark, and sooner on furnaces that short-cycled for years because they were oversized or ran with restricted airflow behind a clogged filter.
- Should I get a second opinion? If you have doubts, yes — but keep the furnace off while you wait. A second technician can inspect a cold, shut-down unit perfectly well.
- Does insurance pay for it? Generally not. Standard policies exclude wear and tear, and a fatigued heat exchanger is exactly that rather than a sudden, accidental loss.
- Is my air conditioner affected? No. The evaporator coil and outdoor unit are a separate circuit and can usually be carried over to the new furnace if they are in good shape.
If a furnace in your house has been tagged, or a technician has told you the exchanger is suspect and you want a second set of eyes, we will inspect it cold and tell you plainly what we find. ZK Mechanical handles heating repairs and replacements across Burlington, Oakville and Hamilton with written pricing before any work starts — call (647) 801-1252 or send us the details.