Anthony — TSSA-Certified G1 Gas Technician
TSSA G1 licence #TBD · 15+ years in gas and mechanical trades · Service area: Greater Toronto Area, Ontario
Last updated: 2026-08-03
How does a technician find a cracked heat exchanger?
Technicians combine a visual inspection with combustion testing. A crack is confirmed when combustion analysis shows carbon monoxide or a shift in flue gas readings while the blower is running, or when a flashlight/mirror inspection or smoke test shows a visible fracture, rust-through, or flame movement tied to blower cycling.
Some cracks are visible directly through an access panel or with a borescope. Others only show up indirectly, because they open and close slightly as the metal heats and expands, which is why a technician will often start and stop the blower a few times during the inspection rather than relying on a single reading.
The most reliable indirect method is testing CO levels in the supply air (the ductwork feeding the house) compared to CO in the flue. If CO shows up in the supply air with the blower running, combustion gas is entering the airstream, which only happens if there is a breach somewhere in the heat exchanger or its seals.
Why can't a cracked heat exchanger just be repaired?
Heat exchangers are sealed, welded steel or aluminized-steel assemblies that are not designed to be field-repaired. Under CSA B149.1, a furnace with a compromised heat exchanger must be taken out of service and the appliance replaced or the exchanger replaced with a factory part, not patched.
Even where a manufacturer sells a replacement heat exchanger assembly for a specific model, the labour, teardown, and reassembly cost is often close to the cost of a new furnace, and the surrounding components (blower motor, control board, ductwork connections) are usually near end of life on the same timeline. Most homeowners end up replacing the whole unit.
What does replacement involve and cost in the Greater Toronto Area?
Replacement means removing the red-tagged furnace, installing a new one sized to the home's heat loss, and having the installation and gas connection inspected before the gas supply is restored. In the Greater Toronto Area this typically runs from CAD 4,000 to CAD 8,500 depending on furnace size, efficiency tier, and whether ductwork or venting modifications are needed.
Higher-efficiency condensing furnaces need PVC intake/exhaust venting and a condensate drain, which can add cost if the existing furnace was a mid-efficiency model with a B-vent. Financing and rental options are available through most contractors and utilities.
| Furnace type | Typical installed cost (CAD) |
|---|---|
| Mid-efficiency (80% AFUE) | $4,000 – $5,800 |
| High-efficiency condensing (95%+ AFUE) | $5,500 – $8,500 |
| Venting/ductwork modification (if needed) | $400 – $1,800 extra |
Can the furnace be temporarily used while waiting for replacement?
No. Once a gas technician confirms a cracked heat exchanger, the furnace is red-tagged (Type A) and the gas supply to it is shut off or the appliance is locked out immediately, because continued use risks carbon monoxide entering the living space. There is no safe interim setting or workaround.
Homeowners are typically left without heat until the replacement is installed, so most contractors prioritize these calls and can often schedule installation within 1 to 3 business days depending on equipment availability.
- Technician confirms the crack with combustion analysis and/or visual inspection
- Furnace is red-tagged and gas locked out the same visit
- Homeowner gets a written report describing the finding
- Replacement furnace is sized and quoted
- New furnace installed and gas connection inspected
- Red tag is cleared and gas service restored
Should I get a second opinion before replacing the furnace?
A second opinion is reasonable, especially for an expensive diagnosis like this one. Ask the first technician for the specific combustion analyzer readings or photos that support the finding — a legitimate crack diagnosis should be documented, not just asserted.
A second technician can re-run the same combustion analysis and inspection within a day or two. Keep in mind the furnace stays off during this process regardless of who confirms it, since only a certified gas technician can clear the tag once the underlying issue is resolved.
Does a cracked heat exchanger affect home sales or insurance?
Yes. A red-tagged furnace found during a pre-listing or pre-purchase inspection is a disclosed safety issue that most buyers' lawyers and insurers will ask about, and an active red tag can hold up closing until the furnace is replaced or the tag is cleared.
Keep the technician's report and the eventual replacement invoice — they document that the issue was identified and properly resolved, which matters for insurance and resale.
Frequently asked questions
Can a cracked heat exchanger be welded instead of replaced?
No. Field welding a heat exchanger is not permitted under CSA B149.1 because the repair cannot be verified as gas-tight under thermal cycling. Replacement of the exchanger or the whole furnace is the only accepted fix.
How long does a heat exchanger last before cracking?
Most heat exchangers last 15 to 20 years, though poor airflow, oversized furnaces, or years of short-cycling can cause cracks earlier. Furnaces past 15 years old with a crack are almost always replaced rather than repaired with a factory exchanger.
Will a carbon monoxide detector catch this problem?
A working CO detector can catch dangerous levels of carbon monoxide from a cracked heat exchanger, but it only alarms after gas has already entered the home. Combustion analysis during a service visit catches the crack before CO reaches alarm thresholds.
Is a cracked heat exchanger covered under warranty?
Many manufacturers offer 10 to 20 year limited warranties on heat exchangers, which can cover the part but not labour. A technician can check the furnace's model and serial number against the manufacturer's warranty database before you commit to a full replacement quote.