Replace a Heat Exchanger (Furnace)

Confirm the crack first (visual/mirror, CO in the supply, or a combustion analyzer), weigh the repair-vs-replace economics, then fully tear the furnace down (gas, electrical, venting, burners, blower), swap the matched heat exchanger, reassemble to spec, and run a MANDATORY combustion/CO test + draft check before it goes back in service.

Approx. 360 min

Before you start

•Crack/failure CONFIRMED, not assumed — visual + mirror/light inspection, a CO climb in the conditioned supply air, or a combustion analyzer showing flue products crossing into the airstream. A cracked HX is a CO hazard: red-tag and shut off until repaired.
•Repair-vs-replace decided: on an older furnace the exchanger cost + 4–6 hr labor often approaches a new furnace — present the economics. Confirm the part is in warranty (many HX carry a long/lifetime warranty) and the exact replacement is available.
•Correct OEM heat exchanger for this model/serial (clamshell vs tubular, NG vs LP, single vs multi-position) — generic exchangers misfit and misfire.

Tools & materials

Combustion analyzer (CO/CO-air-free, O2/CO2, stack temp), CO personal monitor · Manometer (inches W.C.), inspection mirror + light/borescope · Nut drivers, sockets, screwdrivers, gas-rated thread sealant · Electronic gas leak detector + leak-check solution · Replacement OEM heat exchanger + new gaskets/seals/cleat kit

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Confirm the fault & decide

Prove the heat exchanger is the fault
Don't condemn an exchanger on a hunch. Inspect with a mirror/light or borescope for cracks/perforations/scaling, watch for CO rising in the supply registers as the blower starts, or run the combustion analyzer and watch O2/CO shift when the blower energizes (flue products being pulled into the airstream). Rule out a cracked vent, plugged condensate, or a flame-rollout cause first.
Tools: Inspection mirror + light/borescope, Combustion analyzer, CO monitor
Target: Documented crack/perforation OR CO change in the conditioned supply when the blower runs
⚠ A cracked heat exchanger can put CO into the living space — shut off and red-tag the furnace until repaired or replaced.
Weigh repair vs. replace the furnace
Price the exchanger + the 4–6 hr labor against a new furnace, factor the furnace age/efficiency and remaining warranty. On a 15+ yr unit the swap often costs most of a replacement — give the customer the honest comparison before tearing in.
Target: Repair total (part + labor) vs. new-furnace cost, with age/efficiency/warranty factored

Tear the furnace down

Shut off gas + power and disconnect venting
Close the manual gas shutoff, kill and verify electrical power, then disconnect the vent connector/flue from the furnace. Cap or note the gas connection at the valve.
⚠ Lockout/tagout, verify 0 V, and confirm the gas valve is OFF before opening the cabinet.
Pull the burners, gas valve/manifold, and blower
Photograph wiring and gas connections first. Remove the burner assembly + manifold/gas valve, the inducer, the igniter/flame sensor, and the blower as needed to free the exchanger. Keep fasteners and wiring labeled by location.
Tools: Nut drivers, sockets, screwdrivers
Remove the old heat exchanger
Release the exchanger cleats/clamps and slide the old exchanger out, noting orientation and every gasket/seal so the new one seats identically.

Set the new exchanger & reassemble

Set the matched exchanger with new gaskets
Install the OEM exchanger with NEW gaskets/seals and cleats — a reused/compressed gasket leaks flue gas. Verify orientation and that the cells line up with the burners.
Target: OEM exchanger, new gaskets/cleats, cells aligned to the burners
Reassemble gas, burners, blower, venting & wiring
Rebuild in reverse: burners/manifold/gas valve, inducer, igniter + flame sensor, blower, then reconnect the vent and all wiring per your photos. Reconnect gas and leak-check every joint you broke.
Tools: Electronic gas leak detector, Leak-check solution
Target: All assemblies torqued/seated; vent reconnected with correct slope; zero gas leaks at reopened joints
⚠ Never check a gas joint with a flame — use detector/solution.

Combustion test & draft check

Fire and set manifold pressure + temperature rise
Restore gas/power, fire the burners, and confirm manifold pressure to the fuel spec and the supply−return temperature rise inside the nameplate range. Adjust airflow (blower tap), not gas, to land the rise.
Tools: Manometer
Target: Manifold to fuel spec; ΔT inside the nameplate temperature-rise band
Run the MANDATORY combustion/CO test + draft check
Put the analyzer in the flue: confirm clean CO air-free, reasonable O2/CO2, and verify good draft / no spillage at the draft hood or sealed termination. Re-check CO in the conditioned supply to prove the new exchanger is sealed. Do not return to service if CO is high or it spills.
Tools: Combustion analyzer, CO monitor
Target: CO air-free within limit (commonly < 100 ppm air-free), good draft, no spillage, no CO in the supply air
⚠ A furnace producing high CO or spilling flue gas does not go back in service — red-tag it.