Cracked Heat Exchanger Condemnation and Red Tag

Purpose

This standing instruction makes a heat exchanger condemnation an evidence file rather than an opinion, and it separates the person who condemns the appliance from the person who sells its replacement.

Both failure directions are real. Condemn on a visual hunch and a second opinion three days later finds nothing, which costs the shop a customer, a reputation in that neighborhood, and sometimes a licensing complaint. Fail to condemn on real evidence and a family breathes combustion products all winter. What separates the two is not the tech's experience. It is whether the file holds measurements taken before and after the mimics were eliminated, images tied to a named location on the exchanger, and a corroborating test another tech could repeat.

Safety actions that gate the work

• If the appliance is spilling combustion products or ambient carbon monoxide is elevated with occupants present, shut it off and get people out of the space before you investigate anything. Do not keep it firing to gather better evidence with a family in the house.
• Personal carbon monoxide monitor on before you enter the mechanical space, and the ambient reading recorded with the appliance off before any test is run.
• Close the appliance gas shutoff and prove the control circuit dead before any panel, burner assembly or collector box comes off, using live-dead-live per NFPA 70E-2021, 120.5, with the electrical work itself controlled by 29 CFR 1910.333(b)(2).
• Sheet metal edges inside a heat exchanger compartment cut through a thin glove. Cut-resistant gloves and eye protection for any camera or mirror work inside the cells.
• An older furnace cabinet may contain insulation or gasket material you cannot identify. Do not cut, grind or abrade it for a better view; work around it. Disturbing it is a separate scope with its own respiratory control, not something to improvise in a basement at night.

Scope

Covers gas-fired forced-air furnaces and gas package units on residential and light-commercial service, from first suspicion through the tag, the notification and the handoff. Does not cover boilers, oil-fired equipment or commercial makeup air units. Does not cover the response to a sounding CO alarm, owned by the carbon monoxide alarm response visit SOP. Does not cover the no-heat diagnosis that usually precedes this one, owned by the no-heat emergency call response SOP; this procedure begins where that one hands off.

Roles and handoffs

Procedure

1. Secure the appliance before you investigate, if anything is actively wrong. Ambient carbon monoxide elevated, visible spillage at a draft hood, or occupants reporting symptoms means the appliance goes off and stays off while you work. Acceptance: appliance off, space ventilated, occupants clear of the mechanical space. Wrong looks like running it through repeated cycles for the camera while a family sits upstairs. Where nothing is actively wrong, the appliance may run for the tests below, with the monitor on you.

2. Take the full baseline before you form an opinion, because most of these calls are not heat exchangers. Ambient carbon monoxide with the appliance off, flue carbon monoxide air-free once combustion stabilizes, draft, temperature rise measured supply minus return with probes out of the line of sight of the exchanger, and total external static pressure. Acceptance: five recorded values, each written next to the limit or range it is judged against. Wrong looks like opening the burner compartment first, which is how a tech spends an hour looking at metal and never learns the rise was 20 F over the rating plate.

3. Inspect visually and tie every image to a named location. Camera or mirror through the burner ports and, where the design allows access, from the blower side, working cell by cell. Acceptance: images labeled by cell number and position, and a note of every area you could NOT see. Wrong looks like a photo of unspecified metal, which proves nothing to the reviewer and nothing to a customer who asks a second company. What you could not inspect belongs in the file, because an uninspected area is not a clean area.

4. Run at least one corroborating test that a different tech could repeat. Watch the burner flames as the blower energizes and again as it de-energizes, over at least three full cycles, looking for repeatable disturbance, lifting or rollout that tracks blower operation. Where the manufacturer's service literature describes a pressure, vacuum or tracer test for that model, use it. Acceptance: the observation written as what happened, on how many cycles, and at what point. Wrong looks like a single cycle and a judgment word. Never introduce water or a chemical tracer into a combustion path unless that manufacturer's own literature calls for it.

5. Eliminate the mimics, one at a time, and write down each elimination. Restricted airflow raising temperature rise, a blocked or undersized flue, a cracked collector box or induced-draft housing rather than the exchanger itself, a leaking evaporator drain pan dripping onto the cells, and a space depressurized by a kitchen or bath exhaust fan pulling flue gases back down. Acceptance: each mimic named and recorded as present or eliminated, with the measurement that settles it. Stop rule: an unresolved mimic blocks condemnation, full stop.

6. Apply the evidence standard, and apply it as written. Condemn when EITHER a through-wall opening is directly observed and photographed at a named location, OR two independent tests both indicate combustion products entering the airstream, AND in both cases every mimic in step 5 has been eliminated and recorded. A single visual suspicion with no corroboration is a recommendation to investigate further, not a condemnation. Acceptance: the file satisfies one of the two limbs plus the mimic clause. Stop rule: if it does not, the appliance is not condemned today; write what is missing and what it would take to settle it.

7. Tag it, disable it, and notify, in that order and without a gap. Attach the shop's tag at the appliance with the date, the tech's name and the reason. Close the appliance gas shutoff and tag it closed. Disconnect and tape off the thermostat wire at the furnace so a well-meaning occupant cannot restore it with a switch. Acceptance: the appliance cannot be returned to service without deliberate action, and the customer has heard it in person and has it in writing. Notify the gas utility where local practice or code requires it, and record who you spoke to and when.

8. Hand off the file and the heat plan before you leave. The evidence file goes for independent review; the replacement conversation happens only after concurrence is recorded. Acceptance: temporary heat arranged or documented as declined, with electric heaters placed at the manufacturer's clearance to combustibles and plugged into wall receptacles rather than extension cords, and freeze-protection instructions given where outdoor temperatures warrant. Wrong looks like a condemnation and a quote delivered in the same breath by the same person.

The record this produces

One condemnation file per appliance, not per ticket, and it outlives the equipment:

• Ambient CO with the appliance off, and again while firing, with times
• Flue CO air-free with the limit it is judged against, before and after any mimic correction
• Draft, temperature rise against the rating plate range, total external static against the rated maximum
• Images by cell and position, plus the written list of areas that could not be inspected
• The corroborating test, what was observed, and over how many cycles
• Every mimic named, with present or eliminated and the measurement behind it
• Which limb of the step 6 standard was satisfied
• Tag number, date, gas shutoff closed, thermostat disabled, who was told and when, utility notification
• The reviewer's written concurrence

The reviewer reads it to concur or send it back. The customer's second opinion reads it, which is exactly the audience it should be written for.

Worked pass: 18-year-old induced-draft furnace, one CO alarm event

Step 1: ambient carbon monoxide reads 0 ppm with the appliance off. Nothing actively wrong, so the furnace may run for testing with the monitor on the tech.

Step 2 baseline: flue CO 240 ppm air-free, temperature rise 74 F against a rating plate range of 35 to 65 F, total external static 0.88 in wc against a rated maximum of 0.50 in wc. Draft acceptable.

Step 6 FAILS on the first pass. There is a suspicious shadow at the top bend of cell 2 on camera, and the flue CO is high, but static pressure and rise are both well outside their limits, so the airflow mimic in step 5 is unresolved. Stop rule taken: no condemnation, no tag, no quote. The tech writes what is missing: a repeat of the full baseline after airflow is corrected.

Airflow corrected: undersized filter rack replaced with a properly sized media cabinet, and a supply damper closed to a remodeled room re-opened. Re-measured: static 0.44 in wc, temperature rise 52 F, both inside their limits. Flue CO re-measured at 95 ppm air-free, roughly 60 percent below the 240 ppm first reading.

Step 3 and 4, second pass. With airflow correct, camera work at the top bend of cell 2 shows a through-wall opening, photographed and labeled cell 2 upper bend, with the note that the lower third of cells 1 and 3 could not be seen. Flames at cell 2 lift and distort repeatably within two seconds of blower energizing, over four consecutive cycles, and settle when the blower stops.

Step 6, second pass: limb one is satisfied, a photographed through-wall opening at a named location, and limb two is independently satisfied as well by the repeatable blower-linked flame disturbance. Every mimic in step 5 is recorded eliminated. Condemned.

Note what the 95 ppm figure does and does not do. It sits well under the 400 ppm air-free limit the ANSI Z21 series sets for listed vented appliances, and it is not a reason to release the furnace. That limit describes combustion quality in the vent, not containment between the flue gas and the air the family breathes.

Step 7: tagged, gas shutoff closed and tagged, thermostat wire disconnected and taped at the furnace, customer told in person with the images on screen and given the written file. Step 8: manager concurrence recorded before any replacement conversation; two electric heaters placed at manufacturer clearance on wall receptacles.

Checking the pass against the sections above: the step 6 standard requires a limb AND the mimic clause, and the first pass is reported as failing on the mimic clause even though a limb looked satisfied, which is the Boolean as written. The 240 and 95 ppm readings are both flue CO air-free at the same probe location, so the 60 percent comparison is between like quantities, and the post-correction baseline was re-taken in full rather than assumed to have moved with the airflow fix.

References

• ANSI Z21 series listing limits for carbon monoxide in flue products of listed vented appliances, at 400 ppm air-free, and the manufacturer's own limit where it is lower
• Rating plate and manufacturer's service literature for the rise range, the rated maximum external static, and any model-specific heat exchanger test
• NFPA 54, the National Fuel Gas Code, in the edition your authority having jurisdiction has adopted, for venting and shutoff requirements that reach you through the permit
• 29 CFR 1910.333(b)(2) and NFPA 70E-2021, 120.5, for de-energizing and proving dead
• See related: the no-heat emergency call response SOP, and the carbon monoxide alarm response visit SOP