Key Takeaways
- Sort the hazard before you touch the gas: An immediate hazard and a condition that can wait for repair call for different actions, different notices, and different timelines. Know which one you found and say so in writing.
- Numbers carry the shutdown: Ambient CO, as-measured and air-free flue CO, O2, and what happens when the blower starts are the readings a second tech can repeat. A tag without them is an opinion.
- Photos and paper finish the file: Video of the defect, model and serial, a timestamped analyzer report, the action taken on the gas supply, and a signed notice turn a tense kitchen-table moment into a record.
- A good red tag welcomes a second opinion: When the evidence is solid, another qualified tech with an analyzer will find the same thing. Hand the homeowner the file and let the data do the persuading.
Every October the homeowner HVAC forums fill up with the same post. A tech came out, said the furnace is unsafe, shut it down, and quoted a replacement. Now the homeowner is posting photos and asking strangers whether they got scammed. One thread this month about a cracked heat exchanger and a zip-tied gas line drew more than 600 replies.
Plenty of those tags are correct. The problem is that many arrive as a verdict with no evidence attached, and a homeowner staring at a five-figure quote fills that gap with suspicion. The U.S. Consumer Product Safety Commission’s latest estimate counts 274 consumer-product carbon monoxide deaths in 2022, and heating systems were linked to 76 of them.¹ Canadian public health figures put CO deaths at about 300 a year.² A red tag that gets ignored because nobody believed it protects no one. The fix is to build every shutdown like a case file.
Sort the Finding Before You Touch the Valve
Not every unsafe condition is the same emergency, and the rules treat them differently. Ontario’s gas regulation, O. Reg. 212/01, calls both an unacceptable condition and splits them by whether there is an immediate hazard.
With an immediate hazard, the certificate holder shuts off the gas, tells the distributor by phone right away, gives the user written notice not to use it, tags it, and follows up in writing within 14 days. Only someone certified for that work may remove the tag. Without an immediate hazard, the gas stays on, the same notices and tag apply, and the distributor sets a deadline of no more than 90 days.³ Enbridge Gas’s notice gives 42.⁴
In the US, shutoff duties and disconnect authority depend on state law and the utility, so know yours. Either way, make the appliance safe at its own valve with the owner’s knowledge, document what was found, and call the utility when the hazard reaches past the appliance.
For ambient readings, BPI-1200 gives a clear ladder. From 9 to 35 ppm, tell the occupants, open windows and doors, and recommend checking sources and service. From 36 to 69 ppm, also recommend turning off every possible source right away. At 70 ppm and above, stop, evacuate, and call emergency services from outside.⁵ Under 9 ppm, keep testing. A quiet household CO alarm proves less than homeowners think, because UL 2034 alarms are not required to sound until 70 ppm, and even then only after 60 to 240 minutes.⁶
Numbers First: The Readings That Carry a Shutdown
Start with ambient CO at the door and again at the appliance. Then take flue readings the way a second tech would: analyzer zeroed in clean outdoor air, appliance run to steady state before sampling, probe in the hot spot at the manufacturer’s sampling location. Use an access tee or the appliance’s test port. Some older procedures drill the PVC, which certified vent makers such as IPEX prohibit, and a hole under foil tape is a future leak. If the analysis basics need a refresher, the Thinking Tech’s Guide to combustion analysis covers what each number means.
Record CO both as measured and air-free. Air-free CO corrects for dilution: CO measured × 20.9 ÷ (20.9 − O2). A furnace reading 60 ppm at 9% O2 works out to about 105 ppm air-free.⁷ BPI-1200 sets the flue threshold at 400 ppm air-free for central furnaces and boilers and 200 ppm for water heaters and vented room heaters.⁵ Many manufacturers want much lower. Carrier’s 2009 heat exchanger bulletin for its 90% furnaces measures CO at high fire, mid-stream in the vent, after at least five minutes of run time. On the models it covers, under 200 ppm with excess air above 30% means no further heat exchanger inspection, unless the flame changes when the blower starts, which calls for a physical inspection.⁸
The heat exchanger evidence techs skip most often is the blower test. With the probe steady, watch O2 and CO as the blower comes on. A breach that lets supply air into the combustion side shows up as a shift in those numbers. Jason Esteves walked through how small that O2 change can be in his episode on cracked heat exchangers. The carbon monoxide field guide has the full ambient-to-flue testing sequence.
Pictures and Paper: The Rest of the File
Numbers explain the risk. Pictures show it. Carrier’s procedure is blunt: any visible split, crack, or hole means the heat exchanger gets replaced, and the inspection uses a borescope and a mirror. Water, oil, and smoke leak tests are out.⁸ Get the camera into the cells, record video, and narrate what you are looking at. The heat exchanger warranty claims guide covers the same photo habits for getting a claim approved, and they serve double duty here.
A file that holds up contains:
- Date and time on every reading
- Ambient CO at the door and at the appliance
- As-measured and air-free CO, O2, and stack temperature, plus where the probe sat
- Model and serial number
- Photos and video of the defect
- Exactly what was done to the gas supply, and how the appliance was secured
- The written notice, the homeowner’s signature, and who at the utility was called
A timestamped report from the analyzer app, emailed with the work order, beats a thermal printout left in the cabinet that the homeowner never sees.
What a Weak Red Tag Looks Like
Weak tags share a pattern. “It’s 25 years old” is a reason to start planning a replacement. Age alone does not justify shutting off the gas. Rust alone does not condemn a heat exchanger. Carrier’s procedure cleans inlets partly blocked by corrosion and passes them.⁸ A flame that “looks lazy” with no analyzer reading attached is a hunch. A tech who will not show the photos, or leaves no readings behind, invites the exact forum thread nobody wants.
The opposite mistake is real too. Lennox notes that a crack is not always visible, which is why combustion evidence matters in both directions.⁹ Clean numbers on a furnace with a suspicious seam deserve a closer look, not a pass. The goal is a finding anyone can repeat.
None of this is anti-replacement. A failed heat exchanger on a 20-year-old furnace usually means a new system, and selling that system is honest work. The problem is the unsupported scare that turns a correct call into a credibility fight. The furnace tune-up piece makes the same case for inspections built on findings instead of sales scripts.
Hand Them the File and Invite the Second Opinion
Build the file before you write the tag
Enter what you measured and saw on this call. The tool works out air-free CO, checks your readings against published action levels, suggests how to classify the finding, lists what your file is still missing, and prints a one-page notice for the homeowner.
Leave anything you did not test blank. Blanks show up as gaps, which is the point.
Load an example (for training)
Ambient CO and occupants
BPI ambient band
—
Enter a reading
Alarm check: UL 2034 home alarms are not required to sound until 70 ppm, and then only after 60 to 240 minutes. A quiet alarm does not mean low CO.
Flue readings (test in, before you clean or adjust)
Air-free CO
—
CO × 20.9 ÷ (20.9 – O2)
Certification limit
—
ppm air-free
Vent rating check
—
Stack temp vs vent rating
Heat exchanger and venting
Evidence captured
Rust is not a breach. Carrier’s 90% furnace inspection procedure replaces a heat exchanger for any visible split, crack, or hole, and cleans inlets that are only partly blocked by corrosion.
What you did
When the evidence is solid, a second opinion helps you. Give the homeowner the readings, the video, and the notice, and tell them plainly that any qualified tech with a calibrated analyzer should find the same numbers. That one sentence takes the sting out of “is this a scam” faster than any sales technique.
It also protects the business. If the homeowner overrides the tag, the file shows what was found, what was explained, and what was signed. If the furnace gets replaced, the same file becomes the baseline for the new install, and clocking the meter at startup closes the loop.
The furnaces that hurt people are rarely the ones the forums argue about. They are the ones that got a quick look, no readings, and a shrug. Build the file every time, and the red tag carries its own proof.
Additional Sources
- “Non-Fire Carbon Monoxide Deaths Associated with the Use of Consumer Products: 2022 Annual Estimates”, U.S. Consumer Product Safety Commission, Statistical Report, 2026.
- “Poisonings and Injuries Related to Carbon Monoxide Exposure: Data Blog”, Public Health Agency of Canada, Data Report, 2023.
- “O. Reg. 212/01: Gaseous Fuels”, Government of Ontario (Technical Standards and Safety Act), Regulation, 2001 (as amended).
- “Safety Violation Notice: Corrections Required (IS_F_234, Third Party)”, Enbridge Gas, Utility Notice Form, n.d. (accessed 2026).
- “ANSI/BPI-1200-S-2017: Standard Practice for Basic Analysis of Buildings”, Building Performance Institute, Standard, 2017.
- “Smoke and CO Alarm Certifications Explained”, First Alert (BRK Brands), Manufacturer Guide, 2025.
- “‘Air Free’ CO or ‘As Measured’ Carbon Monoxide?”, Jim Davis, National Comfort Institute, High-Performance HVAC Today, 2017.
- “90% Furnace Heat Exchanger Inspection Procedure (DSB 09-0022)”, Carrier Corporation, Dealer Service Bulletin, 2009.
- “What to Do If Your Furnace Has a Cracked Heat Exchanger”, Lennox International, Manufacturer Guidance, 2025.
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