The alarm went off: carbon monoxide response and red-tag. Episode thirty-four of the Gas Service Animated series.

When a carbon monoxide alarm sounds, people come first. Get every occupant out to fresh air. If anyone feels sick, or your monitor shows seventy parts per million or more, call nine one one, and diagnose after they are safe. Treat every alarm as real. It costs a few minutes to be wrong, and it can cost a life to guess.

A household alarm follows U L two thousand thirty-four. At seventy parts per million it must sound within one to four hours. At one hundred fifty, within ten to fifty minutes. At four hundred, within four to fifteen minutes. So a sounding alarm means levels already well above the few parts per million your personal monitor would flag. It is a late warning, not an early one.

But the code alarm and your monitor are not the same tool. U L two thousand thirty-four alarms are deliberately desensitized — below thirty parts per million, sustained, they must not sound at all. Your low-level personal monitor catches carbon monoxide down around five to thirty-five parts per million, long before a household alarm ever trips. A silent household alarm does not mean zero.

Once people are safe, find the source using the isolate and analyze procedure from episode thirty-two. Shut every fuel-burning appliance off, then bring them back one at a time while you watch the reading climb. Confirm with combustion analysis in the flue — as-measured carbon monoxide, corrected to air-free. Never confirm with a combustible gas indicator.

Now the shutdown call. Three findings force it. A confirmed cracked heat exchanger, where the breach disturbs the flame and leaks products into the air stream. Flue carbon monoxide over the air-free limit — four hundred parts per million air-free for most furnaces, corrected with the oxygen formula, not the raw reading. Or spillage at the draft hood that will not clear. Any one of these, and the appliance comes off.

Then you tag it. Red-tag and lock out per your utility's written policy. Tagging thresholds are a utility and jurisdiction convention, not one N F P A fifty-four number. As a common pattern, a yellow tag runs roughly one hundred five to two hundred fifty parts per million as-measured, and a red tag above two hundred fifty as-measured — but these vary by utility. When in doubt, shut down and escalate.

Document everything, before and after. Record the as-measured carbon monoxide. The oxygen. The air-free carbon monoxide. The draft. And your worst-case zone pressure. Combustion analysis bookends the repair — a baseline reading before you touch it, and a verification reading after the fix. Numbers written down protect the occupant, protect the next technician, and protect you.

And do not re-energize on a hunch. A tagged appliance goes back in service only two ways: after the fault is repaired, or the unit is replaced — and then re-verified by combustion analysis. Not because it looks fine. Not because the alarm went quiet. Confirmed by the analyzer, corrected to air-free, and documented. That is the whole job.

Carbon monoxide response: people to fresh air first. The alarm is a late warning, not early. Confirm with the analyzer, never a combustible gas indicator. Then shut down, tag, document, and re-verify. Next episode: the complete series.