Carbon Monoxide Alarm Response Visit

Purpose

This standing instruction answers, in order, the only two questions a carbon monoxide alarm raises: is this building safe to occupy tonight, and what produced the exposure. It answers the first before it starts on the second.

The habit it exists to break is the one every shop drifts into. A tech arrives hours after the alarm, reads zero parts per million at the furnace, writes "no CO found, alarm faulty," and leaves. That reading is real and the conclusion is wrong. A listed residential alarm is built to respond to a dose over time, not to an instantaneous concentration, so an alarm that sounded at two in the morning and a house that reads zero at eight are entirely consistent with a real event that has since ventilated. Closing the call on the zero is how a family gets a second event with the same appliance and no alarm left that anybody trusts.

Safety actions that gate everything else

• On the call, before anything else: everyone leaves the building and stays out, and they call 911. Anyone with headache, nausea, dizziness, confusion or unusual sleepiness goes with them and gets medical attention, because those symptoms are the exposure, not the worry about it. No windows opened first, no hunting for the source, no waiting for the tech.
• Do not enter a building with a sounding alarm that the fire department has not cleared. If they have been and gone, enter with your own monitor on and reading, and leave immediately if it alarms.
• Your own exposure limit is not the family's habitability standard. The OSHA permissible exposure limit of 50 ppm as an 8-hour time-weighted average, at 29 CFR 1910.1000 Table Z-1, governs you as an employee. It does not describe what is acceptable for a household, and it must never be quoted to a customer as a safe level in their home.
• Ventilate before you test, only after you have taken and recorded the as-found ambient readings. Opening a door first destroys the one measurement that describes what the occupants were in.
• Close the gas shutoff and prove the control circuit dead before opening any appliance, live-dead-live per NFPA 70E-2021, 120.5, with the electrical work controlled by 29 CFR 1910.333(b)(2).

Scope

Covers a residential or light-commercial response to a carbon monoxide alarm that has sounded, or to occupant symptoms consistent with exposure, from the phone call through the written report.

Does not cover the condemnation evidence standard when the source turns out to be a heat exchanger, owned by the cracked heat exchanger condemnation and red-tag SOP. Does not cover a routine no-heat diagnosis, owned by the no-heat emergency call response SOP. Does not cover industrial combustion equipment or commercial kitchen makeup air balancing.

Roles and handoffs

Procedure

1. Give the evacuation instruction on the call, in plain words, before you take any details. Everyone out, stay out, call 911, and anyone with symptoms gets medical attention. Acceptance: the caller confirms people are leaving. Stop rule: nothing else on this ticket happens until that is confirmed. Wrong looks like asking about the furnace age first, which is what a technically minded call taker naturally does and which costs the minutes that matter.

2. Take as-found ambient readings before you open a door, touch a thermostat, or start a fan. Work a fixed route so every visit is comparable: outdoors upwind first as a zero reference, then the mechanical room, then the room with the alarm, then each bedroom, then an attached garage. Acceptance: a ppm value and a time at every location on the route, outdoor reference through attached garage, with the bedroom count stated. Wrong looks like one reading beside the furnace. If any reading rises while you stand in it, leave and ventilate before continuing.

3. Interrogate the alarm itself rather than judging it. Record its manufacture date, its location relative to the appliances and to sleeping areas, and any stored peak reading its memory function reports. Acceptance: age, position and peak value recorded. A listed alarm to UL 2034 is designed to respond to concentration over time, sounding at 70 ppm within roughly one to four hours and at 400 ppm within minutes, and is designed NOT to alarm below 30 ppm across a month, so a sounding alarm and a later zero reading are consistent with a real event.

4. Inventory every combustion source and every non-appliance source in the building. Gas or oil furnace, water heater, boiler, range, oven, dryer, fireplace, space heater, attached garage, generator, and any recent change to the building: a new range hood, a finished basement, a replaced window set, a sealed attic. Acceptance: a written list with the fuel and vent type for each. Wrong looks like assuming the heating equipment, which is often innocent, especially in a house where a vehicle idles in an attached garage.

5. Test each fuel-burning appliance under worst-case depressurization, not under easy conditions. Close exterior doors and windows, run every exhaust device at once (bath fans, range hood, dryer, any whole-house fan), close interior doors to bedrooms, then fire each appliance in turn and watch for spillage at the draft hood or barometric damper with a smoke source, while your monitor sits at the appliance. Acceptance: no spillage persisting past the draft establishment time your program or the manufacturer states, and no ambient rise at the appliance. Stop rule: an appliance that spills under worst-case is shut off and tagged before you move to the next one.

6. Where a source is identified, shut it off, tag it, and name the mechanism, not just the appliance. Blocked or disconnected vent, undersized or blocked combustion air opening, a house depressurized past what an atmospherically vented appliance can overcome, an over-fired burner, or a compromised heat exchanger. Acceptance: the mechanism written in a sentence a different tech could act on. Wrong looks like "water heater bad," which tells nobody whether the fix is a vent, an air opening, or an appliance.

7. Where no source is found, that is a documented finding, not a dismissal. Record every reading, every appliance tested and how, and state plainly that a source was not identified today. Acceptance: written recommendations covering alarm replacement where the unit is beyond its service life, placement corrections, and a low-level monitor that displays and logs values a listed alarm is designed not to annunciate. Wrong looks like "false alarm," a phrase that has no place in this report and that a shop should never let leave a truck.

8. Give the occupant instruction in writing before you leave, and say what re-occupancy depends on. Acceptance: the customer holds a written report naming what was found, what is off and tagged, what must be corrected before the appliance returns to service, and who to call if the alarm sounds again. Where an appliance is off and the weather is cold, the freeze-protection and temporary heat provisions of the no-heat SOP apply, with electric heaters at manufacturer clearance on wall receptacles rather than extension cords.

The record this produces

A CO response report, filed to the property rather than to a single appliance:

• Sounding time, who evacuated, whether 911 and the fire department attended
• As-found ppm and time at every route location, before ventilation
• Alarm manufacture date, location, and stored peak
• The combustion source inventory with fuel and vent type
• Worst-case configuration used (which exhaust devices, which doors), and per-appliance spillage and ambient results
• The mechanism, where a source was found, plus tag number and shutoff state
• Where none was found, the explicit statement of that and the recommendations issued
• The written occupant instruction and the re-occupancy conditions

The manager reads it before any follow-up is booked. The customer keeps a copy, and that copy is what a physician, an insurer or a landlord will ask for. The next tech reads the worst-case configuration so a re-test is run the same way rather than an easier way.

Worked pass: alarm at 02:10, family out, tech on site at 08:15

Step 1: caller confirmed everyone outside and 911 called. Fire department attended, measured, and released the building before the shop arrived.

Step 2 as-found route: outdoors 0 ppm, mechanical room 0 ppm, hallway at the alarm 0 ppm, both bedrooms 0 ppm, attached garage 0 ppm, all between 08:15 and 08:24. Recorded before any window was opened.

Step 3: the alarm carries an 11-year-old manufacture date, past the service life printed on it, and its peak memory reports a value in the low three digits. Two findings, and neither closes the call: an old alarm still sounded, and it sounded on something.

Step 4 inventory: 92 percent condensing furnace with a sealed combustion intake, atmospherically vented natural gas water heater sharing a masonry chimney, gas range, electric dryer, attached garage. Recent change: the basement was finished last spring and a high-capacity range hood was added.

Step 5 FAILS. Worst-case set with both bath fans, the range hood on high, interior bedroom doors closed and all exterior openings shut. The furnace fires and shows no spillage and no ambient rise, which is what a sealed-combustion appliance should do. The water heater then fires and spills continuously at the draft hood well past its draft establishment time, and the monitor sitting on the water heater jacket rises from 0 to 38 ppm over 6 minutes. Stop rule taken: water heater shut off and tagged immediately, worst-case configuration reversed, building ventilated, and the remaining survey completed only after ambient returned to 0 ppm.

Step 6 mechanism: the basement finish enclosed the free area that had been serving as combustion air for the water heater, and the new range hood added exhaust capacity the house cannot make up. The atmospherically vented water heater is the appliance that loses that contest, and the sealed-combustion furnace is not affected, which is exactly why step 5 tests each appliance rather than the room.

Step 8: written report issued, water heater gas shutoff closed and tagged, scope written for dedicated combustion air sized to the appliance or a sealed-combustion replacement, alarm replacement recommended on age with a low-level monitor added, and re-occupancy stated as unconditional for the house with the water heater remaining off.

Checking the pass against the sections above: step 2's acceptance condition is a reading and a time at six named locations, and all six are printed, including the zero outdoor reference the route calls for. The alarm's age is reported as one finding among several rather than as the answer, which is the position step 3 takes. The 0 to 38 ppm rise is at one fixed location on the water heater jacket across 6 minutes, not a comparison between two different rooms.

References

• UL 2034, the standard for single and multiple station carbon monoxide alarms, for the alarm response curve and the design intent that alarms do not annunciate low chronic levels
• 29 CFR 1910.1000 Table Z-1 for the technician's own permissible exposure limit, which is an employee limit and not a household habitability standard
• NFPA 54, the National Fuel Gas Code, in the edition your authority having jurisdiction has adopted, for combustion air and venting requirements that reach you through the permit
• See related: the cracked heat exchanger condemnation and red-tag SOP, and the no-heat emergency call response SOP