Carbon Monoxide and Exhaust Safety on a Truck Mount

Purpose

A truck mount is an internal combustion engine running two to four hours at a time, 20 or 30 feet from an occupied house, with a hose through a door somebody propped open. Carbon monoxide has no color and no smell, the early symptoms read as a long day, and by the time a person is confused they cannot get themselves out. It is the hazard in this trade where the tech's own senses give no warning.

This standard puts an instrument between the crew and that gas, sets two separate action levels because the people in the house are not workers, and makes the response unconditional so nobody has to decide anything while impaired.

If a monitor alarms or anyone has symptoms, do this first

Headache, dizziness, nausea, odd tiredness or confusion in anyone inside or at the van is treated as carbon monoxide until proven otherwise, alarm or no alarm.

1. Everyone leaves the building and the area around the van, on foot, at once. Do not stop to shut windows, gather tools or find the source.
2. Shut the engine down only if the switch is on your way out and takes one second. Otherwise leave it running and go.
3. Count heads outside, upwind and clear of the van, and account for children and pets by name with the customer.
4. Call 911 from outside and say carbon monoxide, engine exhaust, and how many were inside. Anyone with symptoms goes to medical care, because symptoms improve in fresh air while the exposure is still doing harm.
5. Nobody re-enters until the fire department releases the building. Not to shut off the machine, not to retrieve a wand.
6. Call the shop manager from outside. The job is over and the unit does not run again until the exhaust path is inspected.

Scope

Covers exhaust placement, monitoring, response and shutdown for any engine-driven truck mount or generator run at an occupied residential or light commercial site.

Does not cover: the machine startup and shutdown sequence (the startup SOP, which gates its own step 1 on this one); hose routing through the doorway (the hose run SOP); and cord-connected portables, which produce no exhaust and are the answer in every enclosed space step 2 rules out.

Roles and responsibilities

Procedure

1. Two monitors go live before the key turns. A personal CO monitor at the tech's collar and a second set inside the house at breathing height, near the hose door. Accept: both power up and self-test, both calibration dates in the future, personal low alarm set at 35 ppm. Wrong: treating the customer's ceiling alarm as the instrument - alarms listed to UL 2034, in the edition they are listed to, are built not to nuisance-alarm at low levels and may take 60 to 240 minutes to sound at 70 ppm. Stop rule: no working monitor, no engine start. Hazard: an instrument is the only warning for this gas, so a monitor left in the other van is a reason to reschedule, not a reason to be careful.

2. Set the van position on distance, wind and openings, never on convenience. Take 20 ft from any window, door or vent as the floor, which is the distance CPSC and engine manufacturers commonly give for portable engines with exhaust pointed away, and treat it as a floor rather than a safe number. Accept: 20 ft or more to the nearest opening, exhaust aimed away, van downwind of the opening the hose uses. Wrong: backing in so the tailpipe sits under a soffit, deck or second-story window. Stop rule: an attached garage, carport, breezeway, underground lot or covered approach is an absolute no - portable or reschedule. Hazard: exhaust does not disperse under a roof, and an engine in an attached garage puts CO through the common wall and door whether the garage door is open or not.

3. Ask what else burns fuel in that building, and who is inside. Note gas water heaters, furnaces, boilers, fireplaces and ranges, plus infants, older adults, pregnant occupants and anyone with heart or lung disease. Accept: appliances and occupants written on the ticket before the engine starts. Wrong: skipping the question because the CO would be yours, not theirs - the blower pulls air out of the house through the hose, and in a tight house that depressurization can pull a natural-draft appliance's flue gases back into the space. Stop rule: a gas appliance sharing a small closet with the hose route in a house with no other opening means the hose door stays wide open as makeup air and the flue is checked for spillage first. Hazard: the occupants this catches worst show symptoms at the lowest concentrations, which is why that list is a field and not a courtesy.

4. Fit the exhaust extension and confirm it is clear end to end. Run it away from the building, downwind, outlet in open air. Accept: seated with no gap at the tailpipe, no crush, no debris at the outlet, outlet not aimed at a neighbor's window or a fresh air intake. Wrong: a hose crushed under a tire or run uphill into a pocket where exhaust pools. Stop rule: a damaged or blocked extension takes the unit out of service, because a blocked outlet backs exhaust into the van box where the tech works. Hazard: it burns on contact within minutes and can ignite dry leaves under it, so it goes on cold, on bare ground, clear of anything combustible.

5. Take a baseline reading five minutes after the engine starts, and record it. Read the in-house monitor at the hose door and the tech's monitor at the van. Accept: in-house reading 0 ppm, no alarm at the van. Wrong: a few ppm treated as background, when nothing else in a clean residence burns fuel. Stop rule: any in-house reading above 9 ppm sustained 2 consecutive minutes stops work and takes step 7; 9 ppm is the EPA 8-hour ambient air quality standard rather than a workplace limit, used here as an investigate-and-move trigger because the occupants are not workers. Hazard: you are in the exhaust stream reading at the van, so read from upwind rather than over the tailpipe.

6. Re-read at every tank change, after any wind shift, and whenever a door or window changes. Accept: in-house reading still 0 ppm at each check, personal monitor silent, readings written with the time. Wrong: assuming the first clean reading holds all day - wind is the variable that moves, and a safe position at nine can be feeding a window by noon. Stop rule: the same 9 ppm sustained gate applies at every re-read, not only at the baseline. Hazard: a monitor detects an exposure, it does not prevent one, so a reading that has climbed has been climbing for as long as the interval between checks.

7. If the in-house gate is crossed, shut down, get people out to fresh air, then reposition. Kill the engine, open the house up, walk the occupants outside, do not restart until the van has moved. Accept: engine off, occupants outside, house ventilated at least 15 minutes, van repositioned farther and out of the wind line, in-house monitor at 0 ppm for 5 continuous minutes before restart. Wrong: opening a window and carrying on, which puts more of the same gas into a space you just proved collects it. Stop rule: a reading that will not return to 0 ppm after repositioning ends the job and sends the unit in for exhaust inspection. Hazard: you are re-entering a space with a known elevated reading, so do it fast, monitor in hand, never alone if anyone is unsteady.

8. Treat a personal alarm or any symptom as the response block at the top of this SOP, not as a reading to interpret. The 35 ppm low alarm setpoint comes from the NIOSH recommended exposure limit, which is a NIOSH recommendation and not an OSHA limit; OSHA's general industry permissible limit is 50 ppm as an 8-hour time-weighted average at 29 CFR 1910.1000 Table Z-1, construction counterpart at 1926.55. Accept: on any alarm or symptom, the response block runs, with no arithmetic first. Wrong: weighing a live alarm against an 8-hour average, which describes a whole shift and says nothing about the next ten minutes. Stop rule: there is no threshold at which a tech works through an alarm. Hazard: confusion is a symptom of this gas, so the decision gets made before anyone is in a position to make it badly.

9. Shut down, restore what you opened, and log it. Cool and stop the engine under the startup SOP, stow the extension once cool, close any window you opened for makeup air, confirm every CO alarm in the house is normal. Accept: extension stowed cold, every window you opened closed again and counted against the number opened, no customer alarm silenced or removed at any point, monitors at 0 ppm at stow. Wrong: leaving a customer's alarm muted because it chirped during the job. Stop rule: a customer alarm that sounded at any point sends everyone out under the response block above; it never means the alarm gets silenced. Hazard: the extension stays hot after shutdown, so gloves, and only once the pipe can be touched with the back of a gloved hand.

The record this produces

The ticket carries: both monitor serial numbers with calibration dates, the personal alarm setpoint, measured tailpipe-to-opening distance, wind direction at start, the building's fuel-burning appliances, the occupant list with anyone at higher risk flagged, the baseline reading with its time, each re-read with its time, any gate crossing with what was done, and windows opened and closed.

The distance and wind fields make an incident reconstructable months later, and they let the office build a per-address profile: a driveway that forced a 22 ft park once will force it again, and that address gets flagged rather than renegotiated by whoever shows up.

Worked pass, with a step that failed

Two story residential, attached garage present and not used. Van on the street, tailpipe measured at 22 ft from the nearest open window, wind from the west putting the van downwind of the propped front door. Personal monitor on the collar, low alarm at 35 ppm; second monitor on a chair 6 ft inside the hose door. Step 3 recorded a gas water heater and a gas range, plus two adults and a 6 month old, so the infant was flagged. Step 4: extension fitted, outlet clear, aimed away from the house and the neighbor's side. Step 5: baseline at 5 minutes read 0 ppm inside, nothing at the van.

Step 6 failed at the second tank change, about 90 minutes in. The wind had backed to the north and the in-house monitor read 14 ppm, holding for the full 2 minutes the gate requires. Against the 9 ppm sustained trigger, 14 ppm is over and the acceptance failed.

Step 7 ran as written. Engine off, both adults and the baby out to the front yard, front and back doors open, 15 minutes of ventilation. The van moved from 22 ft to a measured 40 ft on the far side of the driveway, tailpipe pointed down the street rather than across the yard. The in-house monitor came back to 0 ppm and held for 5 continuous minutes before restart, which is the step's own restart condition and the only reason restarting was allowed.

The remaining hour ran with re-reads every 20 minutes, all at 0 ppm. At shutdown the extension was stowed once cool, the two windows opened during ventilation were closed again, two opened and two closed, and no alarm in the house was touched at any point. The shop manager was called from the yard during the ventilation window, not afterward.

References

• See related: Truck Mount Startup and Shutdown Standard SOP - its step 1 defers van position and monitoring here, and its step 9 owns the cool-down step 9 uses.
• See related: Hose Run and Door Protection Standard SOP - owns the propped door that is also this SOP's makeup air path.
• See related: Portable Extractor Setup in a High-Rise SOP - the exhaust-free alternative for every space step 2 rules out.
• 29 CFR 1910.1000 Table Z-1 for OSHA's general industry 8-hour permissible limit for carbon monoxide, with 29 CFR 1926.55 as the construction counterpart; the NIOSH recommended exposure limit behind the 35 ppm setpoint is a NIOSH recommendation, not an OSHA limit.
• UL 2034, in the edition a given residential alarm is listed to, for why a household alarm is not a work instrument; CPSC and engine manufacturer guidance for the step 2 distance.