The Customer Collapsed and the Furnace Was Running

Why this matters

When a person collapses in a house where a combustion appliance is running, the most likely explanation is still a heart or a fall. But carbon monoxide is the explanation that kills the rescuer too, and it is invisible, odorless and gives the same first picture: a person down, maybe confused before they fell, maybe a headache they mentioned at the door. So the order on this call is fixed and it does not bend for compassion: get out, get 911 coming with breathing apparatus, keep everyone out, and only after the house is released think about why. This case follows a real-shaped call from the doorway to the cause, which in the end was not the appliance the tech had come to look at. Any tech who works near gas, oil, wood or propane appliances (plumbers, electricians, cleaners and pest techs included) can be the first one through that door.

The first ninety seconds

A tech arrives at 8:40 am for a furnace tune-up on a cold morning, 22 F outside. The customer, a woman in her sixties, answers slowly, says she has "a splitting headache, must be a migraine", and walks the tech toward the basement stairs. Halfway down the hall she sits on the floor, then slumps sideways.

The tech's personal CO monitor, clipped to the shirt pocket, chirps as the tech kneels beside her. It reads 140 ppm. The shop's alarm is set at 35 ppm.

What the tech did, in order:

1. Stood up and backed out to the front door, which was still open. The reading at the threshold dropped to 30 ppm.
2. Called 911 from the front step, not the hallway: "Woman collapsed, I'm a heating tech, my CO meter read 140 in the house." That sentence matters. "Possible carbon monoxide" sends the fire department, and the fire department brings self-contained breathing apparatus (SCBA, the air tanks firefighters wear) and its own calibrated meters.
3. Asked the dispatcher whether to go back in for her. The dispatcher's answer was the right one: she was about fifteen feet inside, the door was open, the tech's reading at her position had been 140 ppm and climbing. The dispatcher told the tech to hold their breath, drag her by the shoulders straight to the porch, and not go any further into the house. Fifteen seconds in, fifteen out.
4. Did not go to the basement to shut the furnace off. That is the move every heating tech wants to make and it is the wrong one: the basement was the likely source, the reading would be higher there, and the fire department would ventilate and shut down the appliances themselves.
5. Started on her outside. She was breathing. Recovery position on the porch, a moving blanket from the truck under her, checked every minute until the engine company arrived at 8:47.

What would have flipped step 3. Had she collapsed in the basement beside the furnace, or had the hallway reading been several hundred ppm, or had the tech felt dizzy or headachy, the answer becomes no rescue: wait outside for SCBA. The NIOSH figure for immediately dangerous to life and health is 1,200 ppm, but impairment starts far below it, and a rescuer who loses judgment halfway down a staircase becomes a second patient. A short, straight drag from an open doorway at a moderate reading is the narrow case where a lay rescue makes sense. Firefighters make every other one.

For the general emergency sequence (911 first, CPR, AED, handoff), see related: The Medical Emergency on a Service Call SOP. This case is about what is different when the cause might be in the air.

Containment: who shuts what, and when

The fire department took the house from 8:47. Their readings, which the tech asked for and wrote down: 180 ppm in the hallway, 410 ppm in the basement utility room, 60 ppm upstairs in the bedroom. They shut the gas at the meter, opened windows, and ran a fan in the front door.

The containment questions the tech had to answer, and the honest answers:

• Should I shut the gas off at the meter myself before they arrive? Only if the meter is outside and you can reach it without going back in. Here it was inside, in the basement, so no.
• Should I touch the furnace or water heater after they release the house? Not until the fire department and, where it responds, the gas utility say so. Some utilities send a service person who red-tags appliances at the meter. Preserve what you found; an insurer or the utility may need it as it was.
• Who calls the family? The customer, if she can, at the hospital. If not, the responders. The office can call an emergency contact listed on the account if responders have not, and says only that she was taken to hospital.

At 9:30 the fire department ventilated the house to under 10 ppm, left the gas off at the meter, and handed the tech a release to examine the appliances with the gas utility's service technician present.

The trace, in the order it was run

The tech had come for the furnace, so the furnace was the first suspect. It was the wrong one, which is why the order matters.

Furnace first. An 80 percent efficiency, natural-draft furnace (one that relies on the buoyancy of hot flue gas rather than a fan to vent) sharing a masonry chimney with a natural-draft gas water heater. With the gas restored and a personal monitor running, the tech fired the furnace with the utility tech watching and ran a combustion analysis. Air-free CO in the flue: 38 ppm, inside the range most manufacturers accept for this type of appliance (check the installation manual for the exact limit; many name 100 to 400 ppm air-free as the maximum). Heat exchanger inspected with a camera: no visible crack. Vent connector sound and sloped up to the chimney, as NFPA 54 requires.

A tech who stopped here would have written "furnace OK" and gone home. That is the failure this case exists to prevent: the appliance that was running when she collapsed was not proven innocent of the house's CO by proving it burns cleanly.

Water heater second. The tech held a smoke pencil at the water heater's draft hood, the open collar at the top where room air is drawn into the flue. With the furnace off and the water heater firing, smoke drew up into the hood. Then the tech turned the furnace blower on.

The smoke rolled out of the hood into the room and kept rolling. The water heater was backdrafting: its exhaust spilling into the house instead of going up the chimney.

      CHIMNEY (shared flue)
        |   ^
        |   |  normal draft
        |   |
   +----+---+----+        UTILITY ROOM
   | DRAFT HOOD  | <- room air pulled up
   |  WATER      |    ...until the blower runs:
   |  HEATER     |    then flue gas spills OUT
   +-------------+    here, into the room
                          ^
   FURNACE   RETURN       |
   [blower]==duct==> open panel, missing
      ^                   filter door: blower
      |                   sucks utility-room
      +--- pulls air out  air, room goes
           of this room   negative

Why the blower did it. The furnace's filter rack door was missing and a return-duct panel in the utility room had been left off. With the blower running, the return was pulling air out of the small utility room faster than it could leak back in, so the room went negative: lower air pressure than the rest of the house and the outdoors. A natural-draft appliance only vents if the chimney pulls harder than the room does. A manometer (a gauge that reads small air pressures in pascals, Pa) between the utility room and outdoors read about minus 7 Pa with the blower on, against near zero with it off. Natural-draft appliances generally tolerate only a few pascals of depressurization; consult your program's or jurisdiction's worst-case test standard for the exact limit it uses.

Why it did it that morning. The chimney was cold at 22 F outside after a night of short water-heater cycles, so its draft was weak. The furnace was calling for heat almost continuously. The kitchen range hood had been run during breakfast, pulling the house down further. Three small negatives in the same direction.

The reading that did not settle it

The furnace's clean 38 ppm air-free looked like an answer and was not one, because it measured the wrong thing. Flue CO tells you how well a burner burns. It says nothing about where the products of combustion go. The house reading of 410 ppm in the utility room came from a water heater whose own flue CO would also have been low: it was burning fine and venting into the basement.

This is the cross-trade lesson. A plumber who came for the water heater would have tested it with the furnace off and passed it. A heating tech who came for the furnace would have tested the furnace and passed it. The fault only existed with both running and the return panel off, which is why the test that finds it is a worst-case depressurization test: every exhaust fan and the air handler on, interior doors set to make the appliance zone most negative, then check each natural-draft appliance for spillage. The common field standard is that spillage continuing past about a minute after start-up is a failure; use the protocol your jurisdiction, utility or weatherization program specifies.

The fix, and how it was confirmed

The water heater was red-tagged until the return was sealed. The tech replaced the filter rack door, screwed and taped the return panel back on, and sealed two other return leaks in the utility room. Retested with the furnace blower, the range hood and a bath fan all running: utility room at minus 1 Pa, water heater drafting within about twenty seconds, smoke drawing up into the hood. Ambient CO in the utility room after 30 minutes of both appliances running: 0 ppm on the tech's monitor. The utility service tech released the gas. The tech recommended, in writing, a combustion-air review and a dedicated CO alarm on each level of the house; there had been none.

What goes in the job record

Times: arrival, collapse, 911 call, fire department arrival, release. Readings: the tech's monitor at the collapse point and the threshold, the fire department's readings by location, the furnace's air-free flue CO, the manometer reading before and after, the draft test result. The state of every appliance at the moment of the collapse (furnace on, blower running, water heater cycling, range hood recently run). Who was present for the trace. The red tag. Write it the day it happened, because the customer's insurer, the utility and possibly the customer's doctor will ask what was found, and the honest answer is the one written while it was fresh.

References

• NIOSH Pocket Guide to Chemical Hazards, carbon monoxide: 35 ppm recommended exposure limit (8-hour average) and 1,200 ppm immediately dangerous to life and health
• NFPA 54, National Fuel Gas Code, in the edition your jurisdiction has adopted, which governs venting and combustion air for gas appliances and binds the installer through the local permit
• U.S. Consumer Product Safety Commission guidance on CO alarms and combustion appliances
• See related: The Medical Emergency on a Service Call SOP