Chasing the source. Finding carbon monoxide, appliance by appliance. Episode thirty-two of the Gas Service Animated series.

The carbon monoxide alarm is screaming. The family is already outside, and they called it in. You arrive with two instruments: a personal monitor clipped in your breathing zone, and a combustion analyzer in your bag. At the door the air reads clean. That changes nothing. Somewhere inside, an appliance is making carbon monoxide, and your job is to find it.

Start outside, in fresh air, away from vehicle exhaust. Power up both instruments and let the cells warm up and zero. Confirm the monitor reads zero parts per million. Then walk in with the meter live and sampling, through the door and down the hall to the mechanical room. You are surveying before you reach a single appliance.

Now survey room to room. Living spaces first, then the mechanical room, then the burner zone, which should read near zero. Then the supply plenum, then the flue. A normal home reads zero parts per million; a few parts per million is tolerable with tobacco or candles. Above about six is unusual — investigate. Know your stop rules: above thirty-five parts per million, halt the evaluation; at seventy or more, evacuate everyone and call nine one one.

Read what the numbers tell you. A rise in the supply plenum whispers a cracked heat exchanger: combustion gas leaking into the air the blower pushes through the house. A rise at the draft diverter whispers spillage: flue gas rolling back into the room instead of up the vent. Two failures, two signatures. The map points you at the suspect before opening a panel.

Know every suspect. The furnace: a cracked heat exchanger, dirty burners, or blocked flue. The boiler, atmospheric or power-burner, backdrafts like any natural-draft appliance; cracked sections push carbon monoxide into the breeching; blocked passages, a condensate-plugged flue, or short cycling all raise it. The water heater: an orphaned or backdrafting draft hood, or blocked flue. The range and oven are unvented, so the code grades them as-measured — about two hundred twenty-five at the oven or broiler, and just twenty-five per burner; a self-clean cycle can spike near fifteen hundred. The gas dryer: a blocked vent. Vent-free logs use an oxygen-depletion sensor that trips near eighteen percent oxygen; it does not sense carbon monoxide. Outside sources: a generator over a thousand, a vehicle in an attached garage, a re-entrained vent terminal.

Now isolate. Shut every fuel-burning appliance off, furnace, water heater, range, oven, dryer, fireplace, and shut any engine off too. Let the readings settle. Then bring each appliance back on, one at a time, watching the carbon monoxide climb. The one that raises the reading is your source. One variable at a time separates the guilty appliance from the innocent ones.

Confirm at the flue with combustion analysis. Sample the undiluted gas: on a natural-draft appliance, before the draft hood; on a fan-assisted unit, a port about six inches above the hood or inducer; on a condensing appliance, at the plastic termination — never drill the plastic. Zero in fresh air first, then insert the probe. Let it warm to steady state, about five minutes, and take your reading by the ten-minute mark.

Read carbon monoxide and oxygen as measured, then correct. Multiply the measured carbon monoxide by twenty point nine, divided by twenty point nine minus the measured oxygen. That gives air-free carbon monoxide, the number you compare to the limits: four hundred for a furnace, two hundred for a water heater or space heater. Ranges and ovens are unvented, so the code grades them as-measured instead — about two hundred twenty-five at the oven, and twenty-five per burner. And watch the trend: a reading that keeps rising means an active, worsening fault.

One more test for the cracked exchanger. Watch the carbon monoxide in the supply air with the blower on, then off. If the reading jumps when the blower energizes, combustion gas is being pulled into the air stream through a breach. Crack leakage is often intermittent, coming and going with the blower cycle. Analyze it. Do not assume.

Chasing the source: zero in fresh air, read the map, isolate one appliance at a time, confirm at the flue. The analyzer finds the fault; your monitor protects you. If ambient hits seventy or anyone is symptomatic, evacuate and call 911. Next: worst-case depressurization.