The Battle for Air: Chimney vs Exhaust Fan

Exhaust fans beat the chimney. Every time. A failed makeup-air fan pulled carbon monoxide out of a university boiler room and into occupied floors. The building was evacuated. People went to the hospital. It only takes about 0.03 inches of water column of negative pressure to turn a Category I chimney into a makeup-air duct. In this Boiler Room Detective case, Ray Wohlfarth shows how clothes dryers, bath fans, range hoods, and radon fans steal combustion air from atmospheric boilers and water heaters — in a dorm mechanical room or a tight two-bath house. Same physics. The building does not have to be large. It only has to win the tug-of-war for air. You will see the clues before anyone ends up in an ER: rust under the draft diverter, white streaks on the flue, staining at the draft-hood relief opening, intermittent CO alarms, and the “flu that never quite goes away” when the dryer runs. What is covered: Why Category I draft is weak, and why 0.03 in. w.c. is enough to reverse it A real university depressurization event: makeup air failed, exhaust kept running House math: dryer, two bath fans, range hood, and radon fan can pull about 550 CFM — roughly a 10-inch duct the house does not have What weatherstripping and new windows do to the old leakage path How to investigate: combustion analyzer, ambient CO, smoke pencil at the draft hood Worst-case test steps, and when to stop and ventilate instead What to write in the service report before you drive off. If the house can steal the chimney, catch it before the chimney steals the occupants. Like, subscribe, and send this to a tech who still treats a draft hood as decoration. #BoilerRoomDetective #CarbonMonoxide #CategoryI #Backdrafting #BoilerTroubleshooting #DraftHood #CombustionAir #HVACSafety