Spillage happens when the vent cannot carry the flue gas: a blocked or undersized chimney, a cold chimney at startup, downdraft, or exhaust fans and dryers depressurizing the room. Brief spillage at startup may clear, but continuous spillage means CO and moisture in the building. Techs check at the draft hood with a mirror that fogs, a smoke pencil or a CO meter, and spill switches shut appliances down.
Water heater and boiler both spill when the kitchen hood is on, the mechanical room needs makeup air.
Also heard as
- spill
- flue gas spillage
Related terms
Draft
The pressure difference that pulls or pushes combustion gases through the boiler and out the chimney.
Air-free CO
Carbon monoxide in flue gas corrected to zero excess oxygen, so dilution with air cannot hide a CO problem.
Barometric damper
A weighted swinging gate on the flue pipe that lets room air in to hold chimney draft steady.
Breeching
The duct that carries flue gas from a boiler's outlet to the chimney or stack.