Glossary · Testing & air leaks
Backdrafting and spillage
Also called: backdraft · backdrafting · backdrafting of combustion gases · spillage · flue gas spillage · combustion spillage
When exhaust from a furnace, boiler, water heater or fireplace comes back into the house instead of going up the chimney or vent.
What it means
Spillage is exhaust escaping at the appliance, often briefly at start-up; backdrafting is a sustained reversal where the chimney pulls outside air down. Both are usually caused by depressurization from exhaust fans or by a blocked, cold or poorly sized chimney. Natural-draft appliances with a draft hood are most at risk; sealed-combustion units that take air directly from outside are designed to avoid it.
Why it matters in an audit
Exhaust contains carbon monoxide, so auditors test for this in a combustion safety test and won't recommend tightening a house until venting is safe.