Natural draft comes from hot flue gas being lighter than outside air; forced draft uses a fan to push air in; induced draft uses a fan to pull gas out; balanced draft combines both. Draft is measured in inches of water column, as a negative number where it pulls. A typical oil burner wants about minus 0.02 inches over the fire, with somewhat more pull in the breeching.
Overfire draft is minus 0.01 and breeching is minus 0.06, the heat exchanger's half plugged with soot.
Measure draft with the boiler running and the building closed up; open doors and windows hide spillage problems.
Also heard as
- draught
Related terms
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.
Spillage
Flue gas escaping into the room from a draft hood or damper instead of going up the chimney.
Power burner
A burner with its own fan that forces combustion air into the furnace, rather than relying on natural draft.