Combustion Air, Direct Ducting, and Draft Requirements

Permanent boiler-room louvers or vents are preferred. Direct ducting to the boiler inlet may be used only when the combustion-air system maintains stable pressure and responds immediately to firing-stage changes.

• Maintain neutral or slightly positive pressure at the blower inlet. Do not expect the boiler blower to pull air through long or restrictive ductwork.
• Size ducts and any supply fan for the required combustion-air volume and the nearly instantaneous transition from low to high fire.
• Prevent pressure fluctuations that can produce unstable flame or flame failure.
• A 35°F change in inlet-air temperature is associated in the source with about a 1 percentage-point change in O2. Seasonal retuning may be necessary.
• Protect the intake from rain, snow, and moisture. Prevent cold-air migration into an idle boiler that could freeze pressure-vessel water.
• Chimney draft should remain from 0 to -0.25 in. w.c. across the firing range; a supplemental supply fan should interface with boiler damper controls.