Field reference

Combustion air sizing

Calculation for obtaining outside combustion air (fresh air).

Boiler BTU input and permanent opening size, per NFPA 54 (National Fuel Gas Code) §9.3.3 — formerly cited as §5.3 in older editions.

1Find BTU input

BTU out (BHP × 33,479) —

2Size the opening(s)

BTU IN used below —

One permanent opening

Direct to outdoors — 1 sq in per 3,000 BTU. Maintain 1" clearance on sides & back, 6" on front of the appliance.

Area needed

—

sq in

Area needed

—

sq ft

Square opening

—

in × in

Square opening

—

ft × ft

Two permanent openings

One within 12" of ceiling, one within 12" of floor.

Rate is per opening, not combined.

Each opening

—

sq in

Each opening

—

sq ft

Square opening

—

in × in

Square opening

—

ft × ft