Round Pipe combustion air equivalency

[Ross Neag]

When we see round combustion air inlets are we to use pipe diameter equals XX amount of net free area in square inches, such as 6" round equals 28 sq in OR is there a CFM equivalency?

[James Duffin]

I've always gone with the theory that a square inch is a square inch no matter what shape the pipe is.

[Tom Thompson]

CFM (cubic feet per minute) is for fan calculations, combustion air is usually free atmospheric pressure.

[Ross Neag]

Much obliged gentlemen, Happy 4th of July!

[Gunnar Alquist]

Area = Pi times the square of the radius.

[Jerry Peck]

Ross,

Combustion air is based on the Btu of the appliance(s) in the space, whether or not the space is a 'confined space' or not, and other variables, but, once the net free vent area is determined, and once the minimum opening size is determined, than the shape of the opening only affects the calculation of the area within the opening (and the minimum opening dimension when it applies).

[Ross Neag]

So, back to original question but NEW home. 8" round feeds combustion room (12 x 10 est) with 300k BTU boiler and two 40k BTU HWHs, all Cat 1. 36 door had 25% NFA to home. Comb inlet had insect screen on it (doh). Opinions besides the obvious HVAC referral?