House of Craig - WSSC Combustion Air Requirements Calculator


Enter a number for ONE of the items and hit the Enter key or click the Calculate button.

The other 5 items will be filled in based on the WSSC requirements Gibberish in the input fields will NOT be tolerated! Don't use decimal points!

=Input BTUH of appliance(s):
=Cubic Feet of Indoor Volume
=Square Feet of Indoor Area with 8' ceiling
=Square Feet of Indoor Area with 10' ceiling
=Square In. of Free Area of One of the 2 Pass-Thru Grilles
=Square In. of Free Area of Opening/Duct to the Outside

Usage Example:



The furnace room is not large enough, now what?

The adjacent area is only 800 square feet, now what?

Ok, what about the shortfall?

WSSC LogoWSSC Requirements Used by this Calculator

  1. For all air from inside the structure: The room shall have not less than 1 cubic foot of volume per 15 btuh total equipment load.
  2. For pass-through openings between adjacent spaces: The total net free area of the openings shall be equal to 1 square inch per 500 btuh of the equipment rating.
  3. For air from outside: When connected to the outdoors thru horizontal ducts, the opening area shall equal 1 square inch net free area per 2000 btuh total input rating of equipment
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