How do I know if a space is confined?

A space is confined if its volume is less than 50 cubic feet per 1000 BTU/h of total appliance input. The tool runs this test for you; a confined space needs dedicated combustion air openings while an unconfined space of tight construction may still need them.

Why do indoor and outdoor openings use different sizes?

Outdoor air is denser and more reliable, so the code allows smaller openings, such as 1 square inch per 4000 BTU/h for vertical ducts. Air drawn from an adjacent indoor room is less assured, so it requires a much larger 1 square inch per 1000 BTU/h opening.

What is free area versus gross area?

Free area is the net open area after the blockage of a louver or grille. A metal louver typically passes only about 25 percent of its gross area and a wood louver about 75 percent, so you must enlarge the gross opening to deliver the required free area.

Do I always need two openings?

Most methods require two openings, one within 12 inches of the top of the space and one within 12 inches of the bottom, to create convective airflow. A single combined outdoor opening is allowed under specific conditions and uses the 1 square inch per 3000 BTU/h rule.

Does this cover direct-vent or sealed-combustion equipment?

No. Direct-vent, sealed-combustion, and engineered or mechanical combustion air systems draw air through their own piping and follow separate rules. This tool covers the standard atmospheric and natural-draft openings under IMC 701 and IFGC 304.

Combustion Air Opening Size Calculator (IMC 701)

Every fuel-burning appliance needs enough air to burn cleanly and vent safely, and when it sits in a small room that air has to come through purpose-built openings. This calculator first tests whether your equipment room counts as a confined space, then sizes the combustion air openings using the correct IMC 701 method for your air source, so the furnace, boiler, or water heater never starves for air.

How it works

The calculation has two parts: the confined-space test and the opening sizing.

confined if:   room volume < 50 ft³ per 1000 BTU/h total input
opening area =  total BTU/h ÷ method factor
  indoor (adjacent room):      1 in² / 1000 BTU/h  (min 100 in² each)
  outdoor vertical duct:       1 in² / 4000 BTU/h
  outdoor horizontal duct:     1 in² / 2000 BTU/h
  outdoor single combined:     1 in² / 3000 BTU/h

Most methods call for two openings — one high and one low — to set up natural convection. The tool also gives a round-duct equivalent diameter so you can pick a stock duct, and reminds you to enlarge the gross opening to overcome louver blockage.

Example and notes

A boiler room with 150,000 BTU/h of total input in a 400 ft³ space is confined, because an unconfined space would need at least 7,500 ft³. Using two vertical outdoor ducts at 1 square inch per 4000 BTU/h, each opening needs about 38 square inches of free area, roughly a 7-inch round duct. Always convert that free-area requirement to a larger gross opening based on the louver type, and verify the method and factors against the adopted edition of the mechanical and fuel gas codes and the appliance’s listing.