Combustion Air Calculation Worksheet

The 2018 International Residential Code requires that buildings of unusually tight construction obtain all required combustion air for fuel-burning appliances directly from the exterior of the home. This worksheet is intended to assist in the calculation of the minimum size of the required combustion air vents that are needed for the appliances within the thermal envelope of the home. This worksheet is for vertical ductwork applications only, please refer to Chapter 17 of the 2018 International Residential Code for Horizontal supply ducts or Attic applications.

The code specifies that one combustion air opening shall be within 12” of the ceiling, the other shall be indicated within 12” of the floor. The same intake duct or opening shall not serve both required combustion air openings. Each opening shall provide a free area of 1 square inch per 4,000 BTU’s of total input of all the appliances in the space (2018 IRC M1703.2). Direct-vent (+90% efficiency) equipment that obtains combustion air through a dedicated intake vent that communicates directly with the exterior of the home is exempt from this requirement, therefore, direct-vent appliances do not need to be included within this calculation.

Please indicate the number of BTU’s for each appliance present within the enclosed space. If the line is not applicable, please indicate N/A; for each piece of equipment that is High-Efficiency, please note that on the appropriate line.

BTU's Per 
Furnace #1  
Furnace #2  
Water Heater #1  
Water Heater #2  
Other (Ex. Clothes Dryer)  
TOTAL BTU'S divided by 4,000 
= Free area required for each duct
             Sq. Inches

Free Area of Ductwork

  • 2” Round Duct = 3.14”   Maximum per duct
  • 4” Round Duct = 12.56” Maximum per duct
  • 6” Round Duct = 28.26” Maximum per duct
  • Size of Ducts to be provided:  __________

Revised 3/13/2020