Home Support Frequently Asked Questions ...What size HOM damper do I need?

Btuh to HOM Damper Size Guide Chart

The following chart is based on Table 8.1 of the CSA B149.1-10 Natural Gas and Propane Installation Code. This is typical for most Domestic Canadian applications where the total Btuh of the appliances are less than 400,000 combined and a draft control device is used.

This is simply provided for informational purposes and applicable only for Natural Gas and Propane Installations. For Oil Burning Appliances see note below.
Always check local building codes for your particular situation.


Total input of appliances 
in Btuh(kW)
Required free area of air-supply opening or duct
in inches
2 (mm2)
Damper size* 
in inches (mm)
up to 100,000 Btuh (30) 14 in(9 000) 4"Ø  (75)
up to 150,000 Btuh (45) 22 in2 (14 000) 5"Ø (125)
up to 225,000 Btuh (68) 32 in2 (21 000) 6"Ø (150)
up to 300,000 Btuh (90) 43 in2 (28 000) 7"Ø (175)
up to 375,000 Btuh (113) 54 in2 (35 000) 8"Ø (200)
up to 400,000 Btuh (120) 58 in2 (37 000) 9"Ø (225)

*Based on a maximum equivalent duct length of 20 ft. For equivalent duct lengths in
excess of 20 ft. up to a maximum of 50 ft. Increase duct size by one size.

The HOM Combustion Air Damper is also approved for Oil burning appliances but different calculations are used to determine the size of damper needed depending on your particular circumstances. for Propane and Natural Gas the calculations are roughly 1 square inch per 7,000 Btuh in Canada Oil burning appliances require roughly 1 square inch per 5,000 Btuh. 

Reproduced with the permission of Canadian Standards Association from CSA B149.1-10 Natural Gas and Propane Installation Code which is copyrighted by CSA, 5060 Spectrum Way, Mississauga ON, L4W 5N6 Canada.  While use of this material has been authorized, CSA shall not be responsible for the manner in which the information is presented, nor for any interpretations thereof.  For more information on CSA or to purchase standards, please visit our website at www.shopcsa.ca or call 1-800-463-6727"

*Note: Sizing for customers in the USA: Calculations are roughly 1 square inch per 4,000 Btuh for Propane and Natural Gas. So adjust your sizing accordingly. Always check with your local building codes for your particular situation.

Last Updated (Friday, 04 October 2013 11:17)