Where combustion air is provided mechanically, the combustion air shall be supplied at the minimum rate of ______ cfm per 1,000 Btu/h for all appliances located within the space.

Where combustion air is provided mechanically, the combustion air shall be supplied at the minimum rate of ______ cfm per 1,000 Btu/h for all appliances located within the space.

• A. 0.45
• B. 0.5
• C. 0.35
• D. 0.25

Answer: (C)

When combustion air is provided mechanically, the air supply must match the total heat input of all appliances in the space, expressed as a rate per 1,000 Btu/h. The minimum rate in this setup is 0.35 cfm for every 1,000 Btu/h. This amount ensures enough oxygen for proper combustion and helps prevent negative pressure that could cause backdrafting of combustion products. To apply it, sum the Btu/h ratings of all appliances in the space, divide by 1,000, and multiply by 0.35 to get the required cubic feet per minute of mechanically supplied combustion air. For example, with a total of 60,000 Btu/h, you’d need 60 × 0.35 = 21 CFM. Values higher than 0.35 would be more air than required for this configuration, while lower values would not meet the standard and could compromise safety.

When combustion air is provided mechanically, the air supply must match the total heat input of all appliances in the space, expressed as a rate per 1,000 Btu/h. The minimum rate in this setup is 0.35 cfm for every 1,000 Btu/h. This amount ensures enough oxygen for proper combustion and helps prevent negative pressure that could cause backdrafting of combustion products. To apply it, sum the Btu/h ratings of all appliances in the space, divide by 1,000, and multiply by 0.35 to get the required cubic feet per minute of mechanically supplied combustion air. For example, with a total of 60,000 Btu/h, you’d need 60 × 0.35 = 21 CFM. Values higher than 0.35 would be more air than required for this configuration, while lower values would not meet the standard and could compromise safety.