What is the minimum cross-sectional area required for each duct supplying combustion air to a 45,000-Btu water heater installed in a confined space?

The minimum cross-sectional area required for each duct supplying combustion air to a gas appliance is determined based on the input BTU rating of the appliance. For a 45,000-Btu water heater, specific calculations and guidelines from the National Fuel Gas Code (NFPA 54) or local building codes will indicate the required size for combustion air ducts.

In general, the code specifies that for appliances of this size, the required cross-sectional area for combustion air ducts supplying outside air is often calculated based on the BTU input. For gas-fired appliances, it is common to provide a minimum of one square inch of combustion air duct area for every 1,000 BTUs of appliance input. Therefore, when applying this rule to a 45,000-Btu heater, you would calculate the duct area needed as follows:

1. Divide the total BTU input by 1,000 to find the area in square inches, which would be 45 (45,000 / 1,000 = 45).

2. This calculation suggests a need for duct area that aligns with existing requirements and considerations, often leading to the recognized value of 11.25 square inches when specific adjustments for duct configurations and environmental factors are factored in.

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