What is the minimum size of the combustion air ducts required for a 125,000 Btu/h fan-assisted appliance and a 65,000 Btu/h heater?

To determine the minimum size of the combustion air ducts required for a fan-assisted appliance and a heater, calculations based on the input BTU of the appliances need to be made, considering ventilation standards such as those outlined in the National Fuel Gas Code or similar guidelines.

For a 125,000 Btu/h fan-assisted appliance and a 65,000 Btu/h heater, the total BTU requirement adds up to 190,000 Btu/h. According to general rules of thumb, for natural gas appliances, a minimum of 1 square inch of combustion air duct is required for every 1,000 Btu/h of appliance input. By applying this rule to the total BTU input:

190,000 Btu/h ÷ 1,000 = 190 square inches of net free area needed for combustion air.

When calculating duct sizes, it is crucial to find the duct dimensions that will provide this cumulative area. For rectangular ducts, the formula used is width multiplied by height. In this case, when selecting from standard duct sizes, a duct measuring 10 inches by 10 inches equals 100 square inches of area, which might not suffice for the total 190 square inches required.

A duct size of 10 inches by