How many standard round pipes are required to provide combustion air for a 150,000 BTU/h input rated furnace in a confined space?

To determine the amount of combustion air required for a 150,000 BTU/h input rated furnace in a confined space, it's essential to refer to the guidelines set forth in the International Mechanical Code (IMC) or other relevant standards that specify the necessary air requirements for combustion appliances.

For a furnace of this size, guidelines typically suggest providing at least 1 square inch of free area for every 1,000 BTUs of input. Therefore, for a furnace rated at 150,000 BTUs, the calculation would require 150 square inches of free area for combustion air.

When it comes to standard round pipes, the diameter of the pipes directly correlates to their free area. A 7-inch diameter pipe has a free area of approximately 50 square inches when using standard tables, and since two of these pipes can provide a total of around 100 square inches, they would not meet the requirement needed alone. Therefore, two 7-inch pipes would be adequate to help facilitate the combustion needs of the furnace.

On the other hand, considering larger diameter pipes like 8-inch options would provide even more free area than necessary, leading to excess air. Therefore, using two 7-inch pipes strikes a balance, meeting the combustion air requirement