A vertical duct delivers outside combustion air to a water heater. What is the minimum area of each duct if the water heater is rated at 45,000 btuh?

To determine the minimum area of each duct delivering outside combustion air to a water heater rated at 45,000 BTU/h, it's essential to consider the guidelines set by the National Fuel Gas Code, which outlines the requirements for combustion air.

The general rule is that for every 1,000 BTU/h of input rating, at least 1 square inch of combustion air duct area is needed. Therefore, for a water heater with a rating of 45,000 BTU/h, you would first divide the input rating by 1,000 and then multiply by the minimum square inch requirement for a duct that allows for proper airflow.

In this case, the calculation would be as follows:

45,000 BTU/h ÷ 1,000 = 45 square inches of duct area needed.

When considering duct sizing, you can translate the square inches of the duct area into the diameter of a round duct using the formula for the area of a circle (A = πd²/4). For a given area, you can solve for the diameter.

Using standard duct sizes, the choice of 6 inches would provide an area of approximately 28.3 square inches, which is insufficient. An 8-inch duct results in about