For a 75,000 BTU atmospheric hot water tank in a tight house, how much free air is required using a round duct?

To determine how much free air is required for a 75,000 BTU atmospheric hot water tank in a tight house using a round duct, you need to understand the relationship between BTU output and the necessary ventilation or combustion air supply. In this case, it is crucial to ensure that the combustion process has an adequate supply of air to operate safely and efficiently.

For appliances such as gas water heaters, it is generally recognized that about 1 square inch of free air is necessary for every 1,000 BTUs of input. For a 75,000 BTU appliance, this equates to 75 square inches of free air.

Next, the challenge is to convert square inches of free air into a corresponding diameter for a round duct. To find the diameter of a duct that provides the required cross-sectional area for ventilation, you can use the formula for the area of a circle, which is A = π(d/2)². By rearranging this formula and solving for diameter, one can determine the diameter of the duct that meets the necessary air requirements.

When doing this calculation, it typically results in a diameter that approximates to around 4 inches for the amount of free air specified. This conclusion aligns with common practices for