Gas Fitter Practice Test

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

1. 11"

2. 4"

3. 7"

4. 3"

The correct answer is: 4"

To determine the amount of free air required for a 75,000 BTU atmospheric hot water tank in a tightly sealed house, it's essential to understand the principles of combustion air requirements. An atmospheric hot water tank requires a sufficient amount of combustion air to operate efficiently and safely. When using a round duct for this purpose, specific calculations based on the BTU output of the appliance must be undertaken. For a hot water tank with an input of 75,000 BTU, the general rule is to provide 1 square inch of free air for every 1,000 BTU of input. Therefore, in this case, for 75,000 BTU, the total amount of free air required would be calculated as 75 square inches (75,000 BTU / 1,000 BTU). When converting square inches to the diameter of a round duct, the formula used is: \[ Area = \pi \times (d/2)^2 \] Rearranging this for a diameter gives: \[ d = 2 \times \sqrt{Area/\pi} \] If you input the value of 75 square inches into this formula, the resulting diameter will closely correspond to 4 inches,