HVAC D-2 License Practice Test

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A 6-inch diameter vent measures 20 feet vertical; what is the maximum horizontal length of the 6-inch, single-wall connector?

10' 20' 12' 15'

Correct Answer:

15'

Explanation:

The key idea is how far a horizontal run of a single-wall vent connector can be while still maintaining adequate draft. For a 6-inch diameter vent, the common rule used in practice is that the horizontal length should not exceed about 75% of the vertical rise. With a vertical rise of 20 feet, 0.75 × 20 equals 15 feet. So the maximum horizontal length allowed is 15 feet. This keeps enough buoyant draft and helps prevent backdraft or condensation problems in the vent system. Shorter horizontals (like 10 or 12 feet) would still be acceptable, but they aren’t the maximum allowed. A 20-foot horizontal would exceed the recommended limit.

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