What is the maximum horizontal length for a 6-inch diameter vent when installed with a 20-foot vertical run?

• A. 5'
• B. 10'
• C. 20'
• D. 15'

Answer: (D)

Vent lengths are limited to keep the draft from being too weak and to prevent backdraft or condensate buildup. For a given vent diameter, the allowable horizontal run depends on how tall the vertical rise is. With a 6-inch diameter vent and a 20-foot vertical rise, the standard venting table sets the maximum horizontal segment at 15 feet. That 15-foot limit keeps the pressure drop and resistance within what the appliance and vent system can handle to maintain a reliable draft.

So, 15 feet is the longest horizontal run allowed under these conditions. Shorter horizontals, like 5 or 10 feet, would also be acceptable but aren’t the maximum. A 20-foot horizontal would exceed the limit and could compromise venting performance.