What is the required vent diameter when three elbows are used?

What is the required vent diameter when three elbows are used?

• A. 5 inches
• B. 6 inches
• C. 4 inches
• D. 7 inches

Answer: (B)

Vent sizing for gas appliances is about ensuring enough draft by balancing the vent’s resistance with the appliance’s exhaust flow. Each elbow adds friction and backpressure, effectively increasing the vent’s resistance. The code uses tables that translate that added resistance into a larger required diameter. With three elbows, the added resistance is enough that the minimum vent size becomes six inches. Using a smaller diameter would raise the pressure drop too much, risking poor venting, backdraft, or combustion gases entering the space. A six-inch vent is the standard minimum to compensate for three elbows; larger sizes aren’t required by the table for this many elbows, though they would further reduce backpressure if used.

Vent sizing for gas appliances is about ensuring enough draft by balancing the vent’s resistance with the appliance’s exhaust flow. Each elbow adds friction and backpressure, effectively increasing the vent’s resistance. The code uses tables that translate that added resistance into a larger required diameter. With three elbows, the added resistance is enough that the minimum vent size becomes six inches. Using a smaller diameter would raise the pressure drop too much, risking poor venting, backdraft, or combustion gases entering the space. A six-inch vent is the standard minimum to compensate for three elbows; larger sizes aren’t required by the table for this many elbows, though they would further reduce backpressure if used.