What is the minimum size of a single-wall metal pipe vent on a gas water heater?

• A. 3 inches
• B. 2 inches
• C. 4 inches
• D. 1 inch

Answer: (A)

The vent size is chosen to safely carry the appliance’s exhaust without creating backpressure or backdraft conditions. For a gas water heater using a single-wall metal vent, the minimum diameter is 3 inches. This size provides enough cross-sectional area for the flue gases to escape readily under typical operating conditions, reducing the risk of spillage of combustion products into the living space. Smaller diameters would restrict flow and increase the chances of backdraft, especially at startup or with longer vent runs. Larger diameters, like 4 inches, are only needed for higher input appliances or longer vent lengths, but the smallest allowable size for a standard unit is 3 inches.