The minimum size pipe that may be used on a gravity tank fill line is

The minimum size pipe that may be used on a gravity tank fill line is

• A. 1/2 inch
• B. 1 inch
• C. 1 1/2 inch
• D. 2 inches

Answer: (C)

When sizing a gravity tank fill line, the key is to ensure enough water can flow into the tank without excessive pressure loss. Water flowing under gravity has its pressure converted to flow, so a smaller pipe increases flow velocity for the same amount of water but also greatly raises friction losses along the length. If the line is too small, those losses reduce the actual fill rate, extending the time needed to bring the tank up to level and potentially causing issues with tank venting or air pockets. Choosing a minimum of 1-1/2 inches strikes a balance: it provides sufficient cross-sectional area to keep friction losses manageable and to deliver the required fill rate under typical supply head, while still being practical for installation. Pipes smaller than that would impose too much resistance, making it difficult to fill the tank in the desired time, whereas a 2-inch line would work fine but isn’t the smallest acceptable size.

When sizing a gravity tank fill line, the key is to ensure enough water can flow into the tank without excessive pressure loss. Water flowing under gravity has its pressure converted to flow, so a smaller pipe increases flow velocity for the same amount of water but also greatly raises friction losses along the length. If the line is too small, those losses reduce the actual fill rate, extending the time needed to bring the tank up to level and potentially causing issues with tank venting or air pockets.

Choosing a minimum of 1-1/2 inches strikes a balance: it provides sufficient cross-sectional area to keep friction losses manageable and to deliver the required fill rate under typical supply head, while still being practical for installation. Pipes smaller than that would impose too much resistance, making it difficult to fill the tank in the desired time, whereas a 2-inch line would work fine but isn’t the smallest acceptable size.