What size drain valve must be provided at the lowest point of the waterway?

• A. 1 inch
• B. 2 inch
• C. 1/4 inch
• D. 1 1/2 inch

Answer: (D)

Drain the waterway fully requires a valve that can move the entire column of water out quickly from the lowest point. The valve size governs how fast that water can exit when you open the drain, and a valve that's too small will bottleneck flow, leaving water trapped in the run. A 1 1/2 inch valve provides enough cross-sectional area to empty the waterway promptly under typical service conditions, balancing effective drainage with practical size. Smaller options would drain very slowly and leave residual water; a larger valve would work but isn’t necessary for the standard requirement and adds extra bulk. So, the appropriate size is 1 1/2 inch.