How high must washing machine standpipes extend above the trap weir?

The requirement that washing machine standpipes extend 18 inches above the trap weir is based on plumbing code standards aimed at preventing siphoning and ensuring proper drainage. This height helps to create a sufficient air gap, allowing for proper venting and the smooth flow of wastewater without backflow. By maintaining this minimum height, the standpipe reduces the risk of clogs and potential contamination, as it prevents water from being pulled back into the washing machine from the drainage system.

Choosing a height of 18 inches balances safety and functionality, avoiding too low a position that might compromise drainage performance while not unnecessarily increasing the installation height. The other height options do not align with standard plumbing practices. For example, while a height of 12 inches may seem reasonable, it is not adequate for venting purposes, allowing for the potential risk of siphoning. Similarly, 24 inches and 30 inches exceed the practical specifications for typical washing machines, leading to unnecessary complexity in installation without providing additional benefit in most residential plumbing scenarios. Thus, 18 inches is established as the effective and widely accepted standard for this application.