Dead leg

A section of piping where water does not circulate, such as a branch leading to a rarely used valve. Stagnant water in a dead leg allows bacteria to attach and form biofilm, which can then contaminate the main loop. Good design keeps dead legs as short as possible, commonly less than three pipe diameters from the main flow, and uses zero-dead-leg valves at points of use. Dead legs are checked during installation qualification and system walk-downs.