A temperature and pressure relief valve's discharge pipe should terminate:

The temperature and pressure relief valve is the last defence against a water heater becoming a pressure vessel failure, and every rule about its discharge pipe exists to keep that path open and its operation visible. Terminating near the floor keeps discharged water — which may be near boiling — from spraying at head height onto anyone nearby, and puts it where it will be noticed. No threads on the end prevents a cap or a hose being fitted, and no valve in the line prevents the path being shut off; either would convert a safety device into a sealed system. The pipe must also run downhill for its whole length so it drains rather than holding water, be the same size as the valve outlet, and be a material rated for the temperature. Inspectors check the terminal condition because it is the part most often altered.

The discharge pipe terminates near the floor so hot water is released safely and visibly, and it has no threads and no valve so the path cannot be capped or shut off. Those three conditions keep the relief path open and its operation observable.

TPR valve discharge piping must be full size, run downward to an approved termination, be of rated material, have no valves or threaded end, and terminate a short distance above the floor or an approved receptor so discharge is visible and does not scald occupants.

Down, Open, Low. It runs downhill the whole way, nothing can cap or close it, and it ends near the floor where you will see it.

The most common field defects are a missing discharge pipe entirely, a threaded and capped end, and a pipe that rises before it falls. All three are reportable safety items.

• •Accepting a threaded or capped termination
• •Allowing a valve anywhere in the discharge line
• •Sealing the discharge into a drain rather than terminating above it