The following are 13 requirements for a discharge pipe serving a TPR valve:
1) the pipe should not be directly connected to the drainage system;
2) the pipe should discharge through an air gap located in the same room as the water heater;
3) the pipe should be constructed of materials tested, rated and approved for such use;
4) the pipe should not be smaller than the diameter of the outlet of the valve served, and should discharge full size to the air gap;
5) the pipe should not have valves or tee fittings installed;
6) the pipe should not have a threaded end;
7) the pipe should serve a single relief device, and should not connect to piping serving any other relief device or equipment;
8) the pipe should discharge to the floor, to an indirect waste receptor, or to the outdoors. Where discharging to the outdoors in areas subject to freezing, discharge piping should be first piped to an indirect waste receptor through an air gap located in a conditioned space;
9) the pipe should not terminate more than 6 inches above the floor or waste receptor;
10) the pipe should discharge in a manner that does not cause personal injury or structural damage;
11) the pipe should be installed so as to flow by gravity;
12) the pipe should discharge to a termination point that is readily observable by the building's occupants; and
13) the pipe should not be trapped.
The termination of a relief valve discharge pipe should always be visible and conspicuous. An air gap is necessary to prevent backflow and contamination of the potable water system. The discharge pipe must not be reduced in size, and must not be less than the size of the relief valve outlet. A reduction in size will act as a restriction and would impede the flow rate of the discharge. Relief valves must not be exposed to freezing temperatures. The slow drip of a leaking valve in freezing temperatures could cause ice to form and restrict the discharge, and eventually disable the valve.
An air gap is an unobstructed vertical distance through the free atmosphere between the outlet of the waste pipe and the flood-level rim of the receptacle into which the waste pipe is discharging.
The discharge pipe should have a minimum pressure rating of 100 psi (690 kPa) at 180° F (82° C). Water heater temperature-relief valves are usually set to open and discharge at 210° F (99° C).