TPR Valve Requirements
In the past, water heaters were equipped with only a pressure relief valve. This proved inadequate because if water temperature builds up in excess of 212 degrees Fahrenheit, it will put additional pressure on the tank.
This extra pressure can crack the tank, allowing superheated water to escape as steam and causing an explosion. Today, combination pressure and pressure relief valves, which open at 210 degrees or 150 pounds per square inch, are required.
TPR Valve Location
Temperature and pressure relief valves are required to be installed within the top 6 inches of the tank, where the hottest water is located. A TPR valve has a sensing element that extends into the heater and monitors the temperature in the top 6 inches of the tank.
TPR valves are located either on top of the tank or on the side.
Discharge Pipe Size
A TPR valve is required to have a discharge pipe attached. This is to protect against personal injury, such as scalding, if the valve discharges.
The discharge pipe must be the same size as the discharge opening. Residential water heater TPR valves have a 3/4-inch threaded opening, meaning the pipe must be also be 3/4-inch.
The pipe can be copper, galvanized or chlorinated polyvinyl chloride material, though copper is the most commonly used.
Other Discharge Pipe Requirements
The discharge pipe must terminate no more than 6 inches above the floor. The pipe must be installed so as to flow by gravity and not be trapped.
The discharge piping can only serve a single relief device, meaning each hot water heater must have its own discharge pipe. The pipe may not have a threaded connection at the end.
There may not be tee fittings or valves installed in the discharge line. A closed valve can allow heat and pressure to build up and render the TPR valve useless.
Similarly, the pipe cannot be capped or plugged. And a discharge pipe is required to terminate in the same room with the hot water heater.