A Temperature and Pressure Relief (TPR) Valve is required to be installed on every tank-type water heater to prevent explosion of the tank if the thermostat fails to shut off heating when the water in the tank reaches the desired temperature. It is preset to open when the tank water exceeds approximately 200º F or 150 psi internal pressure, and the settings cannot be adjusted. Discharge piping is also necessary to dump the super-hot water at a safe location on the floor or exterior of the home. See What are the code requirements for the discharge piping from a Temperature-Pressure Relief (TPR) valve at a water heater? for more on that.

    The Thermal Expansion Pressure Relief Valve serves a different purpose. It protects the house water supply piping system from excessive pressure rise due to thermal expansion of the water in the pipes caused by a water heater. The pressure setting is adjustable.

   TPR valves have been code-required since the 1930s, but the requirement for control of the pressure in the pipes due to thermal water expansion entered the code in 2006. It is only required to closed water supply systems.   

    Although a Thermal Expansion Pressure Relief Valve is one option, a Thermal Expansion Tank at the water heater is used more often. The pressure tank has the advantage of continuously controlling the pressure in the piping system, as opposed to just releasing water  when the pressure exceeds a preset limit. Also, see our article What is the little tank on top of the water heater for?