A TPR valve or temperature pressure relief valve is a device that is placed on water heaters to allow the discharge of excessive pressure, or excessive temperature or both.What does all that mean? Well a TPR valve will open when pressure inside of a water heater tank reaches 150PSI or 210 degrees farenheit.The First Line of Protection. Current water heaters contain high-limit controls or thermostaically controlled devices that help prevent the overheating of a waterheater.ell a TPR valve will open when pressure inside of a water heater tank reaches 150PSI or 210 degrees farenheit. The First Line of Protection. Current water heaters contain high-limit controls or thermostaically controlled devices that help prevent the overheating of a waterheater.