A tank-type liquid heater (e.g., a residential water heater) is conventionally equipped with a relief valve assembly that protects against excessive temperature and excessive pressure.  Portions of these relief valves are subject to corrosion and calcification, either of which can render the valve inoperative and unable to prevent a tank from bursting or exploding and sometimes starting a fire.  Such fires and explosions cause structural damage, deaths and injuries each year in the United States and worldwide. 
Overpressure protection is commonly supplied with a valve in which a coil spring acts on an axially translatable shaft to bias a moveable member (e.g., a valve head or disk) against a seat.  The coil spring strength and the disk size are commonly selected so that the valve opens and vents water when pressure in the tank exceeds, for example, 150 psi, which is below a burst pressure of the tank.  This valve is subject to corrosion and calcification because one side of the disk is exposed to the water inside the tank during normal operation and both the other side of the disk and the poppet spring are exposed to water when the valve relieves overpressure, is manually opened for test purposes, or is improperly installed to provide upwardly directed outflow. 
Attempts to deal with the deleterious effects of corrosion and calcification have commonly resulted in a requirement for regular (generally annual) manual test operation of an overpressure valve and scheduled replacement of that valve.  Both of these safety practices are commonly ignored by the consumer, resulting in catastrophic property damages and injuries. 
If the heater does not have a functioning inlet check valve, a runaway heater element may cause dangerous excess temperature not accompanied by excess pressure.  Overtemperature protection is commonly supplied independently of overpressure protection with a wax motor temperature sensor/actuator that includes a protruding plunger or stroke rod portion that pushes against the wetted side of the poppet valve disk when the wax is heated and expands.  The temperature sensor/actuator is also wetted by the water in the tank and is subject to corrosion, calcification and mineral deposits. 
Accordingly, there is a need for an improved T&P valve that is protected from catastrophic failure by preventing corrosion, calcification and mineral deposits from affecting operation of the valve during over-temperature and over-pressure events.