A temperature and pressure relief valve, or T&P valve, is the safety valve on a water heater that opens automatically if the tank gets too hot or the pressure climbs too high. It dumps water to relieve the buildup, preventing the tank from bursting. Code sets it to open near 210 degrees or 150 psi.
A temperature and pressure relief valve, almost always called a T&P valve, is the single most important safety device on a water heater. A water heater is a sealed tank that heats water, and heated water expands and builds pressure. If the thermostat ever fails and the burner or element keeps heating, that pressure can climb until the tank ruptures. A ruptured water heater is dangerous. The T&P valve exists to make sure that never happens.
The valve watches two things at once: temperature and pressure. It is spring-loaded and set to stay shut during normal operation. If the water temperature rises to about 210 degrees Fahrenheit, or the pressure reaches about 150 psi, the valve opens on its own and lets water and steam escape. Releasing that water instantly drops the temperature and pressure back to a safe range, then the valve reseats. It needs no power and no operator. It reacts to the tank's own condition, which is why it is trusted as the last line of defense.
Because the valve can release scalding-hot water, code is strict about its discharge pipe. A pipe threads onto the valve and carries any released water down to a safe spot, so nobody standing near the heater gets burned. That pipe has to run downhill to the floor, a drain pan, or outside, and it must end close to the floor, roughly within 6 inches, so hot water cannot spray. For example, a T&P valve that lets go while someone is doing laundry should dump harmlessly near the floor, not shoot across the room. The discharge pipe must never have a valve, cap, or reducer on it, because any blockage would defeat the whole safety system.
A T&P valve should be tested periodically and replaced if it leaks, sticks, or was never given a proper discharge pipe. A valve that has corroded shut is a silent hazard, because the tank loses its protection without any obvious sign. If a T&P valve is dripping steadily, that is a warning worth acting on, since it can mean the valve is failing or the system pressure or temperature is running too high.