To be proper, it is not a valve. Here's what they do. When you have a PRV or other check valve between your water heater and the city water main, it creates what is termed a closed system. When the water heater heats, the water in the tank expands and needs someplace to go. If the system is an open system, this expansion is absorbed by the city main with no problems. However, with the closed system this can not occur and the rapid rise in pressure occurs in the tank. This causes the T/P valve on the tank to open to relieve that pressure and prevent the tank of exploding. When the pressure is reduced, the T/P should close, but doesn't always. The thermal expansion tank, just like it's name implies, absorbs this expansion. It is placed in the cold water supply between the water heater and PRV.