Relief Valve

• The relief valve is a small valve located above the normal water level of the hot water tank, usually only an inch or two down from the top of the tank. It is designed only to open when water in the tank rises to reach it and exert pressure on it. The water in the tank does not normally reach as high as the relief valve or does not exert enough pressure to open it, so the valve only opens when the water is overflowing.
  
  

Temperature and Pressure

• There are two reasons that relief valves automatically open: too much pressure and rapidly rising temperatures. The two are closely connected--temperatures rising too high and too fast in the water tank will cause pressure to increase. Too much pressure, and the tank can crack or even explode. The relief valve keeps the more serious effects of the pressure increase minimal by providing an outlet for the water.
  
  

Drainage

• The relief valve is usually connected to a small pipe that flows down to a drain, typically located in the concrete floor where the water tank has been installed. Without this drain, the water would stream out from the valve and ruin the surrounding construction. The pipe and the drain give the water a safe place to escape in case of an accident.
  
  

Expansion Tanks

• Relief valves are also used in a plumbing device known as an expansion tank, which transports water back and forth through a system and may be part of some hot water tanks. The expansion tank draws in air to increase pressure, driving water through the pipe, or releases air to decrease pressure and cause the water level to drop. Too much pressure, and the relief valves lets out some of the water before the expansion valve is ruined.
  
  

Indication

• Relief valves also serve as an important indication of hot water tank problems. When pressure increases in a hot water tank it often does so rapidly, and the effects do not last. Owners can check the relief valve to see if it has been opened recently if they suspect a problem.