Normally, the water inlet valve is open and under house pressure all the time.

 

The inner tank is designed to withstand a working (outward) pressure of 84 psi and pressure tested to 130 psi.

 

However, if the water supply is suddenly cut off and the water drains backwards out of the tank, that could create a vacuum inside the inner tank. The tank is not designed to withstand a very great inward pressure differential. If the outside air pressure is much greater than the inside pressure, the vacuum could suck the inside tank inwards, resulting in great damage, possibly requiring the tank to be replaced.

 

Under these conditions, the Air Pressure Suction (Vacuum) relief valve opens, allowing air to be drawn from the outside into the tank to equalize the inside and outside pressure.

 

Although this situation is very unlikely to occur, this valve is sort of like the air bag in a car - usually you don't use it, but if you need it, it's there to provide protection and prevent damage.

 

This valve should be periodically inspected to make sure it is not plugged up with debris. This can easily be done by directing a stream of water from a garden hose into the holes on the valve and making sure they are not clogged.