How Does a Backflow Preventer Work?
A backflow preventer works by using a one way valve, often called a check valve, to prevent water from backing up into the supply line. Basically, it only allows water to flow through the valve in one direction.
What It Does
When no supply is present, the valve automatically closes so that water on the other, or downstream, side of the valve cannot pass through the valve and contaminate the supply line.
Letting Water In
Most backflow preventers are of the ball check valve kind. A ball that is slightly larger than the diameter of the supply line is used to block the supply line and stop water flow. That ball is connected to a spring that maintains it in the shut, or off, position. When water enters from the supply line side of the valve, the water pressure is strong enough to force the ball back from the opening and allow the water to pass through the valve and into the pipe beyond.
Keeping Water Out
When there is no water pressure present in the supply line, the ball will remain in the closed position, preventing any water from the pipe beyond the valve from entering into the supply line. In cases where water is coming from both directions, whichever side has the most water pressure will win. If more pressure is coming from the supply line, the ball will move to the open position and the incoming water will force the water from the downstream side of the valve back into the pipe. If more pressure is coming from the downstream side of the valve, it will force the ball into the closed position, preventing the flow of water from either side.