In an Atmospheric Vacuum Breaker, what happens when the circuit valve closes?

• A. The float falls to seal the line to the valve and potable water supply
• B. Water flows back to the city main
• C. The valve remains open
• D. Water stops at the sprinkler head

Answer: (A)

An atmospheric vacuum breaker prevents backflow by using a float-operated valve that stays open while water is flowing and seals when flow stops. When the circuit valve downstream closes, the flow ceases and gravity causes the float to fall, seating the valve and cutting off the line to the potable water supply. This stops any potential backflow into the drinking water. The top vent then allows air to enter, preventing a vacuum from forming and helping maintain the seal. So the key outcome is the float falling to seal the line and protect the potable supply from backflow.