An anti-siphon unit is an extremely important part of any plumbing system. An anti-siphon device is recommended for use in installations such as 'below waterline' applications of engines with wet exhausts, toilet systems and pumping systems to name but a few.
Under normal operating conditions, the anti-siphon valve is closed as the water passes in the required direction, pushed passed by the pump. Under fault conditions the weight of the column of water in the hose can create a vacuum causing more water to be drawn into the system and starting a flow of water. Under these conditions the anti-siphon valve will be drawn off its seat and allow air into the pipe, breaking the water column and stopping the siphoning effect. Non-valved anti-siphons work in a similar way, but under normal conditions a small quantity of water flows out of the vent pipe. Air is drawn in should a vacuum be created.