Definition
An anti-siphon valve is a device fitted at the top of a raised loop in a marine cooling or exhaust circuit that admits air to break a water column, preventing seawater siphoning back into the engine when it is stopped.
On many installations the engine sits at or below the waterline, which means a continuous column of water can form between the sea and the engine. When the engine stops, that column can siphon — and seawater entering a cylinder through the exhaust destroys an engine quietly, while it sits at a mooring.
The defence is a vented loop: the hose is routed above the waterline and an anti-siphon valve at the top admits air to break the column.
What goes wrong
The valve is a small component that lives in a salty, humid space. Salt crystals, corrosion and age block the air path, and a valve that cannot admit air is not protecting anything. It gives no warning when it fails, which is why it belongs on an inspection schedule rather than being trusted indefinitely.
Where you meet it
On a service quote for exhaust or cooling work, in the commissioning notes of any new installation, and in the diagnosis of an engine that has hydrauliced or filled with water at rest. See wet exhaust systems.