Anti Siphon Loop

What is an anti-siphon valve, and why do I need one?

Anti-siphon valves are often referred to as vented loops and they prevent back siphoning of water in plumbing systems that lead to the outside of a boat. You need vented loops, as part of any plumbing system that is below the waterline such as: engine exhaust systems, plumbing for bilge pumps, or the marine head.

The reason you have them is to stop a siphon or backflow of water in the system. With a siphon, the water can travel the wrong way in a plumbing system. In the case of a marine head the water can go back into the toilet and overflow it.

How does an anti-siphon work?

An anti-siphon loop is fitted with a one way valve. This permits water to be pumped through the loop. However when the pumping is stopped and no more water is pushed past the bend, air is let into the pipes and the water level drops away from the loop on both side, thus preventing a siphon from occurring. The vented loop allows air in but not out.

This universal anti-siphon valve has variable push fit hose connectors to suit most commonly used sizes of hose: 13mm (1/2″), 19mm (3/4″), 25mm (1″) and 32mm (1 1/4″). At the top of the U-bend there is also a vent line connection to allow any odours to be vented outside the boat. The vented loop should be fitted so that is always approximately 200mm (8″) above the waterline.

Vented loops are simple devices that very rarely go wrong, and can prevent disastrous water intake.