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Vented Loops

| Maintenance | October 14, 2015

Vented Loop

Vented loops or Anti-Siphon Valves: are inverted U-shaped pipes with a vent at the top to let air escape. Vented loops are found in toilet discharge lines or in systems which are mounted below the waterline.


Vented loops are a necessity for many marine systems to prevent back siphoning . You need vented loops (anti-siphon valves) as part of the engine exhaust system, or 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 and in the case of a marine head the water can go back into the toilet and overflow it. With an engine the water can flow through the exhaust into the engine causing much damage an flooding.

Vented loops are simple devices and very rarely go wrong, but when they do the results are catastrophic. The boats potentially sinks! Mount the vented loop so it will remain above the waterline at all heel angles. Clean the anti-siphon valve regularly to keep it functioning.

What is a siphon?

I remember siphons from draining a swimming pool as a kid. The pool was on top of a slope so using the principle that water wants to flow downhill. A hose was filled with water and both ends blocked off. One end of the hose is placed in the pool and the other down the slope. Both ends are now unblocked and a siphon is formed resulting in the water flows downhill fully draining the pool.

So how does a siphon work in a boats plumbing system or exhaust system. The diagram right from Jabsco demonstrates the problem.

In top diagram you can see that water will flow into the lower bowl. In bottom diagram the hose has a loop. In this case the hose is filled with water and a siphon is created, so water will still flow into the bottom bowl.

Now if a vented loop is added at the top of the loop the siphon is broke and water will not flow to the bottom bowl.

How does a Vented Loop fitting work?


A vented 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 vented loop, 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.

Detail of a vented loop showing the Duckbill one way valve. The valve allows.

Forespar supplied this diagram of how the vented loop works.

Implication of siphons in boats

Jabsco Vented loop diagram

Obviously the raw water or sea water intake of any boats water system is below the waterline. Also in many cases so are the boats toilets, heads, engines, generators etc. This requires a through hull fitting or seacock to be added to the raw water intake. In case of any failure in the hose or any other part of the water system. The seacock can be shut off to stop water incursion in case of an emergency. However during operation of the boats systems, the seacock is left open. This allows for water to enter the inlet if a siphon occurs. Once the water comes in, a toilet or head can flood.

Boats Heads, Toilets

Marine Heads are often installed below the waterline. Therefore its important to have a vented loop. Most often this is installed on the discharge side of the heads plumbing system. See the diagram courtesy of Jabsco showing the vented loop installed above the boats heeled waterline (if its a sailboat) by 8 inches minimum.


Vetus Vented Loop Head

Vetus Vented Loop

If you add or have a Vented Loop in the discharge side of the toilet, add a breather, to vent odors overboard.

I have seen heads & holdings tanks that have vented loops but the vent breather is not plumbed overboard and odors linger below. The picture left shows a Vetus Vented Loop with a overboard breather as part of the package.


Engine or Generator Vented Loop

engine vented loop

In an exhaust system without an anti-siphon valve, when the engine is shut down, raw water continues to siphon into the exhaust system until it reaches the same level as the outside, i.e., the waterline. If the engine is installed below the waterline the water will flow back up the exhaust pipe and into the engine itself.

This can be avoided with a vented loop. The vented loop, or anti siphon breaker is a Loop with a small valve that closes under raw water pump pressure, when the engine runs, and opens when the engine stops, allowing air into the pipe, thus preventing the water from siphoning.

Therefore it must be installed somewhere in the raw water line between the raw water pump and the mixing elbow. This diagram shows the vented loop (siphon break) between the pump and the manifold. It is also possible the loop be after the manifold and before the mixing elbow (where the engine exhaust & raw water mixes)

The vented loop which was installed in the seawater cooling system, by the boats manufacturer will prevent a siphon from building. You should find out where it is and add into your yearly maintenance schedule. Check that the valve is clan and working.

Servicing a Vented Loop

Look at your boats systems and find the location of the vented loops. Note their position and add them to maintenance schedule. These should be inspected to make sure they are clean and the valve is seated properly. If the valve is blocked it wont prevent syphoning.

Servicing a Vented Loop

For servicing procedures look at the  Groco installation and service manual.

For servicing of vented loops, Include them in your yearly! maintenance program that include all seacock’s and through-hull fittings.




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