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Anti-Siphon Valve Basics

Managing your potable water flow is important, but perhaps even more important is managing your raw-water flow. A critical component is the anti-siphon valve.

Vented loop open closeup illustration

Most vented loops operate by allowing air into a passageway to break the siphon effect. When flow from the pump stops, air is introduced through the duckbill valve. (Illustration: ©2018 Mirto Art Studio)

If you have an inboard engine or a marine head, chances are you have an anti-siphon valve. If that anti-siphon valve stops working, its failure may destroy your engine or trash your boat with toilet discharge or sink the entire boat. The good news is that anti-siphon valves are simple to inspect and easy to maintain.

How It Works

In your marine head, the anti-siphon valve works to help stop backflow of discharge water into the head and then possibly into the boat. A similar valve also works to help stop siphoning of water in the head intake line when the head is below the maximum-heeled waterline. The anti-siphon valve should be installed above that point. Follow the head manufacturer's instructions with this.


Encrusted deposits around the exterior surface of a vented loop is a sure sign that service is required.

For the engine, the anti-siphon valve's purpose is to stop raw cooling water from flowing into the exhaust manifold and through the exhaust ports in the block and then down into the cylinders when the engine is shut down. It does this by providing a siphon break in an elevated loop at the highest point in the line, which is normally installed with the engine. It incorporates a valve that lets air into the hose to break the siphon effect when water stops flowing, and that keeps flowing water from spilling out of the hose. (Anti-siphon valve assemblies are sometimes referred to as "vented loops" because the valve is typically in a pipe assembly in a loop of hose.) These are not found in all boats. There are other anti-siphon valves for fuel systems and other purposes. These are beyond the scope of this article.

Vented loop bronze illustration 1

Vented loop bronze illustration 2

Vented loop bronze closed illustration

Vented loop bronze open illustration

Illustrations: ©2018 Mirto Art Studio

How To Fix And Maintain

Locate each one on your boat. They usually live behind bulkheads or are stuck away in some corner of your engine space. A clue to their whereabouts is a hose leading to it and one going from it, with perhaps a smaller tube leading overboard. It looks like a looped pipe and is vented at the top.

1. Make sure there's no weeping where the hoses join the looped pipe. This is indicated by salty deposit, green stain corrosion, and on the head hose, odor and perhaps other "stuff." If there is weeping, tighten the hose clamps. It's best to remove the hose from each side of the pipe to make sure there's no corrosion or wear in the pipe nipples. This is particularly likely to occur with the caustic material coming from the head and metal nipples. If the hoses leading to the loop are too tightly spaced, you may have difficulty doing this.

2. Remove the vent at the top of the looped pipe and inspect it. This is done by removing the tube coming from the vent spout (if there is one) and usually unscrewing the bronze, plastic, or PVC fitting at the top of the loop. Sometimes this is done by unscrewing bolts that hold down the top to the vent.

Illustrations: ©2018 Mirto Art Studio

3. The valve is often a rubberlike joker valve (also called duckbill valve) or a diaphragm valve. Each one is supposed to go to the open position when water flow ceases in order to let in air through the vent to avoid a siphon. When water is flowing (as by pumping the head or running the engine), the valve should be closed to prevent water from coming out of the air hole in the top of the vent.

4. For a joker valve, carefully pull it out, taking care not to tear it. Check for tears or deformities. Frequently you'll need to replace it because it will be stiff from calcium and other deposits, although you may be able to carefully clean these off. Blowing with low-pressure air may help.

5. For a diaphragm valve: This is likely to be a flexible rubberlike membrane, which opens and closes over a stainless frame. The job will probably require removal of some bolts to unseat the valve. Replace the valve if it is deformed, stiff, torn, or showing any other signs of working poorly. Sometimes just cleaning it will suffice. Clean the area in which it seats. A small brush may help, but often you can do it with just your fingers.

6. For other valves: Some valves have a spring-loaded ball or flap on a hinge or other device or float designed for this purpose. Clean any spring and surfaces or other components according to manufacturer instructions. Check for malformation of components. Lubricate, but only with a product recommended by the manufacturer. If there is any question, replace the valve.

7. If you have a vent tube leading overboard, it's critical to see that this isn't clogged (such as by bugs), as this could have the same effect as the valve remaining closed, thus allowing a siphon flow.

8. If a valve fails, the consequences could be dire. I and other people have sometimes removed the valve from the loop in the engine system (NOT the head discharge) and installed a hose from the air vent in the loop to a vent overboard where a small amount of diverted water flow can be easily seen when the engine is running. With this arrangement, you don't worry about the valve hanging up. The vented passage is always open. And you can easily tell when the water is running and when it stops. This flow, however, must not be of such volume that it adversely affects the cooling of the exhaust.

9. Flushing regular doses of vinegar through the head will help to diminish any deposits.

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Tom Neale

Technical Editor, BoatUS Magazine

One of the top technical experts in the marine industry, Tom Neale, BoatUS Magazine Technical Editor, has won nine first-place awards from Boating Writers International, and is author of the magazine’s popular "Ask The Experts" column. His depth of technical knowledge comes from living aboard various boats with his family for more than 30 years, cruising far and wide, and essentially learning how to install, fix, and rebuild every system onboard himself. A lawyer by training, for most of his career Tom has been an editor and columnist at national magazines such as Cruising World, PassageMaker, and Soundings. He wrote the acclaimed memoir All In The Same Boat (McGraw Hill), as well as Chesapeake Bay Cruising Guide, Vol. 1. These days, Tom and his wife Mel enjoy cruising their 2006 Camano 41 Chez Nous with their grandchildren.