Replacing Boat Antisyphon Valves: A Step-By-Step Guide

The anti-siphon valve is an important component in boats with inboard engines or marine heads. It helps manage the flow of raw cooling water, preventing it from flowing into the exhaust manifold and through the exhaust ports when the engine is shut down. This valve is usually installed with the engine and is often located behind bulkheads or in some corner of the engine space. While it is simple to inspect and easy to maintain, it is crucial to know how to replace it in case of failure, as a malfunctioning anti-siphon valve can destroy your engine or cause serious problems like down-flooding or even sinking.

Locate the valve

To locate the anti-siphon valve, start by checking behind bulkheads or in the corners of your engine space. Look for a hose leading to and from it, with perhaps a smaller tube leading overboard. It will look like a looped pipe and is vented at the top.

On your fuel tank, you should see a tube that goes to the bottom of the tank with an anti-siphon valve on top. The flexible line attaches to the barb of the valve.

If you have an inboard engine or a marine head, you likely have an anti-siphon valve. In your marine head, the anti-siphon valve helps stop the backflow of discharge water into the head and then possibly into the boat.

For the engine, the anti-siphon valve stops 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 at the highest point in the line, which is usually installed with the engine.

Bilge pump risers will prevent downflooding, but they won't prevent a siphon, so they should be equipped with anti-siphon valves. Anti-siphon valves are typically installed between the heat exchanger and the injection point on the exhaust elbow, between the raw-water flush pump and the toilet bowl, and anywhere along the discharge line for the bilge pump.

Check for damage

To check for damage to your anti-siphon valve, you'll first need to locate it. These valves are usually tucked away behind bulkheads or in some corner of the engine space. A clue to their whereabouts is a hose leading to and from it, with perhaps a smaller tube leading overboard. It looks like a looped pipe and is vented at the top.

Once you've located the valve, the next step is to check for any signs of weeping where the hoses join the looped pipe. This can be indicated by a salty deposit, green stain corrosion, and in the case of the head hose, an odour and perhaps other "stuff". If there is weeping, tighten the hose clamps. It is also a good idea to remove the hose from each side of the pipe to check for any corrosion or wear in the pipe nipples, especially if the hoses are too tightly spaced. This is more likely to occur with the caustic material coming from the head and metal nipples.

Next, remove the vent at the top of the looped pipe and inspect it. This is usually done by removing the tube coming from the vent spout (if there is one) and 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.

The valve you'll find could be a rubberlike joker valve (also called a duckbill valve) or a diaphragm valve. The joker valve is supposed to go to the open position when water flow ceases to let in air through the vent to avoid a siphon. When water is flowing, the valve should be closed to prevent water from coming out of the air hole in the top of the vent. Carefully pull out the joker valve, taking care not to tear it, and 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.

For a diaphragm valve, you'll likely need to remove 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, and you can do this with a small brush or your fingers.

For other types of valves, such as those with a spring-loaded ball or flap on a hinge, clean any spring and surfaces or other components according to the manufacturer's instructions. Check for malformation of components and lubricate only with a product recommended by the manufacturer. If there is any question about the valve's condition, replace it.

Finally, if you have a vent tube leading overboard, it's crucial to ensure this isn't clogged (for example, by bugs), as this could have the same effect as the valve remaining closed, allowing a siphon flow.

Remove the vent

To remove the vent, start by locating the anti-siphon valve on your boat. It is usually found behind bulkheads or tucked away in a corner of the engine space. Look for a hose leading to and from it, with perhaps a smaller tube leading overboard. The anti-siphon valve looks like a looped pipe and is vented at the top.

Once you have located the valve, follow these steps to remove the vent:

• Remove any tube coming from the vent spout.
• Unscrew the fitting at the top of the loop. This could be made of bronze, plastic, or PVC. Sometimes, this may involve unscrewing bolts holding down the top of the vent.
• Inspect the valve inside. It is often a rubber-like joker valve (also called a duckbill valve) or a diaphragm valve.
• Carefully remove the valve, taking care not to damage it. Check for any tears, deformities, or deposits that may be causing issues.
• If the valve is a diaphragm valve, you may need to remove some bolts to unseat it.
• Clean the valve and the area where it seats. You can use a small brush or your fingers for this.
• If the valve is damaged or cleaning does not improve its function, replace it with a new one.

Remember, it is crucial to ensure that the vent tube leading overboard is not clogged, as this could have the same effect as a closed valve, allowing a siphon flow.

Inspect the valve

Inspecting the anti-siphon valve on your boat is a simple and important task that can prevent serious issues, including engine failure, toilet discharge, and even sinking. Here are some detailed steps to guide you through the inspection process:

Locate the Anti-Siphon Valve:

First, you need to find the valve. It is usually located behind bulkheads or in a corner of the engine space. Look for a hose leading to and from it, and possibly a smaller tube leading overboard. The anti-siphon valve looks like a looped pipe and is vented at the top.

Check for Weeping:

Inspect the area where the hoses join the looped pipe for any signs of weeping. This can be indicated by a salty deposit, green stain corrosion, or, in the case of the head hose, an odour or other "stuff." If you notice any of these issues, tighten the hose clamps to address the problem.

Remove the Vent:

To gain access to the valve for inspection, you will need to remove the vent at the top of the looped pipe. Start by removing the tube coming from the vent spout, if there is one. Then, unscrew the fitting at the top of the loop, which is usually made of bronze, plastic, or PVC. In some cases, you may need to unscrew bolts holding down the top of the vent.

The anti-siphon valve is typically a rubber-like joker valve (also known as a duckbill valve) or a diaphragm valve. Both types of valves are designed to move to the open position when water flow stops to let air in through the vent, preventing a siphon. When water is flowing, the valve should be closed to prevent water from coming out of the airhole at the top of the vent.

For a joker valve:

• Carefully pull out the valve, being careful not to tear it.
• Inspect the valve for any tears or deformities.
• If the valve is stiff due to calcium or other deposits, try cleaning it carefully. Blowing with low-pressure air may help remove the deposits.
• If cleaning does not restore the valve to proper functioning, replace it.

For a diaphragm valve:

• This type of valve is likely to be a flexible rubber-like membrane that opens and closes over a stainless frame.
• Removal may require unscrewing some bolts to unseat the valve.
• Inspect the valve for any deformities, stiffness, tears, or other signs of poor functioning.
• Clean the valve if necessary. You can use a small brush or your fingers to clean the area where it seats.
• If the valve is damaged or cleaning does not improve its function, replace it.

By regularly inspecting and maintaining your boat's anti-siphon valve, you can help ensure the proper functioning of your marine head and engine systems and prevent potential disasters.

Clean or replace

Antisiphon valves are typically installed between the heat exchanger and the injection point on the exhaust elbow, between the raw-water flush pump and the toilet bowl, and anywhere along the discharge line for the bilge pump. They are simple to inspect and easy to maintain.

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

Joker Valves

Carefully pull the valve out, taking care not to tear it. Check for tears or deformities. You'll likely 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.

Diaphragm Valves

This is likely to be a flexible, rubberlike membrane that opens and closes over a stainless frame. The job will probably require the 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.

Other Valves

Some valves have a spring-loaded ball or flap on a hinge or other devices or floats 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.

Frequently asked questions