How Does an Anti-Siphon Valve Work? (Answer in 2020)

Do you have an engine or toilet installed on your boat? In that case, there might be an anti-siphon valve in there as well. The work of this valve is to ensure water is flowing in one direction, so there’s no contamination or unwanted entry or re-entry of water, but how exactly does it do that? Read on to find out.

What Is an Anti-Siphon Valve?

An anti-siphon valve is simply a piece of equipment that lets air in, while stopping the exit of water that is under pressure, thus delivering the effect of breaking the siphon. Virtually all generators, engines, and marine heads below the waterline use anti-siphon valves.

Realize that anti-siphon valves are usually not interchanged with raw water check valves, as the latter are susceptible to failure.

In the event that the anti-siphon valve fails, the inboard engine might stop working, or the boat might get trashed with discharge from the toilet, or even worse, the whole boat might sink. Fortunately, it’s easy to examine and maintain an anti-siphon valve. Let’s see how it works.

How an Anti-Siphon Valve Functions

In the toilet system:

The valve’s functions to stop the discharge water from backflowing into the toilet. A second valve stops water siphoning in the intake system. When installing the valve, it’s essential to follow the manufacturer’s instructions to ensure its effectiveness.

In engines:

The valve stops the flow of cooling water to the exhaust manifold then ultimately into cylinders once you shut the engine. To achieve this function, the valve provides an anti-siphon effect in a raised top loop on the line’s topmost point.

There’s a valve which allows air to enter the hose to end the siphon effect once the water ceases to flow. It also prevents the spilling of water out of the hose.

You might hear of the anti-siphon valve fabrications being termed vented loops, as the valve is usually housed in a hose loop. But, not all boats have such an assembly.

Point of installation

For engines:

Between the injection point on the exhaust elbow and the heat exchanger.

For heads:

Between the toilet bowl and the raw water flush pump. There are many folks out there installing them on the pump inlet, but that is incorrect installation, and it leads the valves to suck air and fail.

For heads, an anti-siphon might be installed somewhere along the bilge pump and the discharge line.

A Few Rules for Anti-Siphon Valve Installations

  • The most vital rule is that the valve should be installed above the waterline. In fact, some manufacturers dictate that the installation be at least 2 feet over the waterline to make sure the operation is efficient. Be sure to check the manufacturer’s installation instructions; this is very important.
  • The anti-siphon valves ought to be examined regularly. They also need to cleaned and serviced at least once per year. This will go a long way in preventing clogging and increasing the valve’s efficiency and longevity.

How to Fix and Maintain Anti-Siphon Valves

The first thing to do is located them. Usually, you will find them by following hoses leading to away from the engine or the head. An anti-siphon valve looks like a lopped pipe, and it’s vented at the upper part.

Look out for leaks

If the valve has an issue, you should notice leaks where the hose joins the looped pipe. There may be some deposit or corrosion that is stained green and perhaps, some odor and stuff on the head hose.

Are there leaks? If so, tighten the loose clamps and remove the hose from the pipe, making sure there is no corrosion in the pipe nipples.

Take off the vent located at the looped pipe’s top and examine it. You can do that by taking off the tube that comes from the vent spout (in case there’s one) and by unbolting the plastic, PVC or bronze fittings at the loop’s top.

At times, also, the vent can be removed by loosening the screws holding down the top of the vent.

How it should be

Often, the valve will be a rubber-like joker/duckbill valve. It could also be a diaphragm valve. When the water flow stops, the valve should open to let air in and prevent a siphon. In contrast, as the water flows, the valve ought to stay closed, so the water doesn’t come out of the air hole.

Taking it out

(i) If it’s a joker valve: Remove it carefully, being careful not to tear it. Look for any tears or other deformities on the valve. If the valve is torn or deformed in other ways, the best thing you can do is replace it, but if there’s only a little calcium deposit, you could just clean it. A high-pressure air pump will help you blow off deposits.

(ii) If it’s a diaphragm valve: it’s frequently a rubber-like item that opens/closes over a frame. You will possibly have to remove a few bolts to unseat it, and if the valve is torn, stiff, or deformed in other ways, the best you can do is replace it.

Sometimes, however, cleaning the valve is enough. If you’re to clean it, be sure, also, to clean the place where it sits. Although you can do the cleaning with your fingers, a little brush will come in handy.

(iii) For other types of valves: look for any malformations and replace or clean the components of the valve, like the spring (if there’s one), according to what the manufacturer instructs. You can also lubricate the valve but use only the products that the manufacturer recommends.

If there’s a vent tube heading to the vent overboard, be sure to check whether it is clogged, as clogging (by bugs and other stuff) might produce a similar effect to when the valve remains closed – that is, the siphon flow is not stopped when it should.

Pro tip: flush vinegar dozes through the head regularly to deal with deposits. This will ensure the valve remains effective for a long time.

Frequently Asked Questions (FAQs)

1. What are vented loops?

Vented loops are structures that stop back siphoning in water systems. They are essentially anti-siphon valves and are normally a vital part of marine toilets and other plumbing systems.

2. How often should I examine the anti-siphon valve?

You ought to examine them as often as you can but not less than once every year. Also, you need to clean them at least once per year to ensure there is no clogging. That way, you will keep them working properly.

3. How do I know that an anti-siphon valve is not working properly?

If the valve has a problem, there’s likely to be a leak where the hose connects to the looped pipe. The best thing to do is replace the valve but sometimes, cleaning it can reverse the issue. 

If you have anything else to know about anti-siphon valve. Then inform us via the comment section.

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Last Updated on April 21st, 2020

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