How does an anti siphon valve actually work.

[Tranona]

Lavac installations have the syphon break (albeit a rather smaller hole than in a Vetus break) on the 'suction' side without problems. It depends if your pump can suck more water than air.

That, however, performs a different function. It is there to provide a controlled release of the vacuum which draws the water in. The size of hole determines when the vacuum stops and water is no longer drawn into the bowl. The vacuum is created initially by pumping and the bowl lid sealing onto the bowl. If the hole is too small the vacuum is released too slowly and the bowl can overfill. Two different size "holes" are provided to suit different pipe runs.

 

[starfire]

Not a "need". A useful feature.
If you can see water discharging while the engine is running you know the vent is not blocked. If it can be discharged to a cockpit drain it's easily seen (although the crew will be off to the heads more frequently) but even if overboard it can be checked.

With valved types you have no way of knowing if they are blocked or not ... except thet they often dribble

And if overboard, a useful source of warm water if you are cleaning the decks while on passage !

 

[alahol2]

That, however, performs a different function. It is there to provide a controlled release of the vacuum which draws the water in. The size of hole determines when the vacuum stops and water is no longer drawn into the bowl. The vacuum is created initially by pumping and the bowl lid sealing onto the bowl. If the hole is too small the vacuum is released too slowly and the bowl can overfill. Two different size "holes" are provided to suit different pipe runs.

I'd have to partially disagree because if that hole were not in the inlet pipe then the water would continue to syphon into the bowl/boat. I agree the size of the hole affects the volume of water ending up in the bowl. It's a dual function hole.

 

[Dave_Seager]

A very interesting thread.
My YM30 engine has an anti-syphon valve on the inlet side of the water pump. There is a small diameter hose coming off the valve and is have assumed that this goes to the output side of the water pump - but have not checked this.
I am now wondering if this is failing and letting air into the inlet side and reducing flow.
I will check this out next weekend but any thoughts or experiences & observations would be appreciated
Thanks
Martin

Anti-syphon vents must be open to the atmosphere to work. If yours is connected to the outlet side of the pump then, when the engine is running, water would flow from the output side to input and when the engine was stopped it would probably have no effect.

 

[VicS]

I'd have to partially disagree because if that hole were not in the inlet pipe then the water would continue to syphon into the bowl/boat. I agree the size of the hole affects the volume of water ending up in the bowl. It's a dual function hole.

You are right. It functions as a syphon break if the toilet is below the water line but it would still be needed to break the vacuum and control the volume of water left in the bowl even if the toilet was above the water line.

 

[VicS]

A very interesting thread.
My YM30 engine has an anti-syphon valve on the inlet side of the water pump. There is a small diameter hose coming off the valve and is have assumed that this goes to the output side of the water pump - but have not checked this.
I am now wondering if this is failing and letting air into the inlet side and reducing flow.
I will check this out next weekend but any thoughts or experiences & observations would be appreciated
Thanks
Martin

If its in the pump suction it will allow the pump to draw in air reducing the amount of water being pumped.

If the vent line is connected to some other point in the system it will almost certainly prevent it operating as a syphon break!

The Vetus catalog includes recommendations about the height of the water injection into the exhaust below which a syphon break is required. (15cm minimum)
They also give a recommended height above the w/l at which the antisyphon valve should be mounted. ( 40 cm minimum)

 

[Hazymoonshine]

Anti-Syphon

Further to VicS's post, my Vetus anti-syphon in my Jeanneau is positioned as described - 40cm or so above the waterline. It runs a small black tube down into the bilges. This tube acts as the air inlet to break the syphon - and also drains a small amount of water out.

My question is, can I shorten this pipe, because it appears to have been allowing a lot more water through and I would like to keep an eye on it. I plan to shorten it and have it drip into an empty coke bottle or similar. Surely it doesn't matter whether the tube is 1m or 10cm long??

Best wishes

Julian

 

[alahol2]

Further to VicS's post, my Vetus anti-syphon in my Jeanneau is positioned as described - 40cm or so above the waterline. It runs a small black tube down into the bilges. This tube acts as the air inlet to break the syphon - and also drains a small amount of water out.

My question is, can I shorten this pipe, because it appears to have been allowing a lot more water through and I would like to keep an eye on it. I plan to shorten it and have it drip into an empty coke bottle or similar. Surely it doesn't matter whether the tube is 1m or 10cm long??

Best wishes

Julian

You are correct in thinking that the length of pipe has no influence on the operation of the anti-syphon. Yes you could shorten the pipe and drain into a bottle BUT make sure the pipe doesn't go below the level of liquid in the bottle or it will cease to function as an AIR inlet. It may be time for you to take the top off the anti-syphon loop and clean the valve to prevent it 'dribbling'.

 

[Hazymoonshine]

Thank you for that advice - I assumed that was the case but always better to check! I plan to check it frequently, as I will move it to a much more accessible spot - so obviating the risk of it ending up under water.

I had the unit apart last summer to make sure it was fully 's', but perhaps I ought to take another look. Good point.

(Thank you also for your Solent guidance over the years, much appreciated!)

 

[Halo]

If the anti syphon valve fiited (higher than the stariner on my engine water inlet) has failed could it be letting in too much air into the and so reducing the water flow rate ?

 

[Tranona]

Purely coincidence I have just had my valve apart for cleaning - Volvo 2030 where it is fitted between the pump outlet and the entry to the heat exchanger. Has a diaphragm valve in it and the vent goes in a pipe to the Volvo water trap silencer. Surprised how tiny the hole was - and the cost of the repair kit (£40+ from Volvo) although diaphragm, washer and o ring "only" £3.60 from Keypart. Overnight in a tub of vinegar cleaned them up nicely.

 

[VicS]

Further to VicS's post, my Vetus anti-syphon in my Jeanneau is positioned as described - 40cm or so above the waterline. It runs a small black tube down into the bilges. This tube acts as the air inlet to break the syphon - and also drains a small amount of water out.

My question is, can I shorten this pipe, because it appears to have been allowing a lot more water through and I would like to keep an eye on it. I plan to shorten it and have it drip into an empty coke bottle or similar. Surely it doesn't matter whether the tube is 1m or 10cm long??

Best wishes

Julian

I am worried that if the vent tube is extended into the bilges it might even set up it own syphon and in any case if the (narrow) tube became full of water it would stop the thing breaking the syphon in the cooling system.

Shorten it to be well above the water line and allow the dribbles to collect in a suitable container.

 

[Hazymoonshine]

Thank you very much for this. I will be down on my boat this Friday and it is my first task. The second is to run the engine at operating revs under load and watch for drips/a stream.

Apologies for the slow reply to your advice, much appreciated.

Best wishes

Julian