How does an anti siphon valve actually work.

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mocruising

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The name would suggest that there is a valve in their some where. Yes I know the purpose is to stop sea water back siphoning back through the engine or head/toilet bowl but how does it achieve this.

Why I am asking this question. I have just had an electric loo fitted to the boat. The yard at my request fitted an additional sea water pump so we don't gobble up our FW. The toilet manufacturer recommend that an anti siphon valve be fitted in a loop above the water line on the inlet side of the pump between the seacock and the pump. The yard have done this but they have fitted a Vetus ASDH valve (I think) and not a Vetus ASDV valve. They look identicle on page 79 of the vetus manual.
 
Some makes have a sprung valve, others a flap valve. The idea is that positive pressure in the pipe willclose the valve, whilst any draw on the hose will suck it open, allowing air in to break the syphon effect.

If you have the wrong one, it has no valve but a small hole and will pee all over you whenever it is working! Usually this bleed off is piped to a convenient place such as a cockpit drain so you can see it is working.

Rob.
 
The valves on page 79 of the Vetus catalog should be fitted in a pressurised section of pipe.

You ceratinly dont want the "H" type as it will allow air to be drawn in when the pump operates

Even the "V" will open under suction and allow air to enter.

IF your system is suitable you need the V type fitted in a loop in the pump discharge not the pump suction

They allow air to enter when the pressure falls . the H type because it is open . the V type because a small valve opens when under a slight suction. the valve is held shut by the pump pressure when the pump is operating
 
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It is correct that there are two types of anti-syphon device. One has a constant discharge and should only be used with the appropriate skin fitting which has a very small bore to restrict the flow. The other has a spring which holds the valve open with no water pressure and closes when the pump puts pressure on it. Both types should be fitted AFTER the engine driven pump on an engine, never on the suction line and the advantage of the valved type is that it does not discharge water into the boat. I am a fan of the constant discharge type though as the valve type can jam shut sometimes and there is a risk of catastrophic failure if water does syphon back to the engine. Neither of these situations relate to the use on a toilet though. If one is fitted to a toilet in my opinion either type could be used but not between the seacock and the pump. It should be fitted at the top of a loop between the pump and the toilet so that positive pressure from the pump keeps the valve shut (or discharges a small amount of water with the valveless type) when running but is vented to air when not running to prevent syphoning.

Edit: Just read Vics post.... I have repeated his advice I think!
 
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mocruising said:
The toilet manufacturer recommend that an anti siphon valve be fitted in a loop above the water line on the inlet side of the pump between the seacock and the pump. The yard have done this ...
Click to expand...

Again, wrong place.

All you need do is download ANY toilet manufacturer's instruction manual for a manual head and you'll see where it SHOULD go: between the hand pump and bowl, NOT between the seacock and the pump.
 
Many thanks for all your useful advice however I am confused. I have in front of me the Sanimarin installation manual (Sanimarin SFA 797 04.09 covering sanimarin 31,35,35 ST and 48 and Sani Marin Elite models) it clearly states there should be a siphon valve between the pump and the toilet. ie Seacock/pump/hose in a U rising above the water line with the valve at the top of the U then down again to the toilet. It shows a similar arrangement on the discharge side after the toilet.
 
mocruising said:
I have in front of me the Sanimarin installation manual (Sanimarin SFA 797 04.09 covering sanimarin 31,35,35 ST and 48 and Sani Marin Elite models) it clearly states there should be a siphon valve between the pump and the toilet.
Click to expand...

That's correct, but in your first post you said the valve was "on the inlet side of the pump between the seacock and the pump".

A lot of people do that with manual toilets, because they don't realise the hose from pump to bowl is removable, but it's wrong and doesn't work.

If it's actually fitted as described in your second post, that's fine.

Pete
 
prv said:
That's correct, but in your first post you said the valve was "on the inlet side of the pump between the seacock and the pump".

A lot of people do that with manual toilets, because they don't realise the hose from pump to bowl is removable, but it's wrong and doesn't work.

If it's actually fitted as described in your second post, that's fine.

Pete
Click to expand...

Fine provide it's the "V" type valve .

An "H" type valve will discharge water from the vent when the pump is running.

Which brings us back to the original question how to tell them apart:

The "H" type will discharge water from the vent when the pump is running! the "V" type wont.

You could always unscrew the vent outlet and look to see if there's a valve in it or not!

Even if the thing has been installed in the pump suction all is not lost.
Just put your finger over the vent to stop air being sucked in while you flush the toilet!. :)
 
After 32 years of use, my latest survey insisted that an antisiphon valve be fitted to the toilet.

I pointed out to the surveyorthat the loo had functioned perfectly satisfactorily for 32 years without, and that recently a new manual pump with locked position was fitted to prevent such siphonage.
I suspect that surveyors must find something to comment on, and certainly the original Jabsco installation instructions make no mention, but I am sure they do now, which rather begs the question, "if they are that important, why are they not part of the toilet itself"?
 
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.
 
My original post was correct. The vent has been put in on the suction side of the pump, which is why the only way I can get it to work is by screwing a ss self tapping screw in to the vent pipe. They will have to change the arrangement.
 
mocruising said:
My original post was correct. The vent has been put in on the suction side of the pump, which is why the only way I can get it to work is by screwing a ss self tapping screw in to the vent pipe. They will have to change the arrangement.
Click to expand...

Yep - instead of sucking up water, the pump is just sucking in air through the valve. It's been fitted in the wrong place by a numpty who doesn't know how it works.

If they fitted the "hose" type of fitting rather than a valve, they'll need to plumb in the bleed hose as well, or replace it with the valve kind which is more normal on a toilet.

Pete
 
Two comments-my boat went 30 years without one then one day the auto shower pump came on-toilet was syphoning down into shower tray and through to waste water trap.
I fixed it and it cost me hardly anything.Depending on size of toilet supply pipe use a 22mm copper T with 22mm tail in and tail out.Reduce the third outlet/get one with reducer down to 10mm/8mm pipe and crush end of pipe tail flat with a pair of pliers.If there is too much of a hole left you will have no suction on pump but get it small enough by trial and error and you have a perfect toilet antisyphon loop.
 
mocruising said:
My original post was correct. The vent has been put in on the suction side of the pump, which is why the only way I can get it to work is by screwing a ss self tapping screw in to the vent pipe. They will have to change the arrangement.
Click to expand...

Your post original was not correct. It said "The toilet manufacturer recommend that an anti siphon valve be fitted in a loop above the water line on the inlet side of the pump between the seacock and the pump. The yard have done this....."


However I agree the yard will have to cahnge the installation to waht the toilet manuafacturer really says, namely, "there should be a siphon valve between the pump and the toilet."

Make sure at the same time that it is the "V" type of antisyphon valve that they fit. Otherwise you will have it discharging water every time you flush the toilet. A useful feature when used as an antisyphon device on an engine cooling system but unnecessary and a nuisance on the toilet system
 
pappaecho said:
f...which rather begs the question, "if they are that important, why are they not part of the toilet itself"?
Click to expand...

The reason is simple. The heads are sold with a short, white 3/4 inch hose between the pump and the bowl, because the manufacturers do NOT know whether the toilets will be used above or below the waterline. In MOST boats they are installed below the waterline, hence, requiring the anti-siphon (vented loop) between the bowl and pump. Since the manufacturers can't know what particular arrangement you have on your boat, YOU have to buy the hoses and the vented loop and install it yourself, PER their installation directions.

If you have an RV (caravan), or a megayacht with the head above the waterline, you don't need the vented loop.

RTFM. Please.
 
VicS said:
<snip> A useful feature when used as an antisyphon device on an engine cooling system but unnecessary and a nuisance on the toilet system
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Why (not the need for an anti-syphon device, but the need for a small stream of water)?
 
Comrade Red said:
Why (not the need for an anti-syphon device, but the need for a small stream of water)?
Click to expand...

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 :mad:
 
VicS said:
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 :mad:
Click to expand...

Phew, mine dribbles (a bit like me sometimes).
 
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
 
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