Anit-Siphon Loops - How do they work ?

[LadyStardust]

I am replacing hoses in the heads on my boat and I am confused by the anti-siphon loops, the toilet bowl is below the water line and both the sea water inlet and the grey water outlet have an anti siphon fitting, fitted at the top of a looped hose. The fitting is roughly on the water line. At the top of the fittings is a small hole that is used, i assume, to stop siphoning occurring, one of the fittings has a metre of small hose attached to it which goes above then below the water line and is sealed off. The other (inlet hose) is just open and water squirts out when pumping sea water in
Does anyone know or can point me to an explanation of how these systems are meant to work in this kind of installation.

 

[bruce]

water flows down, except under pressure, the water will rise to water level the boat is sitting. by having a hose looped higher than lowest point the boat may lean, the water must be pumped in to get over the hump and not flooding the boat if toilet valve fails. the loop with vent should be at the high point of the loop, but yours sounds different, prob should have a local marine shop or fellow boater to have a look just to be safe. another way to look at it is to take a straw and glass of coke, the coke flows up the straw only to the level of the coke in the glass which would represent the waterline of your boat. the a.s.loop would go above this line.

 

[Anonymous]

I'm glad you asked - mine is similar and I haven't got around to working out how it works yet....my anti-syphon fitting is well above the water line and has a hole in it. When I flush, I can hear air being sucked in. I can operate the flush while putting a finger over the little hole which, unsurprisingly, stops the air getting in /forums/images/graemlins/smirk.gif but a syphon doesn't actually start with my finger over it. There is a little white float inside the pipe (transparent plastic pipe) on the toilet side of the AS fitting that behaves like a valve. Presumably that's to stop any foul water from going back that way? I'd be glad to get more details of what should be in such a system and whether there is any recommended regular maintenance.

 

[HeadMistress]

Re: Anit-Siphon Loops - How they work

An anti-siphon loop serves two purposes...the loop creates a "hill" in a line that's far above the waterline at angle of heel to prevent sea water from rising it over it to flood the toilet, engine or other device.

The air valve (there should be one, not just a hole in the nipple at the top of the loop) pulls air into the line to break a siphon created by the priming of a pump.

So a vented loop is actually two devices in one--a high loop AND a siphon break.

An anti-siphon device is only needed in lines through which water is PULLED...because pulling water is what starts a siphon. It has no effect whatever on liquids pushed through a line--although in the absence of an air valve, whatever is being pushed through the line will squirt out the hole where the air valve--which is a one-way valve that only allows air to be pulled IN, nothing out--should be.

Which should explain why a vented loop in a toilet intake line should NOT be installed between the thru-hull and pump, because air is pulled into the line by the anti-siphon valve (or hole) that prevents the pump from priming. It has to be installed between the pump and bowl where it cannot interfere--but can squirt if no air valve is installed--with flushing the toilet because the pump is no longer PULLING water, but is now PUSHING water to the bowl.

So...if an anti-siphon only functions in lines through which water is being PULLED, why would a vented loop be needed in a toilet or any other discharge line? The first reason: the loop above the waterline. But also because an open seacock can allow enough water to be pushed up it to connect with any water already in the line, producing a continuous flow of water that could start it working the other way..the siphon break injects air into the line to stop it.

 

[Anonymous]

Re: Anit-Siphon Loops - How they work

Peggy,

My pipes go down into areas that I haven't been able to look into at the time of writing but as far as I can see, my A/S device is on the inlet, between the sea cock and the toilet inlet pump. What seems to happen is that a small amount of air does get sucked into the line but not enough to bother the pump. Presumably the idea is that there is enough air to prevent a syphon though I cannot prove that since I cannot start a syphon even by putting my finger over the A/S air hole. This seems to be a deliberate design by Nauticat so presumably there is a school of thought that advocates that design?

David

 

[alahol2]

I suspect the anti syphon loop with the small bore tube is actually a syphon break intended for the cooling water outlet of a diesel engine (before it rejoins the exhaust elbow). The small bore pipe would normally exit through the topsides and act as a telltale with a small flow of water. There is normally no valve in this type of break. If your small bore tube is sealed, I don't think it will act as a syphon break.

 

[Ships_Cat]

Re: Anit-Siphon Loops - How they work

I suspect you might be on the right track when you say that yours works by not letting enough air in to affect the pump suction.

There are a number of ways to stop the anti-siphon valve letting enough air in to destroy the pump suction. One is to just have a very small hole (which may be inclined to block though). Another, and is what we did, is to have a big hole in the valve but a piece of permeable foam under it. The foam lets enough air through to destroy any siphon (should one occur) but does not let enough air in to affect the pump (manual) suction.

The benefit of going to the trouble of having the anti-siphon on the suction side is, of course, it normally results in less and tidier more easily hidden pipework.

John

 

[Gunfleet]

Re: Anit-Siphon Loops - How they work

I know you're an absolute expert in this Peggy but it sounds very complicated (to my non expert mind). Is it the case that there should be a loop with a siphon break above the water line after the pump on the inlet side - ie between the bowl and pump? This could be difficult to arrange. My loo siphons from the inlet but I decided it was just as easy to simply shut the cock on the inlet side when you are not actually pumping.
John

 

[HeadMistress]

Re: Anit-Siphon Loops - How they work

It's not really that complicated...but you're quite right that the whole problem can be solved by keeping the seaocks closed when not actually in use. The problem is, many boat owners don't...and that's not just an idictment of owners, but of boat builders who--because decor has trumped safety and seacocks are anything BUT "decor" items--have taken in recent years to putting seacocks in places that are so inaccessible as to make opening and closing them as needed so difficult that it's d'd near impossible. So owners rely on the wet/dry valves in the toilets to keep their boats afloat instead...which is VERY risky because a) wet/dry valves--especially in cheaper toilets--have a nasty habit of failing...and b) people, especially guests--have an even nastier habit of leaving 'em in the wet position.

So because one can never go too far wrong erring on the side of caution, IMO it's best to bite the bullet and install vented loops. They only have to be at least 4-6" above waterline at any angle of heel...and if someone should forget to close the seacock and/or leave the wet/dry valve in the wet position, your boat will be safe.

 

[rwoofer]

Re: Anit-Siphon Loops - How they work

"Which should explain why a vented loop in a toilet intake line should NOT be installed between the thru-hull and pump, because air is pulled into the line by the anti-siphon valve (or hole) that prevents the pump from priming. It has to be installed between the pump and bowl where it cannot interfere--but can squirt if no air valve is installed--with flushing the toilet because the pump is no longer PULLING water, but is now PUSHING water to the bowl. "

Peggy, thanks for explaining why my toilet is like this. Interestingly Nigel Calders book does not describe this particular setup, which is why I fretted over it when replacing my heads, which is as you described.

Nigel describes two setups 1) with anti syphons in both the inlet (seacock to pump) and outlet 2) Something that isn't above and different from 1). Why might he have got it wrong?

 

[Ships_Cat]

Re: Anti-Siphon Loops - How they work

I have not seen the figure you refer to but if it is as you say then he has not got it wrong because if you read my earlier post you will see that it is possible to put the siphon breaker between the seacock and the pump but you have to incorporate some method such that the pump maintains its prime.

It is frequently done - our own boat is done that way and from my understanding from Lemain's post, his is too. Also having the break before the pump becomes especially easy (and usually necessary) if one has an electric toilet as then one can just use a solenoid valve on the vented loop, with the valve closing when the pump runs and opening venting the loop when the pump stops running.

John

 

[HeadMistress]

Re: Anti-Siphon Loops - How they work

Nigel didn't get it wrong (he never does)...however, not all toilets are designed to allow a vented loop to go between the pump and the bowl. The cheaper "bicycle" pumps--which are the toilets most OEMs use and most boat owners buy--have a hose coming off the the top of the pump to the back of the bowl which is replaced by the line with the loop in it. But expensive manual toilets like the Blake which have a foot pedal or other separate valve control for flush water, and electric macerating toilets with integral intake pumps don't have that line between the pump and the bowl...so the only place a loop CAN go is in the line between the seacock and the pump. And as John says, electric toilets with integral intake pumps require an electric solenoid valve instead of the standard vented loop, to protect the intake impeller from excessive wear. The only solution for some manual toilets is, put your finger over the hole in the loop while pumping the toilet.

 

[LadyStardust]

Re: Anti-Siphon Loops - How they work

Thanks Peggy for your description. I will go back and look to see if I can place the incoming vented loop after the pump, but I think that will be difficult. I have moved the loop to be as high as possible without major surgery, and it should be above the water line now. It does not have an air valve, and I wonder if that explains the poor pumping i was getting?
The reason for this re-plumbing was, beside oldish hoses, I arrived at my boat to find the bowl overflowing and filling the bilges quite nicely last week. Reasons were, 1-The boat was heeled over slightly putting the loop below water level, 2-the seacock was not as i expected, 90deg to hose means nearly off, 75deg was the true off, and thirdly I hadn't set the toilet valve to 'dry' as the toilet, and in fact the boat, are both new to me, I assumed turning the seacock off would do the trick!
Lucky I wasn’t away for a long time!
On the outgoing side, the vented loop has this sealed small hose connected to the top of the loop, again, no valve, just a small hole in the fitting. I suppose this will stop any waste being sprayed around the locker it's in, but will this sealed pipe undo the effects of the anti-siphon loop.

 

[Ships_Cat]

Re: Anti-Siphon Loops - How they work

Peggy

Not wanting to appear to be unravelling of your advice as I hold it in high esteem, but it is possible to put the antisiphon loop on the suction side of the pump, even on the "bicycle" pump ones such as the cheap manual Jabsco ones. Indeed the instructions do say to remove the pipe between the pump and the bowl and fit the antisiphon there, but it is possible to ignore that and place it in the suction line as long as some "trick" is used to protect the priming of the pump.

The main advantages of setting it up with the antisiphon on the suction side are, of course, that it isolates the whole toilet including the pump from the sea and with fewer connections, and (more importantly if appearances are of concern) results in much less visible and not visible pipework, there only being two hoses to the toilet (the seawater inlet and the discharge, with just the short manufacturer supplied hose remaining between the pump and bowl).

Obviously it has to be done so as the suction of the pump is protected but that can be achieved along the lines I set out (I am also told that the prime can be preserved with judicious pipe layout too, but I have never seen that done).

Regards

John

 

[HeadMistress]

Re: Anti-Siphon Loops - How they work

Paul, you said, "I arrived at my boat to find the bowl overflowing and filling the bilges quite nicely last week. Reasons were..."

The REAL reason was: you left the bloomin' seacock open! /forums/images/graemlins/smile.gif Relying on a wet/dry valve and/or making sure it's in the dry position, or even a vented loop that can very easily become clogged--preventing it from functioning as an anti-siphon device--is a VERY good way to sink a boat! Although adhering to the strict rule of keeping all seacocks closed except when actually in use can be so difficult as to be impossible, NEVER leave the boat with seacocks open.

John, you said, "but it is possible to ignore that and place it in the suction line as long as some "trick" is used to protect the priming of the pump."

Ok...other than putting your finger over the hole while pumping, what are the tricks? (That's NOT a sarcastic question...I'm always interested in learning something new. In fact, some of my best ideas came from other boat owners.)

 

[LadyStardust]

Re: Anti-Siphon Loops - How they work

You are quite right Peggy, I did leave the sea cock open, but I think this is an important lesson, I had deliberatley closed the sea cock until i had time to check out the plumbing, learn how it worked and what to do etc. My mistake was in assuming that the seacock, because it looked like all the others, worked in the same way. I also assumed that the boat yard that had removed, tested and replaced it had done a good job. I will be having the boat hauled next month and the seacock replaced, then I will find out why my assumptions were wrong.
With boats, especially the bits that might sink you, the lesson must be - Don't Make Assumptions!
but the exercise has at least taught me a bit about anti-siphon engineering, and I think the loop was to low, now rectified.
When i finishg re-plumbing tomorrow I will know if my pump can prime or not
Thanks P

 

[Ships_Cat]

Re: Anti-Siphon Loops - How they work

As in my earlier post, the simplest way is to put a small piece of porous soft foam in the siphon vent instead of the valve. For example, using one of the Forespar loops as typical, one takes the little rubber duck bill valve out and replaces it with a small cubical piece of soft foam and screws the cap back down onto it. A small piece cut from the foam strip used for draft strip (but the very small celled stuff) is ok.

It will let enough air through to break the siphon (you can feel the air sucking in through it after finishing pumping) but not enough to trouble the pump. The foam does not seem to get salt caked in it, I assume because the wet side is always below atmospheric pressure when working. Obviously, one cannot do (and would have no need to do) the same thing in any pipe that is subject to above atmospheric pressure.

On our own boat, for example and with a "bicycle pump" toilet, it enabled the siphon break to be hidden in a locker between the sea water services manifold (all our seawater, apart from the engine cooling which has a dedicated supply, comes in through one seacock) and the toilet without having to pipe to the toilet, then back to the siphon break, then back to the toilet again. The foam has never required replacing or rinsing since the boat was built for us 8-1/2 years ago.

{Edit: Perhaps I can add that the same sea water service manifold has a number of other pumps off it - small centrifugal for freezer, several hand pumps, and a diaphragm deck wash pump - none of which are troubled by the same siphon breaker which is on their suction side.}

John

 

[Anonymous]

Re: Anti-Siphon Loops - How they work

[ QUOTE ]
...other than putting your finger over the hole while pumping, what are the tricks? (That's NOT a sarcastic question...I'm always interested in learning something new. In fact, some of my best ideas came from other boat owners.)

[/ QUOTE ]Peggie, I have two Jabscos as installed by Nauticat Finland in December 1998 and neither they nor the plumbing has been altered apart from a new holding tank that I had fitted last September. Both toilets have a loop of transparent plastic pipe looped well above the waterline, with a plastic device, like an engine anti-syphon with small inlet hole, on the top, in line with the toilet flush on the low pressure side (inlet) of the manual pump. Both toilets work the same; there is a quiet hissing on each stroke when flushing but there is sufficient suction, despite the air, to allow water to pass on each stroke. I had never put a finger over the hole until the other night when I did so to see what would happen - in fact the piping is behind decorative trim that is only removed for maintenance. I don't think that the hole is very critical - the pump clearly handles the air without any problems and would take more, though the amount of water per stroke would obviously reduce. When we bought the boat in August she had been out of the water for several months and the toilets both primed themselves as soon as she was launched. David

 

[HeadMistress]

Re: Anti-Siphon Loops - How they work

John, have you used the foam in a loop in a discharge line--one in which water and/or waste would squirt out the hole without an air valve? If so, does it work without any leaking? Or stink after a while?

'Cuz I'm not a fan of putting a vent line on the nipple, as many people do...especially on a loop in a toilet discharge. The hole and line are so small that waste squirting into it quickly builds up into a blockage that turns the loop into an UNvented one with no anti-siphon function any more.

Lemain, if your Jabsco toilets have no trouble priming with the loop between the seacock and the pump, count your blessings. It does occasionally work there, but rarely...and that's not the recommended location. All the toilet installation instructions show it between the pump and the bowl.

 

[Ships_Cat]

Re: Anti-Siphon Loops - How they work

No Peggie (would I be so silly /forums/images/graemlins/smile.gif).

The siphon breaker is in the suction hose between the seawater inlet seacock for the flushing water and the suction side of the pump. As I said in my post, the "piece of foam" method can only be used under suction (not that there is any reason to want use it on the pressure side, of course, as the normal valve in the breaker is then fine).

I have also seen a small hole used as Lemain describes - small enough to not need ones finger to go over it. I believe that what Lemain is describing (perhaps he can say if this might be so) is that the valve is extended upwards on a piece of hose so that it will not tend to dribble through the small hole when water is passing through the loop.

John