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Old 19-10-2015, 05:30   #1
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Anti Siphon on the waste output

Hi all
Please see attached diagram, my question is;

Do I need an anti siphon on my output from the holding tank, the pipe with the manual hand pump ( in my diagram )

I would only ever open the seacock when I was pumping out, this setup would rely on the one way valve inside the pump.

The tank may or may not be below heeled waterline.

Make sure tank is empty before removing manual pump! or fit a shut-off valve directly under the tank.

The diagram is not to scale or arrangement ( I don't have to crawl under floorboards or tanks to pump out! )

I'm just trying to have a very simple design with short pipe runs.

many thanks
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Old 19-10-2015, 08:30   #2
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re: Anti Siphon on the waste output

If any part of the tank is below waterline, I'd say yes...a vented loop between the pump and the thru-hull is needed. However, if you can mount the pump above the top of the tank, that could let you get away without one. Despite your assurances that you'll NEVER allow anything that needs its protection, nobody's infallible and stuff happens to the best equipment and installations . You can never go wrong erring on the side of caution.

Howevr, if the tank is entirely above waterline, the answer is no...a loop would only prevent gravity from draining the tank....in which case, you wouldn't need the pump either.
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Old 19-10-2015, 14:02   #3
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Re: Anti Siphon on the waste output

Many thanks Peggie, here is a possible mod.

If this idea is good, would the red 'T' juction in the diagram be a 'Y' valve, or ok as a simple 'T' junction?

I do like the idea of a gravity discharge system ( no pump ) but worry about the extra length of pipe going up to it's top?

I like the one in the diagram because the long pipes should sit empty ( it seems to me )

thanks again.
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Old 19-10-2015, 14:26   #4
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Re: Anti Siphon on the waste output

It wouldn't HAVE to be y-valve, but it should be.

I do like the idea of a gravity discharge system ( no pump ) but worry about the extra length of pipe going up to it's top?

You lost me with that. Or maybe not...

You'd have a length of pipe/hose coming out of the top of the tank going straight to the deck pump out...there'd also have to be a pickup tube inside the tank that reaches to the bottom (I'll give you details on how to install that if you need 'em). A second pipe (hose) connected to a fitting IN the bottom (NOT "at" the bottom) of the tank that goes directly to a below waterline thru-hull. It can even stay open to allow you to flush directly over through the tank when at sea. Just rinse out the system fairly often to remove sludge in the bottom of the tank and the discharge hose.
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Old 19-10-2015, 14:43   #5
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Re: Anti Siphon on the waste output

When I said 'worry about the extra length of pipe..' I mean from the toilet up to the top of a gravity tank, ( not in my diagram ) I guess it depends how high I would need to mount a gravity tank.

I prefer the non gravity version in my diagrams, because I can have a bigger tank, and a shorter run from toilet to tank.

also in my diagram, I have assumed my deck pump out pipe did go to the bottom, so is the pickup itself?
Have I got confused there?

thanks for your time.
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Old 20-10-2015, 02:29   #6
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Re: Anti Siphon on the waste output

Here is an update with the Y Valve instead of T junction.
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Old 20-10-2015, 06:08   #7
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Re: Anti Siphon on the waste output

Do I now need an anti-Siphon valve between Y-Valve and Pump, as in diagram?
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Old 20-10-2015, 09:23   #8
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Re: Anti Siphon on the waste output

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Originally Posted by mtompson View Post
Do I now need an anti-Siphon valve between Y-Valve and Pump, as in diagram?
Anti-siphon won't work on the suction side of the pump as it's designed to open with negative pressure, it must be on the discharge side. Our Cape Dory configuration is like you're proposing except the holding tank is built in to the keel and the anti-siphon is between pump and valve btw.

One other thought, depending how long the solids sit in the tank they may need the force of a pump to suck the tank out that siphon alone won't provide. Pretty nasty business
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Old 20-10-2015, 09:28   #9
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Re: Anti Siphon on the waste output

Mine is set up just like your last drawing, has worked satisfactorily for the past 12 years, in coastal and off shore waters, and it eliminates another maintenance/ mechanical item (the anti siphon ). Just remember to close the sea-cock after pumping, otherwise it could fill your holding tank.
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Old 20-10-2015, 09:38   #10
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Re: Anti Siphon on the waste output

This is why my system's outflow is above the waterline (well, not much, but the logic is that pumpout is not going to give offense because I'll be alone). Water is brought in via the standpipe to the Lavac, and either goes up via a bulkhead mounted pump to the holding tank, or may be diverted where legal out of the boat. Wave action does the washdown.
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Old 20-10-2015, 10:37   #11
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Re: Anti Siphon on the waste output

Thanks everyone, wow it's a minefield of systems & ideas this subject!
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Old 20-10-2015, 13:37   #12
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Re: Anti Siphon on the waste output

new version, with a question on having an anti siphon straight off the toilet.
I have ordered 'Get Rid of Boat Odors' so after I've read that I may not need to ask so many questions! but I really appreciate everybodys time.
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Old 22-10-2015, 02:55   #13
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Re: Anti Siphon on the waste output

Ok, I've done a bit more reading, I'm now putting the vented loop between the overboard (manual) pump and the seacock.
(Just as a side note, these diagrams are concentrating on the discharge side, I will have a vented loop on my intake side.)

All vented loops sit above heeled waterline.
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Old 22-10-2015, 09:02   #14
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Re: Anti Siphon on the waste output

Quote:
Originally Posted by mtompson View Post
Ok, I've done a bit more reading, I'm now putting the vented loop between the overboard (manual) pump and the seacock.
(Just as a side note, these diagrams are concentrating on the discharge side, I will have a vented loop on my intake side.)

All vented loops sit above heeled waterline.
Vented loop won't work on the intake side, you'll just suck air via the vent.
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Old 22-10-2015, 09:18   #15
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Re: Anti Siphon on the waste output

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Vented loop won't work on the intake side, you'll just suck air via the vent.
I think he means the TOILET intake. If so, it doesn't belong in the intake line between the thru-hull and the pump..it has to go between the pump and the bowl because the pump PUSHES water to the bowl. And there needs to be an air valve in the loop to prevent squirting.

The loop in the tank discharge line should go in AFTER the pump. It too needs an air valve.

Too many people don't even know what an air valve is, or that vented loops are supposed to have one. So they think the solution to a loop in a discharge line that squirts is a vent line....not realizing that because that line is so small--typically only 1/4" ID--it quickly becomes clogged up with sea water minerals or waste...turning the loop into an UNvented loop that no longer has any ability to break a siphon. And because the vent line has solved the squirting problem, it's "out of sight, out mind"...never occurs to anyone to clean it. Air valves also should be removed and cleaned and replaced when worn out.
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