Vented Loop for raw water to head. - SailNet Community
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post #1 of 12 Old 06-24-2013 Thread Starter
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Vented Loop for raw water to head.

I have a dedicated thru-hull to provide raw water to the head, which is also at least at times below the water line. Prior owner had no vented loop from the thru-hull to the head. I'm installing a new head (goodbye Headmate!) and several schematics I have seen show it is advised to have a vented loop well above water line between the raw water inlet and the head.

My main question is just how does a vented loop work? It seems to me that with a vented loop between the pump and water source, all that will pump is air; with insufficient suction to have the raw water even reach the vented loop, to say nothing of the head.

What am I missing?

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post #2 of 12 Old 06-24-2013
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Re: Vented Loop for raw water to head.

Vented loops should be connected to every thru-hull except the engine intake. They stop siphoning. Once a hose is filled, there is a potential for the water to reverse and siphon in any time the inside end is lower than the thru-hull. The little air vent in a loop lets air in once there is no pressure in the line and the water wants to drain down due to gravity. Suction lets air in so no siphon can take place. They need to be located as high as possible. I put the Marelon plastic loops on all my thru-hull fittings.

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post #3 of 12 Old 06-24-2013 Thread Starter
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Re: Vented Loop for raw water to head.

I have taken a fresh look at several sources and simply must have gotten it wrong. Thre different references give the same advice... put the vented loop between the pump and bowl.

Sorry for the false alarm but I have the info I need.

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post #4 of 12 Old 06-24-2013
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Re: Vented Loop for raw water to head.

You can put a vented loop between the raw water intake and the bowl. You just have to put your finger over the hole on top of the vented loop when you pump water into your toilet. With a vented loop there, you won't have to worry about siphoning or forgetting to close the valve.

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post #5 of 12 Old 06-24-2013
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Re: Vented Loop for raw water to head.

The vented loop on Aeolus is between the pump and toilet, not the through hull and pump. Since this through hull remains open, it present the possibility of leaking at the pump itself, but since the system is closed until it gets to the toilet, there is no way for the boat to flood because the siphon break is between the pump and toilet bowl. I think this is the standard set-up, but the mess of hoses associated with the head can be pretty baffling.

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post #6 of 12 Old 06-24-2013
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Re: Vented Loop for raw water to head.

Nobody wants to suggest a composting head? ... I'm not, I love plumbing.

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post #7 of 12 Old 06-24-2013
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Re: Vented Loop for raw water to head.

The raw water intake is a suction line and you do not want to use a vented loop on a suction line. The vented loop is to allow air in to break a siphon flow. If used on a suction line it will pull air into the line and the pump will loose prime. it may work with some pumps but in the end I think will cause problems.

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post #8 of 12 Old 06-24-2013
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Vented Loop for raw water to head.

I have an electric jabsco and installed a loop to help control the water refill level in the toilet after flushing. The toilet bowl and thru-hull for the raw water intake are both at the same elevation and I was getting too much water in the bowl , especially when sailing.
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post #9 of 12 Old 06-24-2013
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Re: Vented Loop for raw water to head.

Years ago I got tired of sitting down into a bowl of cold water in the morning so insulated the head from outside with a manifold made of 3/8 copper Ts and els . The up and over discharge vent is connected to one and a vent on the raw water intake and the hose from pump to bowl take the other two. The top hose leads up has high as possible , preferably out thru deckhead. The vacuum break for the raw water was obtained by filling the outlet of the manifold with solder and drilling out a suitable breather hole. the system tends to flush itself with the pumping water and all drains back to the bowl.Worked for me.
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post #10 of 12 Old 06-25-2013
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Re: Vented Loop for raw water to head.

I shut the seacock for the raw water and use a cup filled from the lavatory sink to fill the bowl with fresh water from the tank. No need for a vented loop, siphon worries, or nasty smelling sewater decomposing in the sewer lines.

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