Siphoning holding tank - SailNet Community
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post #1 of 19 Old 10-24-2008 Thread Starter
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Siphoning holding tank

In the last year and a half, I have replaced my holding tank, head, all the hoses, etc. I matched the original setup with all the new stuff. Last summer I was visually checking on the holding tank and noticed it was chuck full of clear seawater. The tank had been emptied recently and there was very little use. Not near enough to account for this volume. The only way I assume the seawater could have gotten in is through the holding tank vent is whenever I was sailing healed over. The vent has is located on the side of the hull well above the water line had the vent hose (inside) is looped even higher. I would like to address this problem this winter. Any suggestions would be appreciated.
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post #2 of 19 Old 10-24-2008
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It seems unlikely that it could be coming from the vent hose unless it is submerged while heeled for quite some time. As soon as it is not underwater there could not be any siphoning. I would suspect either the discharge or intake line but it's difficult not knowing the setup and without seeing it.

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post #3 of 19 Old 10-24-2008
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I agree with John, I would check on the discharge side.

Last edited by bubb2; 10-24-2008 at 12:16 PM.
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post #4 of 19 Old 10-24-2008 Thread Starter
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The head hose connects to a two way valve. In one direction the waste flows through a vented loop and out the bottom of the hull. If the valve is selected in the other direction, all waste goes into the tank. If I close the through hull for the waste and pump the head, it will become difficult to pump because there is no place for water to go. I surmise that this indicates the new two-way valve is ok. I can also open an inspection port on the tank and visually and aurally confirm this.
I did have a whole day sailing healed over to the extent that the vent was underwater for hours. I did sail many times after this event without the vent submerged and no new water in the tank. If it is the vent, I would think there has to me some other way for outside air to get into the tank in order for it to siphon.

Could a "check valve" in the vent possibly be a solution?

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post #5 of 19 Old 10-24-2008
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I agree check your discharge line , close that tru hual when not in use. if you don't have one well theres a good winter project. I use to have the simular problem my head would fill up when healed over to port. I installed y valves one on my direct ob and one on the discharge from the tank. they are easier to get to than the thru huals.

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post #6 of 19 Old 10-24-2008
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Check that vented loop If is froze that could be the cause of your siphon.
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post #7 of 19 Old 10-24-2008
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Do you have siphon vents on the intake and discharge lines, and if so, are they plugged?

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post #8 of 19 Old 10-24-2008
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Ditto check the break valves.

I would wonder whether your vented loop in the discharge is located properly - can a half open Y valve allow seawater to siphon back the discharge hose into the tank?

See Installing a Head by Don Casey for how casey assumes the location of a Y valve.

Now I believe my Y valve is before the discharge hose vent, but I could be wrong...

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post #9 of 19 Old 10-24-2008
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I wouldn't put a check valve in a vent line...that kind of defeats the purpose of the vent line, since check valve won't normally stay open and air won't flow.

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post #10 of 19 Old 10-24-2008
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Quote:
Originally Posted by erps View Post
Do you have siphon vents on the intake and discharge lines, and if so, are they plugged?
Do not install a siphon on the intake line.

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