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Old 23-09-2018, 17:31   #16
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Re: Head flushing overboard through air vent

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Originally Posted by a64pilot View Post
10 yr old aluminum waste tank, your sitting on a time bomb and it’s very likely about to go off.
You really need to come up with a plan to remove it and replace it with a plastic tank, before that decision is forced upon you in a nasty inopportune manner.
I'm in complete agreement. I'm researching my options right now. I've emailed plasti mart and now comparing stock sizes at Ronco Plastics.

Other suggestions welcome.

OP: What is the path of your vent tube? Does it come from the top/bottom/side of the tank?
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Old 23-09-2018, 19:33   #17
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Head flushing overboard through air vent

My tank was odd ball shaped.
Triple M plastics made an exact duplicate to my drawings. I had an extra 1.5” hole cut in the bottom so I could use a macerator to empty the tank, and increased the vent size to 1” based on Peggy’s recommendation.Click image for larger version

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Old 23-09-2018, 21:03   #18
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Re: Head flushing overboard through air vent

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Originally Posted by a64pilot View Post
My tank was odd ball shaped.
Triple M plastics made an exact duplicate to my drawings. I had an extra 1.5” hole cut in the bottom so I could use a macerator to empty the tank, and increased the vent size to 1” based on Peggy’s recommendation.Attachment 177858
Triple M Plastics are building me a couple fuel tanks right now.

I need to have them quote the holding tanks again. First try was more than the fuel tanks which are 4x bigger with just as many, if not more connections plus inspection port.

I'm going to reconfigure my holding tanks a bit. At a minimum get rid of the unnecessary Y valve. All waste will enter the tank and when appropriate have the overboard thru hull valve open to drain the tank. Pump out from the top as the other method.
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Old 29-09-2018, 16:47   #19
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Re: Head flushing overboard through air vent

Hi Peggie et al

This is Howler (aka Dan) the original poster of this thread. I read your advice and did the recommended troubleshooting. I even bought your book and went through its troubleshooting section.

Here’s the situation.

1. The holding tank is near empty.
2. The vent line is not clogged and has no sags (the boat is a brand new 2018 Beneteau)
3. The issue seems to occur after sailing in big seas. For example today we were sailing in 5 foot waves coming from abeam, and heeling quite a bit in 20+ knot winds, after which the problem recurred.
4. The air vent is connected to the top of the holding tank as it should be.

After that, when emptying the head, the waste comes out of the air vent thru-hull. And it’s more than just an amount that could accumulate in the air vent hose from seawater entering.

The only thing that solves the problem is to empty the (tiny quantity) of waste actually in the holding tank through the gravity discharge valve. After that it operates normally again.

This leads me to think that there’s some kind of air pressure build up from the waves or heeling, which is relieved when the holding tank is emptied. But it’s still an enigma how the head can empty directly to the air vent just because of some air pressure. Again, the holding tank was near empty both times this occurred.

Any addition additional ideas? Thank you.
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Old 29-09-2018, 18:32   #20
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Re: Head flushing overboard through air vent

The issue seems to occur after sailing in big seas. For example today we were sailing in 5 foot waves coming from abeam, and heeling quite a bit in 20+ knot winds, after which the problem recurred.
The air vent is connected to the top of the holding tank as it should be.


AHA! I THINK I may know what's happening... but I have so many questions that need answers before I toss it out here, that it would be easier to find out if I'm right if we could actually TALK instead of swapping a bunch of emails. So I'm gonna send you a PM with my email address and when I can be available tomorrow (Sunday)--or Monday if Sunday won't work for you--and ask you to reply via email with a day/approx time frame that works for you and your phone #.

Stand by for incoming PM...

--Peggie
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Old 29-09-2018, 18:32   #21
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Re: Head flushing overboard through air vent

It sounds as though some clown has connected up the pipes incorrectly.

The system usually works by flushing the toilet into a Y valve--one branch of which leads to a sea cock and a through-hull fitting via a siphon-breaker. That is your ocean outlet which bypasses the holding tank entirely.

The other branch of the Y valve goes to the top inlet of the holding tank. I suspect in your case it goes to the deck breather pipe hence your problem. In my case the macerated sewage goes into the holding tank directly, since the outlet to the tank was via a siphon breaker but I have seen some where the tank was located high enough to warrant a siphon breaker on the inlet side. That stops sewage returning top the bowl of the heads. I like them to be as low set as it is practical to set them. When full they are heavy.

Some have a valve that outlets directly to a sea cock and through-hull. I never kliked thios idea at all--and my lower outlet for emptying the holding tank went to a pump, and from thence through a siphon breaker to return below waterline to the separate outlet from the ocean discharge outlet. It would probably have been OK to put the two hoses into the came outlet, I chose not to do so and had one on each side of the hull, both below waterline.
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