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Old 21-09-2018, 04:13   #1
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Head flushing overboard through air vent

When flushing the head (Jabsco electro-flush on a recent model Beneteau Oceanis), I noticed black water gushing out of the air vent on the side of the hull. Of course this is illegal, embarrassing and doesnít make friends in the marina.

The holding tank was nearly empty and nothing had been put in the head other than fluids. It was operating fine and showed no evidence of clogging anywhere.

I pumped out the small amount that was in the bottom of the holding tank and tried again. For now the problem has subsided but itís very concerning. The only thing I can think of is air pressure buildup in the holding tank that diverted the discharge to the vent line, but if the vent line was so obstructed to cause air pressure to build up, I doubt the discharge would have been able to flow through it.

Any similar experience or advice would be welcome.
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Old 21-09-2018, 04:19   #2
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Re: Head flushing overboard through air vent

Maybe there is a low spot in the vent line?

If so, I could see the vent line filling up on your last full tank, then retaining some black water.

Next, you build pressure in the empty tank with a new flush. That pressure forces out the black water in the vent line just like a snorkeler coming up for air.

Check to see the vent line is all uphill from the top of the tank to the thru hull.
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Old 21-09-2018, 06:49   #3
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Re: Head flushing overboard through air vent

Chotu is on the right track: Overfilling the tank--or perhaps heeling--pushed waste into the vent line where it pooled--and may have even hardened if the boat sat a while before you used it again--in a sag. So I question whether a pumpout was actually able to empty the tank without suction from the pumpout pulling the blockage out of the vent line, 'cuz if a pumpout can pull in air to replace contents as they're pulled out, the pumpout will pull a vacuum that won't allow it pull out more than a gallon or two. Flushing the toilet would pressurize the tank enough to clear the blockage in the vent line, but the tank would still be close to full.



So you need to make sure the tank IS empty. Before attempting to pumpout again, shorten and straighten the vent line to eliminate any sags where liquid can pool. If there is no sag, remove the vent line from the tank and clean out any buildup in that end of the vent line and the vent fitting on the tank...that's one of the two most common locations for a vent blockage. The other one is the vent thru-hull, so using an ice pick, screwdriver blade or whatever works, scrape it out. And I'd replace the "vent" thru-hull with an open bulkhead thru-hull that you can stick hose nozzle against and backflush the vent line every time you wash the boat, preventing any more vent blockages.


And finally...I'd install a tank level indicator to let you know how full the tank is getting so you won't overflow it again. The SCAD Scad Tank Monitors are top rated, the single tank "solo" model is inexpensive, and easy to install because the sender goes on the OUTside of the tank and provides an accurrate 1/4, 1/2, 3/4 and FULL reading.


So you have a bit preventive maintenance work to do, but doing it will prevent the need to cure this problem again.



--Peggie
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Old 21-09-2018, 09:45   #4
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Re: Head flushing overboard through air vent

Thanks for your analysis and suggestions, Peggie!
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Old 22-09-2018, 12:33   #5
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Re: Head flushing overboard through air vent

Your holding tank is full and overflowing through the breather vent--and it has also probably damaged your carbon filter irreparably, so you will need a new cartridge for it or a new filter if it is the disposable type.

Emptying your holding tank would be a good start. How do I know this? It happened to me. I discovered that the "Full" warning light had been disconnected.
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Old 22-09-2018, 13:21   #6
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Re: Head flushing overboard through air vent

We had a similar experience when we bought our boat. Unfortunately for me, the vent is in the base of the stanchion right outside (and facing) the head porthole that was open when I used the head for the first time.... Not the kind of shower I was looking for.

Make sure there is NO sag in the vent line and that it is as vertical as possible. It is not meant to act as a 'P' Trap like your home plumbing uses to block odor. The best way to prevent black water tank odor is to get air into it. Lookup Peggy Hall, Head Mistress on the internet and read her recommendations.
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Old 22-09-2018, 13:58   #7
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Re: Head flushing overboard through air vent

Peggie is the authority on marine sanitation.

You have received the very best advise available.


Thank you,


Ask her about venting your holding tank...
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Old 22-09-2018, 19:42   #8
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Re: Head flushing overboard through air vent

Lookup Peggy Hall, Head Mistress on the internet and read her recommendations.

I appreciate the plug but I've already "got this." Apparently you missed my reply (post #3) to this thread's OP.

As for your problem...venting water and fuel tanks through a stanchion is a great iea because that prevents sea water from contaminating the fuel and potable water supply...but it's not the way to vent a waste tank. However, it's a fairly simple job to install a vent thru-hull (although I wouldn't use a "vent" thru-hull) and reroute the vent line to it...something I also suggested to the OP in my previous reply.

--Peggie
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Old 22-09-2018, 19:43   #9
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Re: Head flushing overboard through air vent

I had a boat with an aluminum holding tank. the vent lines came out the bottom but had a tube running up inside the tank. Apparently the tube disolved or something so anytime there was anything in the holding tank, it came out the vent.
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Old 22-09-2018, 19:52   #10
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Re: Head flushing overboard through air vent

I don't THINK it was the vent line that came out of the bottom of the tank with a tube to the top inside the tank....that had to be the the discharge fitting that should have had the line to the deck pumpout fitting connected to it. If the vent line was connected to it, that would definitely explain why waste spurted out the vent every time the toilet was flushed. A PO obviously had NO idea how to plumb the tank!

In an aluminum tank, it's not surprising that the pickup tube would have corroded through 'cuz urine is so corrosive that it'll not only eat through a metal "dip tube," it'll turn any metal tank into a colander in an average of about 10 years.

At the risk of being accused of blatant self-promotion, I think most of y'all could benefit from the information in my book. The title (my publisher's idea) is a bit misleading...although it does deal with every source of odor on a boat and how to cure, or better yet, PREVENT 'em, it's actually a comprehensive "marine toilets and sanitation systems 101" manual that explains the laws, describes all the types of systems and will help you learn how to operate and maintain your system to prevent 99% of problems instead of having to cure 'em. 'Cuz you get to do any preventive maintenance on your terms when it's convenient...the need to cure a problem never happens when it is! There's a link to it in my signature (just click on the title).

(end blatant self promotion)

--Peggie
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Old 22-09-2018, 20:57   #11
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Re: Head flushing overboard through air vent

My 10 year old aluminum holding tank has the same issue. I haven't pulled it apart, but will soon. The tank is either full (as indicated by the light) or the vent tube doesn't reach to the top as it once did.
The drawing clearly shows the vent overboard enters the tank at the bottom. I personally feel the light is faulty (dirty sensor maybe) and the vent tube inside the tank has failed.
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Old 23-09-2018, 06:32   #12
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Re: Head flushing overboard through air vent

The drawing clearly shows the vent overboard enters the tank at the bottom.



Only air should pass through a tank VENT--air pushed out by incoming contents, air pulled in to replace contents as they're drawn out--which means the vent line has to be at or on the top of the tank (That applies to all tanks--water, fuel and waste). It doesn't make any sense to put it at the bottom of the tank. So I suspect there's some terminology confusion at work here and it would be very helpful if I can see that drawing...any chance you can send it to me as an attachment to an email? I'll send you a PM that includes my email address.


--Peggie
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Old 23-09-2018, 06:45   #13
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Re: Head flushing overboard through air vent

Quote:
Originally Posted by peghall View Post
The drawing clearly shows the vent overboard enters the tank at the bottom.



Only air should pass through a tank VENT--air pushed out by incoming contents, air pulled in to replace contents as they're drawn out--which means the vent line has to be at or on the top of the tank (That applies to all tanks--water, fuel and waste). It doesn't make any sense to put it at the bottom of the tank. So I suspect there's some terminology confusion at work here and it would be very helpful if I can see that drawing...any chance you can send it to me as an attachment to an email? I'll send you a PM that includes my email address.


--Peggie
Emails sent. The vent is at the top, but the tube runs inside the tank from the top and exits the bottom. Drawing will explain better than my words.
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Old 23-09-2018, 11:25   #14
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Re: Head flushing overboard through air vent

Quote:
Originally Posted by FlyingScot View Post
Emails sent. The vent is at the top, but the tube runs inside the tank from the top and exits the bottom. Drawing will explain better than my words.
Thanks for the drawing! There shouldn't be a tube or anything else inside the tank on the vent fitting...that's guaranteed to force waste--even when there's only an inch or two in the tank--out the vent every time the toilet is flushed. The only thing that should be attached to a vent fitting is the vent line to the vent thru-hull.

So the solution to your problem is: remove the pipe from the vent fitting.

Although the "standard" location for the discharge fitting on any tank is on a vertical surface as close to the bottom of the tank, when there's enough clearance above the tank for a 1.5" hose fitting, it's not uncommon for a holding tank to have the discharge fitting on the top of the tank, which would require a pickup tube inside the tank that goes to the bottom of the tank. Because waste will always rise in a discharge/pumpout line to the level of the tank contents, a line coming off a discharge fitting at the bottom of the tank to the deck pumpout fitting is likely to be the first one to permeate with odor. Putting the discharge fitting on the top of the tank with a pickup tube inside the tank prevents that.

So I have a strong suspicion that whoever spec'd the fittings on these tanks mistook the vent fitting for the discharge/pumpout fitting... it's the only explanation that makes any sense.

I'm still trying to sort out what's going on with the rest of the plumbing on the drawing you sent me.

--Peggie
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Old 23-09-2018, 15:19   #15
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Re: Head flushing overboard through air vent

10 yr old aluminum waste tank, your sitting on a time bomb and itís 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.
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