Head flushing overboard through air vent

[FlyingScot]

Originally Posted by a64pilot

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?

[a64pilot]

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.

[FlyingScot]

Originally Posted by a64pilot

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.

[Howler]

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.

[peghall]

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

[Mike Banks]

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.

[dasf]

Hi All,

Having the same issue (apologies for opening this up after 4 years!). I've just purchased a 2yo Merry Fisher 895. It has an 80L holding tank and the vent pipe is at the top and doesn't have any bends to the through-hull fitting.

Every time I use the head, waste comes out of the through-hull.

I've checked the vent pipe for build up - nothing. I've flushed the vent pipe and it will always send back the water yellowed and smelling.

The tank is only about 15% full (I can see through it) so I'm not sure what's causing the waste to leak out of the vent.

Diagram here: https://www.manualslib.com/manual/10...page=66#manual

Any ideas?

[peghall]

(See your quoted post below my reply):

I'm guessing that there's a blockage in the vent THRU-HULL and possibly at the other end of the vent--in that end of the vent line and/or the vent fitting fitting in the tank.

All tank vents have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized...incoming flushes are forced out the vent.

You've confirmed this when you said I've flushed the vent pipe and it will always send back the water yellowed and smelling. The pressure from inside the tank isn't allowing the water to do anything else. If flushing the toilet won't allow the flush to get into the tank, it's more proof of a vent blockage.

To clear it, Start by cleaning out the thru-hull...use a screwdriver blade, ice pick--whatever works..if there's a screen in it,knock it out...screens create more problems than they prevent. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the deck pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...reconnect the vent line. If there's a filter in the vent line, remove it...once a filter gets wet--which can happen when a tank is overfilled or even due to heeling--the charcoal swells and blocks the vent...one of the reasons why vent filters aren't recommended for use on most sailboats.

To prevent future vent blocks, replace the "vent" thru-hull with an open bulkhead, aka "mushroom", thru-hull. That not only will improve the tank's ability to "breathe" (essential to preventing odor out the vent), but will also allows you to put a hose nozzle against it to back flush the vent line every time you wash the boat and/or pump out or dump the tank. Anything that's tried to set up housekeeping will get a water slide ride into the tank.

A blocked vent also makes pumping out or dumping the tank impossible, 'cuz without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.

You're welcome to give me a shout via PM if you need to discuss in more detail than is practical in a discussion forum.

--Peggie

Originally Posted by dasf

Hi All,

Having the same issue (apologies for opening this up after 4 years!). I've just purchased a 2yo Merry Fisher 895. It has an 80L holding tank and the vent pipe is at the top and doesn't have any bends to the through-hull fitting.

Every time I use the head, waste comes out of the through-hull.

I've checked the vent pipe for build up - nothing. I've flushed the vent pipe and it will always send back the water yellowed and smelling.

The tank is only about 15% full (I can see through it) so I'm not sure what's causing the waste to leak out of the vent.

Diagram here: https://www.manualslib.com/manual/10...page=66#manual

Any ideas?

[Uncle Bob]

Originally Posted by dasf

Hi All,

Having the same issue (apologies for opening this up after 4 years!). I've just purchased a 2yo Merry Fisher 895. It has an 80L holding tank and the vent pipe is at the top and doesn't have any bends to the through-hull fitting.

Every time I use the head, waste comes out of the through-hull.

I've checked the vent pipe for build up - nothing. I've flushed the vent pipe and it will always send back the water yellowed and smelling.

The tank is only about 15% full (I can see through it) so I'm not sure what's causing the waste to leak out of the vent.

Diagram here: https://www.manualslib.com/manual/10...page=66#manual

Any ideas?

Another possibility is that at some time in the past the hoses have been replaced and the vent and deck pump out connections have been transposed, so now the vent is connected to the dip tube which runs to the base of the tank and the pump out to the top of the tank, neither will work . As the tank has contents added it becomes slightly pressurised, which pushes the contents up and out the vent. The easiest way to find out is try to pump out via the deck fitting, if it works as designed then I am wrong, if it wont pump out then try swapping the two connections at the tank and try again. Distasteful work but obviously something is wrong and needs to be rectified,

[peghall]

That would be highly unlikely because the diameter of most pumpout lines is considerably larger than that of most vent lines.


--Peggie

[Uncle Bob]

Originally Posted by peghall

That would be highly unlikely because the diameter of most pumpout lines is considerably larger than that of most vent lines.


--Peggie

Yes but that is the only conclusion I could think of given the details provided, 15% full tank, adding to contents goes out the vent overboard and flushing the vent line returns more dirty water. As said, I may be wrong but I can't think of an alternative that meets all conditions.