(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
Quote:
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?
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