Holding Tank issues

[EmeraldBay]

Goodmorning All,
Any help or suggestions much appreciated.
We are having an over flow issue on our forward holding tank. When pumping into it, waste water flows out of the vent - however the tank is nearly empty. I am presuming that pumping in builds up the pressure and this is what causes the out flow, however, I am guessing that the pressure is building up because the vent is not working properly, but it works well enough to let water out???
Any sensible advice?

Regards Emerald Bay

 

[RoyS]

Sounds like it is full to me. The vent pipe is on the top of the tank.

 

[Peggie Hall HeadMistress]

The vent fitting on a tank is always on or at the top of the tank...so a tank that's nearly empty cannot be overflowing out the vent. If you think it is because you've recently pumped it out, the pumpout wasn't able to remove more than a gallon or two before a blocked vent caused the pump to pull a vacuum. The vent IS blocked...flushing the toilet is pressurizing it enough to force at least a little out the vent each time.

Holding Tank Vents 101: The vent on all tanks--water, fuel, waste holding--has two essential functions: it allows air IN to replace contents as they are drawn out, and it allows air displaced by incoming water, fuel or waste to get out. Therefore, it's absolutely essential that all tank vents be kept open, because unless air can be pulled in through the vent to replace the contents being pulled out, a vacuum is created in which nothing more can be removed--but in which strong suction can crack the tank (not likely in fresh water and fuel systems, but highly likely if a waste holding tank is connected to a strong pumpout). A pressurized tank can have equally disastrous consequences--anything from a geyser when the deck fill or deck pumpout cap is removed to a backup in the toilet, to blown out fittings, to a burst tank.

99% of all vent blockages occur at the through-hull. Dirt daubers build nests in the opening, insects fly or crawl in, get stuck and die, dust, dirt and pollen clog screens in the thru-hull...

On sailboats, however, heeling can send tank contents into the vent line, also creating a blockage at the tank in both that end of the vent line and the vent fitting on the tank. Overfilling the tank can also cause this on any boat. This will require removing the vent line from tank (it would be a very good idea to open the deck pumpout fitting cap to relieve any pressure before disconnecting any hoses!

So start by using a screwdriver blade, icepick--whatever works-to clean out the vent thru-hull. If that doesn't clear the vent, you'll have to clean out the other end of the vent line too. Do not use the toilet again until the vent is clear.

I suggest you also check out the link in my signature here...it goes to a comprehensive "marine toilets and holding tanks 101" manual that will help you learn how to cure most "head aches" including blocked tank vents, but PREVENT 'em...and as you're already learning, prevention is a lot easier that cure!

 

[EmeraldBay]

Definitely not full. I macerate until empty, and then knock on top of tank and it sounds very empty.

 

[dlochner]

Definitely not full. I macerate until empty, and then knock on top of tank and it sounds very empty.

Open the deck pumpout fitting to relieve pressure and then open to the inspection port to verify that the tank is indeed empty.

 

[Peggie Hall HeadMistress]

Open the deck pumpout fitting to relieve pressure and then open to the inspection port to verify that the tank is indeed empty.

I'd be sure to be downwind of the deck fitting and have a hose at the ready when you open it.

Btw...according to your profile, you're in Oban in the US...but the only Oban I can find is on the west coast of Scotland (have been there, in fact)... Where in the world are you actually located?

 

[EmeraldBay]

Was Oban Scotland, data input error, now in English Harbour Antigua.
I regularly open up the deck fitment and relieve some pressure. I think I'll replace vent line as it smells and is a probably from new.

 

[Ken Cross]

Was Oban Scotland, data input error, now in English Harbour Antigua.
I regularly open up the deck fitment and relieve some pressure. I think I'll replace vent line as it smells and is a probably from new.

I don't think there should be anything to relieve by opening the deck fitting, if the vent is working. It should be easy to check if you can see the top of your tank. Pump the toilet and see if the tank bows up at the top. If it does, stop immediately and fix the vent.
Since the deck fitting goes to the bottom of the tank, any relief from there could result in a brown guiser.
It sure sounds like a plugged vent to me too.

By the way, Peggie's book is excellent, well written and easy to understand.

Ken

 

[Peggie Hall HeadMistress]

I think I'll replace vent line as it smells and is a probably from new.

If the vent hose stinks, definitely replace it. But if the only odor is what comes out the vent when the toilet is flushed, replacing the vent line won't help (Holding tank odor is a whole 'nother subject!).

Boat builders use the same vent thru-hulls on all tank vents. They're designed to prevent sea water contamination of the fuel and fresh water supply, which isn't necessary for holding tanks. So I'd also strongly recommend that you replace the vent thru-hull with a different type--an open "bulkhead" or "mushroom" thru-hull that you can stick hose nozzle up against and backflush the vent line every time you wash the boat. That will prevent any more vent blockages. You won't take on more than a few drops of sea water through it unless that thru-hull is in a location that puts it under water for more than a minute or two when the boat is heeled.

I don't think there should be anything to relieve by opening the deck fitting, if the vent is working

You're right...a pressurized tank is a sure sign of a blocked tank vent. So is a manual toilet that gets harder to pump and/or "burps" and backs up or an electric toilet that does. However, because pressure often escapes back through the toilet and/or manages to seep out the vent when the toilet isn't used for a while, it can sometimes take a few flushes for those symptoms to develop again, allowing users to miss that sign.

 

[dlochner]

Since the deck fitting goes to the bottom of the tank, any relief from there could result in a brown guiser.

By the way, Peggie's book is excellent, well written and easy to understand.

Ken

Better a brown effluent geyser on deck than a cracked hose or fitting from an over pressurized hose below decks!

Peggy's book is well worth the price. It sits proudly on Second Star's bookshelf.

 

[EmeraldBay]

Unfortunately no inspection hatch. I take on board all comments, appreciated, will start with vent hose clean, then possible replacement.

 

[Peggie Hall HeadMistress]

A blockage in the vent line is rare...they're almost always at both ends of it--the thru-hull, the vent line fitting on the tank and that end of the hose. So start with the vent thru-hull...if that doesn't clear it, open the deck fitting cap to relieve any pressure and clean out both parts of the other end.

 

[dlochner]

A blockage in the vent line is rare...they're almost always at both ends of it--the thru-hull, the vent line fitting on the tank and that end of the hose. So start with the vent thru-hull...if that doesn't clear it, open the deck fitting cap to relieve any pressure and clean out both parts of the other end.

I'd be sure to be downwind of the deck fitting and have a hose at the ready when you open it.

Shouldn't that be "Upwind" of the fitting?

 

[Peggie Hall HeadMistress]

OMG...you're right--I mistyped it!

Standing downwind would be more entertaining though.....

 

[IH82BL8]

If you're getting overflow out of your vent hose the vent hose is not blocked.

Also, did you actually see effluent from the vent hose or did you only hear it? If you didn't see which opening it's coming from, is it possible that what you forget to set your Y valve to the tank and are actually pumping directly overboard?

 

[Peggie Hall HeadMistress]

If you're getting overflow out of your vent hose the vent hose is not blocked.

Not necessarily true. If the tank is seriously pressurized, that pressure is gonna try to relieve itself wherever it can. Usually that's back through the toilet--it can turn a joker valve inside out...but some waste can make it out the vent. It won't be a full flow, could only be a dribble....but that can be enough to mislead people into ruling out a blockage. And btw, it doesn't matter whether the tank is being pumped out or dumped, the vent has to allow enough air into the tank to replace contents being pulled out, an overboard discharge pump will pull a vacuum that prevents it from pulling more than a gallon or two, same as the pumpout. It occurs to me as I write this that if the problem isn't an actual blockage but a kink in the vent line, enough pressure could push a pretty decent stream of waste through it.

 

[Ron20324]

[QUOTE="Peggie Hall HeadMistress,... I'd be sure to be downwind of the deck fitting and have a hose at the ready when you open it.

Shouldn't that be "Upwind" of the fitting?[/QUOTE]

Yeah I was thinking it was "three SHEETS to the wind".

 

[Peggie Hall HeadMistress]

Your spell checker must have been working overtime!