Tank Vent problem

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pdecat

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I have a weird problem. When using overboard pump out the anti shiphon U fitting vent line pushes out "stuff" when the tank pump out starts to suck air. I think that when the air is pushed up to the U anti siphon fitting it is easier for the air to push liquid out the smaller vent hose than all the way back down to the 2" underwater thorough hull. This doesn't happen when the system is pumping liquid only when it starts to suck air. Any suggestions??
Bruce
 
Is this a vent line that begins at the waste holding tank or is it connected to the vented loop? Have you cleaned the duckbill valve in the vented loop?
 
I rewrote the question to be clearer. It is the vented loop vent line and i dont believe there is a duck valve in it.
 
is it a new problem ?

if so, i'd check for blockage in teh trhu hull, sea cock, like, etc...

odd.
 
I think Pascal's on the right track. For general info purposes, not all vented loop fittings have a built-in check valve. Some of them, particularly those used in sewage lines, simply vent out the side of the hull. Overall they provide a better means of venting the line, but sometimes water and/or effluent will work its way up the vent line. If the vent line is run properly, the water will drain right back down, after the pump has stopped running. But if it isn't run right, if the line droops or is run downward at any point, there's a good chance for water to be retained in that line - which will stop the line from actually venting. The exit of the vent line (from the boat) should be high on the side of the hull, up near the toe rail. If the line is run properly, water will go up it sometimes, but will drain right back down afterwards. If it isn't run properly, there could be a problem. If liquid is coming out the side of the hull, then there's a problem somewhere - as Pascal noted.
 
i haven't tracked my vented loop line (if it aint' broken, dont' fix it...) but from the vented loop, it goes right back down below the floor. I assumes it connects to the tank vent....

ok.. darn.. you got me curious and i looked. indeed if comes back down and connects to a Tee on the vent line, near the top of the tank. I had never looked for the Tee as it's out of the way, further forward under the floor. there is no check valve of anything in the line. Te loop is just a U with a small tee at the top. i don't see room for a check valve in there.

Vic is right, if anything gets pushed up the vented loop, it will come back down but Tee into the vent line and drain back in the tank.

Simple, but it seems to work. the whole system was redone in the 90s by a Vacuflush dealer when all 3 heads were replaced with VF and a single HT installed.

check it you have a second vent hole on the hull, for the vented loop. if not, follow the vented loop line and see where it connects.
 
Vic:
it is not a little water, it comes in big spurts coincident with the overboard pump. somehow it is easier when pumping air to force liquid out the anti siphon vent than down under the boat 1' or more under water and 4' or so below the U fitting.
 
restriction somewhere... check the seacock and the thru hull
 
it pumps out quickly so I dont think there is a restriction. nothing out the vent line until the tank empties and the pump sucks air
 
I suspect that when pumping liquid the weight pushes stuff out the through hull even when it may partially push up into the vent line but when air mixes in the weight is less so there is nothing to push down and the air plus fluid is forced up the vent leaving the line from the through hull to the U fitting to drain via gravity.
What i cant figure out is how this doesn’t happen to every system of this design???
 
so it only happens as the tank gets almost empty ?

start by tracing the vent fitting at the top of the syphon break. where does it go?
as mentioned, mine goes back down to a tee in the tank vent line just above the tank. this ensures that anything that woudl get into the hose at the top of the syphon break will come down, and back in the tank.

maybe yours is connected too high up the vent line.
 
yes, only when the pump starts to suck air.
the antisiphon U fitting vent line in question goes out the hull side following a slight uphill path
what do you mean too high??
The T is an interesting idea though it could thee flow in both directions and I wouldnt want liquid going to the vent odor filter.
 
Beyond what's already been said, I haven't a clue! Sorry! I feel fairly certain that it's something in the way the lines are run, but I don't know for sure. About the only thing left to consider is where the through-hull is located. If it is all the way down near the keel area, the head pressure of the outside water may be making the output from your macerator pump take the path of least resistance - up the vent from the vented loop fitting - but why it only happens when the tank is empty or nearly so, I honestly don't know; I'd think it would happen all the time, if that were the case.
 
so you have two vent fittings ? one for the main vent, via a filter and another one for the vented loop ?

it seems like when you're pumping, some stuff goes up the vented loop vent line.. but then why does it only get pushed out when the pump sucks air ?

when you are pumping stuff, the tank may under a slight vacuum especially with a filter. could that be the differnce ? have you tried running the macerator with the pump out fitting open ? worth a try... i'm not really sure how it would make a difference with this is so odd anyway...
 
yes two fittings

I only do this off shore so i dont see it often. But yes only when the pump hits air at the bottom then it pulses strongly out the U fitting vent hose.

I dont have a macerator just a sea land discharge pump.

Probably no tank vacuum as I replaced the filter with a straight pipe tube as an experiment.
 
I am going to do some plumbing so I thought I would resurrect this thread.

Some added info:
The U fitting vent line that spurts is a 5/8" dia hose. Is that unusually large for a vent?
I dont think the U fitting has an anti siphon valve it just looks like a thread to barb fitting tapped into the top of a U fitting. Is there an ant siphon valve that I could insert inline in the vent hose so that i don’t have to open the seals on the U fitting??
 
its big until poo and tissue harden in it and clog, i switched mine to 1"
 
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