Holding Tank Vent Question

Dec 5, 2015
115
Oday 272LE Louisville, KY
My 15 gallon plastic holding tank current has about 1.5 gallons of water in it this season. There are two vent hoses coming off of the top of the tank to a "T" fitting and then one tube to a vent on the side of the boat. The outside vent is clear and I replaced the tube between the "T" fitting and the outside vent on the starboard side of the boat. I replaced all of the sanitation hoses with new Raritan hoses last year. There have never been any solids flushed during my 8 years of ownership. I noticed clear water in the new vent tube. The head flushes well. What is forcing water into the vent tube when the holding tank is almost empty?
 
Dec 2, 1997
8,777
- - LIttle Rock
My 15 gallon plastic holding tank current has about 1.5 gallons of water in it this season. There are two vent hoses coming off of the top of the tank to a "T" fitting and then one tube to a vent on the side of the boat.
Teeing two vent lines together to go to a single thru-hull prevents either one of them from accomplishing anything.
Holding tank vent lines should be short (ideally no longer than 5' but that can be hard to achieve depending on the location of the vent fitting on the hull) straight and rise no sharper than 45 degrees. Replacing the "vent" thru-hull with an open bulkhead fitting (the kind your bilge pumps use) eliminates the need for a second vent and also allows you to prevent vent blockages by putting a hose nozzle against it regularly to back flush vent line...anything that tries to set up housekeeping in it will get a water slide ride into the tank.
As for the source of the water in your tank...that's gonna require more conversation than is practical in open forum 'cuz I have questions that need answers before I can offer any solutions. If you'd like to pursue it, send me a PM.

--Peggie
 
Jan 7, 2011
4,951
Oday 322 East Chicago, IN
I would guess condensation, rain water or lake water (if you buried the rail ).

The original owner on my O’Day 322 added a second vent line to the holding tank, but added a second hull fitting also.

Greg