Holding tank venting issue.

[chuck53]

I have a problem with my holding tank vent.  Sometimes it vents ok, but most of the time, it doesn't.
Whenever I do a pump out, I'll hold my thumb over the vent hole for a few seconds creating a vacuum.  As soon as I let go, I can hear the air being sucked in.  Sucking air in is no problem.  Air going out is the issue.  Most of the time my tank is bulging because air can't escape and I can't figure out why as sometimes, not often, it will.
Any ideas guys?

[Stu Jackson]

Most likely there is a small amount of fluid in it, and the OEM installation has the vent line hose with a slight dip in it on its way to the stanchion.  The higher pressure of the pump out overcomes whatever is in there.  Solution: straighten the hose out so it goes UP, not down and then up.  New hose wouldn't hurt either.

[chuck53]

Not saying you are wrong, but me standing on the holding tank, all 200 lbs, should push a few ounces of water up and out the vent opening. Once in a while, I can stand on the tank and the air will rush out.

[britinusa]

Hmmmm. when under pressure it blocks but under vacuum it opens.

Replace the hose and while at it eliminate any sharp bends (kinks) in the hose.

Paul

[chuck53]

There are no kinks in the hose.  As I said, on occasion it vents as it should, but mostly not and the vacuum pulls air thru just fine with no problem.

[Stu Jackson]

There are no kinks in the hose.  As I said, on occasion it vents as it should, but mostly not and the vacuum pulls air thru just fine with no problem.

Chuck, you're asking a question and then arguing with the answers.  C'mon.  Know how a check valve works, right?  You could well have something that blocks one way and not the other.

[Noah]

Take the hose off the stanchion and ream or blow it out or replace it. Also pump and flush out your tank really well, then fill ii with fresh water until it comes out the stanchion vent. After that, if you are atill not "happy", and have problems, move the vent to the side of the hull at the rub rail. There is no magic that I know of here. As they say "#### happens"! I would also NOT recommend standing on the tank!

[KWKloeber]

I have a problem with my holding tank vent.  Sometimes it vents ok, but most of the time, it doesn't.
Whenever I do a pump out, I'll hold my thumb over the vent hole for a few seconds creating a vacuum.  As soon as I let go, I can hear the air being sucked in.  Sucking air in is no problem.  Air going out is the issue.  Most of the time my tank is bulging because air can't escape and I can't figure out why as sometimes, not often, it will.
Any ideas guys?

Chuck

Plain and simple, you have a blockage.  There's no other magic.
You have to make sure of 2 things,  the stanchion is completely clear (they tend to get clogged up) and the line is completely clear and NO DIPS in it.   Remove the hose and blow it out from both directions -- trace it to make sure there are NO dips.

While the hose is off, ream out the stanchion -- like with a straightened coat hanger - not with a hook on the end, but bent so when you twist it you are reaming the tube.  The stanchion is tube within a tube, so you might have to remove it to really get it cleaned out.  You can try a compressor or air tank using a needle nose, rubber tip blower on the vent hole.

-ken

[chuck53]

Ok, I thought about a check valve...there is none.  No kinks.  Blockage?  Why is ok sometimes?
I've pumped the tank many times since we have had this boat and always add fresh water to the tank to rinse.  No, I've never filled the tank to the point of water coming out the vent.
However,  on more than a few occasions while pumping out, I've held the water hose up to the vent and flushed water thru the vent while pumping out.  Water was being sucked thru the vent... no gurgling or backing up.  If there was something in the line, I would think this would wash it out.

Despite warnings not to do it, I just stood on my tank.  The bulging slowly went down as air escaped thru the vent.  It's back to it's normal size.  I can always tell when my tank isn't venting as the board covering the holding tank raises up above the settee.  Not much, maybe an inch.

[Dave Spencer]

Chuck,
I hate to pile on but clearly there is a blockage that is acting as a check valve.  I would work it through systematically. 
1 - remove the vent hose from the fitting on the tank.  If the it's not blocked, the problem is between the hose and the stanchion.
2 - put the hose back on the tank and remove it at the stanchion.  If it's not blocked, the problem is in the stanchion.
Like others, I think the stanchion is the most likely place for the blockage since the diameter of the vent narrows quite significantly at that point.  I think it's also a favourite place for spiders and spider webs can be tenacious.  The hosing that you did with the vent wouldn't necessarily clear spider webs.  Think about how effective (of, more accurately - ineffective) it is to remove spider webs from other spots on the boat using a hose. 
All I can say about the bulge in the tank is "Yikes!"  The board lifts approx an inch.  There must also be significant pressure on your joker valve and the head must be very difficult to flush.
Good luck - you'll get it solved eventually.

[chuck53]

Ok, I just disconnected the hose at the holding tank.  I cleaned the end some and blew thru it.  NO resistance at all.  I unscrewed the fitting on the tank and looked it over.  It's fine. 

[Ed Shankle]

If you are pouring some water down the vent to flush it after a pump out, you could be inadvertently creating the blockage, since there is a subtle dip in the hose between the top of the tank and where it runs under the lip of settee cutout. If yours runs that way, you may need to secure it against that lip keep it elevated just a bit more. I don't know how others have dealt with that design flaw, there's probably a couple of different ways to approach it.

Regards,
Ed

[chuck53]

Yes, there is a dip in the line, but there is no way a couple of ounce of water is going to block the line enough to allow pressure inside the tank build to the point that the sides of the tank will bulge out.

[chuck53]

Ok, here's what I have found.
I took the hose off the tank again and this time used my water hose with pressure nozzle to flush the hose from the tank side.  I did this at least a half dozen times, each time a little dirt coming out of the vent along with a strong stream of water.  In between flushing I tried to run a pretty flexible wire thru the vent hose both from the vent end and tank end.  Either way, I could only get the wire in about 3 feet or so.

This tells me there is some kind of blockage somewhere in the middle of the hose.  I'm guessing a mud dauber.  The only real bend in the hose is about a foot from the stanchion vent hole and I can get the wire well past that meaning there are no bends or kinks in the hose.

My next plan is to get another wire that is stiffer than what I tried today and try to get it past the blockage so we can pull the wire from either end.  If I can do that, I will put a knot in the wire, pull it back and forth to try to break up the blockage. 

Yes, I could just replace the vent hose, but that job is way easier said than done.  Breaking up the blockage will be 10 times easier.

Anyway, I think I have already cleared enough crap out of the vent hose that I won't have anymore problems, at least, in the near future.

[KWKloeber]

If you can get water past that blockage, I think I'd try a dock water hose coupled to it, and water flowing coupled with the pressure might break a dauber free -- or whatever else it is.

ken