Vacuum in Head

[Madwand]

I must be missing something. Catalina 30 TR. I have a y valve off the head that goes to the overboard thru hull and to the holding tank. The thru hull has a valve. If I turn the thru hull off and direct the flow to the tank, I get a vacuum when I flush. I would expect this tank is sealed and the only other direction air or water can go is to the next thru hull after the macerator, which stays closed. So how are you supposed to flush to the holding tank?

[StuM]

There will be an air vent to the holding tank. If you are getting a pressure buildup, the vent is blocked.

[Madwand]

Ha, I forgot about this big obvious hole on the top...


It goes up through this cabinet in the corner.


Then it seems to go into the hull where I can't follow it any more. Could this be attached to the underside of the stanchion? The one on the other side of the boat overflows when I fill my water tanks.

[HopCar]

You said you get a vacuum when you flush to the tank. Did you mean you have a hard time pushing the sewage into the tank? If so Stu is probably correct, blocked vent line.
If your fresh water tank vents through a stanchion, your holding tank probably does too.
Another option would be a blockage between the Y valve and the tank.

It's probably time to change that vent hose. It looks like old vinyl hose which can get brittle with age.

[Madwand]

Ugh. I can't even imagine how I'm supposed to feed that through a space that I can't access.

Yes, the plunger feels like there's a blockage right at the flapper valve, but if I go to the Y valve and turn it there's a loud commotion and it clears.

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[jeepbluetj]

The vent indeed goes into a stanchion on the port side of the boat. You'll see a hole in the inboard side of the stanchion. It's an inferior vent arrangement. (As Peggie H reminded me on another board..... )

If that vent is blocked, you can't pump into the tank.

On my boat I had almost the same problem. If I tried direct overboard discharge I could not empty the head. It came down to the previous owner's idea of installing a Y valve was faulty. Either the head was connected to the holding tank or the discharge thru hull was connected to the holding tank. (Bottom of Y went to tank)

Since there's no reason really for direct discharge here I just removed the Y valve completely.

[jeepbluetj]

So a few things to check:
1. Plugged vent. You can test by removing it at the top of the holding tank.
2. Y valve installed incorrectly.
3. Plugged pipe from y valve to tank.

You can pretty easily get to that vent line. Mine terminates at the stanchion in the cabinet in the head. I shoulda put in a better vent when I replaced it. (It's now pretty far down on the list 'cause it's not broken... just not a good design)

[gulfstar37]

Previous boat was Catalina 30. They vent the holding tank through hollow stanchion on port side. Yes they can plug up and prevent undesirables from going in holding tank. What goes in displaces, pushes, smelly air out. If you really force the issue you will build up enough pressure to blow off vent hose if your lucky or find the weakest point on your tank and burst it, bad day, our next time your wife uses it and switch to flush mo

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[gulfstar37]

Mode she might get a surprise. A really bad day. Need to remove vent from tank and figure where the blockage is and fix. You may also find out you will have trouble emptying the holding tank at the pump out station. Same reason. What come out needs to have air to replace it.

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[Madwand]

Yeah, when I got the boat, I tried to do a pump out offshore in the transit here and nothing came out. Didn't realize why at the time.

I don't know how jeep blue is getting to his, there's nothing behind my head cabinet but a hull wall.

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[Madwand]

I can't imagine how this hose that comes up underneath the deck is attached to my stanchion. So I guess the first step in finding out is to remove this stanchion which is only held on by two screws and I assume it is bedded to the toerail?

Has anyone ever pulled one of these off? Is it going to rip off fiberglass? Is the hole underneath big enough for the vent hose to pass through? Or is it flush up against the bottom? I hate starting a project where I have no clue what I am getting into.

[gulfstar37]

The stanchion tube goes through the base through a hole in deck to inside of boat where the vent tube connects. Just info, this is an area where water can migrate into deck core especially if bedding fails. When you pull out you may want to dig core out a little and coat with epoxy.

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[Madwand]

This seems to be what I'm dealing with.
https://www.catalinadirect.com/index...-base-vent.cfm

Looks like a lot of hardware for this.
https://www.catalinadirect.com/index...il-mounted.cfm

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[gulfstar37]

You got it--however on our cat 30 the stanchion base was flat not the high low you pictured but same idea.

Good luck.

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[Madwand]

I'm pretty sure I have the high low. Because of my toe rail. I still can't imagine how I am supposed to reach nuts on the underside of the deck from the top of the deck. It'll be next weekend before I can dig in it.