Mystery Holding Tank

[Sailj]

Hello, we got onto our saba 50 a few weeks ago and one of the heads has never been right.

We have 4 heads with electric flushes and all with their own holding tank. Each holding tank has a standard open / close valve that uses gravity to empty when they’re open.

One of the heads works fine with the holding tank valve open, however when the tank valve is closed, the head will not empty anything from the toilet bowl.
We thought maybe it was a pressure issue (a blocked overflow perhaps) but the issue persists even with the deck opening of the holding tank unscrewed and open.
We have run a hose down the deck opened and the overflow isn’t blocked, clear water runnings out.

We’re at a loss how can this be? The holding tank isn’t blocked as the head empties when it’s open….

Maybe someone has had a similar issue before?

Thanks

[wyb2]

You’re 100% sure the valve you are closing is not between the head and the tank?

[Sailj]

Yeah it’s the same big red valve as all the other holding tanks and it’s located directly below the holding tank so I’m 99% sure it’s the holding tank valve

[peghall]

Most likely the problem is a blocked tank vent (which you may be mistakenly calling an "overflow"...nothing but air should ever go through it)...it's a common problem.

Two most common vent blockage locations are the vent thru-hull or the other end of the vent line--that end of the hose and/or the vent fitting on the tank. Opening deck pumpout cap won't help...vent lines are above the surface of the contents, deck pumpout lines originate below the surface at the bottom of the tank...they don't provide any escape for air displaced by incoming flushes nor any source of air to replace tank contents as they're pumped out, causing dock pumpouts to pull a vacuum that can even implode a tank if the pump is strong enough.

To clear the vent line, Start by cleaning out the vent thru-hull...use a screwdriver blade, ice pick--whatever works. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the deck pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...reconnect the vent line.

Fwiw, all "vent" thru-hulls are designed to keep sea water out of fuel and fresh water tanks...they're totally wrong for use on holding tanks, but boat builders use 'em on all tanks 'cuz buying the same ones in bulk saves 'em $$. I recommend replacing holding tank vent thru-hulls with open bulkhead thru-hulls for two reasons: 1. they allow much air exchange with the gasses in the tank (the key to odor prevention)...and 2. You can prevent vent blockages by putting hose nozzle against 'em to back flush the vent line every time you wash the boat and/or pump out. Any waste that's spilled into the vent line is flushed into the tank and anything that tries to set up housekeeping in the vent line gets a water slide ride into it.

--Peggie

[Sailj]

Thank you Peggie for your detailed reply.

Sorry for my lack of knowledge but I’ve got a follow up question - When on the dock last week we used a water hose down the deck pump out cap and clear water flowed out of the vent (what I wrongly called overflow). Would this not imply that it is unlocked?

[ggray]

Stupid question...but you do see a holding tank installed for that particular head?

My cynical mind can see the hose from the head going to the "holding tank valve" and then overboard, and the holding tank deck pumpout hose going to the vent thruhull.

This setup would produce what you are seeing.

Perhaps I should first ask "how much water do you run down the deck pumpout fitting before clear water comes out the vent fitting ?" before I make such an outrageous accusation!

Maybe it's a teeny tiny holding tank!

[peghall]

I just noticed that you're a brand new cruisersforum member...Welcome aboard!

Were you able to successfully flush the toilet after you did this? To get a valid test result, you'd have first needed to reduce the level in the tank enough to prevent any incoming flush from overflowing out the vent again.

Something in the system is creating back pressure that's preventing the toilet from flushing...a blocked tank vent is the most common, but not the only thing that can do it. Do your toilets flush with sea water or fresh? If sea water, when if ever have you checked the toilet discharge lines for sea water mineral buildup? If the problem toilet is the most used, buildup will be thicker in its discharge line than the others. I've seen it reduce the inner diameter of a 1.5" hose to less than 1/2." That can definitely create back pressure. Not hard to cure without removing any hose, just a bit of a PITA that can require some time and effort.

You have macerating electric toilets...is there any sign that one is struggling? Have you had any guests aboard that might have flushed a wet wipe?


We'll get to the bottom of your problem eventually....



--Peggie