dehler holding tank - should it do this ?

[PaulR]

over last few weeks have had unpleasant problem with heads/holding tank - when boat heeled to extreme (as on saturdays beat) backflow from tank into heads bowl and even (ugh) overflow.


heads (jabsco manual) appears to be working normally and have checked both valves at bottom of heads pump to outlet and both seem fine so suspect problem is in holding tank.

my suspicion is that holding tank not emptying normally and hence build of liquid and when healed to extreme extra height of contents of holding tank compared to when boat level is putting pressure onto heads valve and hence backflow - certainly seems to be too much liquid to have been solely in pipe between heads pump and holding tank.
Dehler tank is rectangular stainless steel with 3 pipes in/out all at bottom of tank- dehler drawing shows outlet pipe is straight onto hole in bottom of tank and then to seacock, inlet pipe to fitting on bottom of tank with internal pipe running from bottom of tank to near top inside , same for breather pipe which vents to through hull fitting at waterline.

fearing blockage in tank outlet I removed outlet pipe from seacock and used plumbers snake (flexible coiled wire thing) to prope up it - no problems - snake went all the way up with no obstruction and thankfully no sudden gush of ugh!

Before trying that i removed the breather pipe connection (this with outlet seacock open) and was somewhat surprised when I could then hear air being sucked into the tank for what seemed like a minute - then I checked if through hull for breather clear and it is (can blow air through it using dinghy pump ) and could push drain snake up the breather but not get it to turn corner at top of internal pipe fitting.

Key question - I guess the air being sucked in was because the tank emptied itself when I released the breather and as contents went out air sucked in but if I am right cannot work out what was stopping it emptying before as no obvious blockage in breather pipe - any suggested checks / solutions ?

 

[mph]

It could be worth checking the breather hull fitting, I have come across some of these which have a small filter....which can get blocked.

 

[boatmike]

Yes its obviously blocked somewhere and the filter is an obvious place to look. Most breather pipes are far too small in dia too. Should be at least 1" dia IMHO.

 

[Birdseye]

The breather may or may not be blocked, but that is unlikely to mean that you only have a problem when heeled.

My guess is that all that is happening when you heel is that the back pressure on the toilet valves increases and they then leak. That should not be the case - there's no use in a holding tank that you have to empty in harbour before you get under way. So I reckon its back to the bog valves

 

[davel]

Blocked breather pipe would mean that the tank wouldn't empty. Boat heeling could force contents back through the inlet pipe and past the valve in the toilet. God knows how you managed to get the stuff into the tank in the first place though. A blocked breather would have made pumping out the bowl would have been tough and if the tank was already full ....?????

BTW stainless is a crap (Sic) material for a holding tank as the welds get corroded by the contents. I had to replace mine after about 5 years. Now have a plastic tank fitted.

 

[Guest]

I am confused about the "breather" being at the water line. If you are sailing the water line is not where it should be!

So as the boat pitches you will be pressurizing the whole system in a cyclic manner. Poor heads valves would not know what to do. I would need a diagram, but with a constant cyclic pressure and a slightly leaky head valve you could get a pumping action setup.

If the inlet of the tank goes to the top; then the tank must have been filled. Otherwise you would have only seen just the pipe contents expelled. So there are 2 problems; a pressurizing tank and and a back filling tank.

Heads hose is about 1litre/m contents so it could just have been the pipe.

If this is the case and the tank is stainless then I would suspect all the welds on the tank. Stainless is dissolved by the gasses from heads. Also the repetitive movement of a "panting" tank will work harden the weak points and weld edges.

 

[alan]

I have had a problem with the toilet (Jabsco) bowl 3/4 filling up with brown liquid when I left the boat for some days, even though the holding tank was empty. It is surprising how much liquid is "held" in the pipe from the bowl to the tank.
Dismantled outlet valve from bowl (Joker valve) and it appeard to be in perfect confition; however, as it was in bits I decided to replace the rubber joker valve with a new one ...................... problem solved!!
Could it be that you are getting liquid leaking back into the bowl due to a "weak" joker valve, and this is accentuated when the boat is heeled over ???
Just a thought and it is only a ten minute job to replace the joker valve.

Alan.

 

[Norman_E]

That has to be the answer. If tank contents are returning to the bowl, then regardless of any other problem they must be passing back through the joker valve, and replacing it should solve the problem.

 

[PaulR]

thanks everyone - will try new joker valve and really hope that is solution