Lavac water drains without flushing

[kalessin]

I have a Lavac problem and haven’t managed to find a solution on here, not even in the posts from my dear late husband Samwise.

The Lavac flushes successfully to sea and the vacuum is created as usual. Once you can open the lid, there is the usual amount of clean water in the bowl. However over the next few minutes the water drains away, leaving just a little in the pipe.

Last year the toilet wasn’t really working at all and I deduced that not enough water was coming in. Over the winter the inlet seacock was replaced and a new pump was fitted. Pipes at that point were all checked and appeared reasonably clear. After that the toilet worked much better until this draining problem has arisen in the past couple of weeks. The toilet is still usable but less pleasant.

The toilet set up (see attached image) has been the same for the past 18 years. We have encountered many other issues but not this one before!

Any suggestions please?

 

[RobbieW]

I think it's siphoning, has the syphon break at the top of the loop blocked ?

 

[Boathook]

The inlet pipe at the top of the loop should have a s small black bit, a bit like a biro end, with a small hole in it. When pumping out, you should here a hissing noise if it is working correctly.

 

[geem]

The inlet pipe at the top of the loop should have a s small black bit, a bit like a biro end, with a small hole in it. When pumping out, you should here a hissing noise if it is working correctly.

That is correct, but it has nothing to do with why the bowl would empty. The syphon break is on the inlet not the discharge pipe.

 

[Boathook]

That is correct, but it has nothing to do with why the bowl would empty. The syphon break is on the inlet not the discharge pipe.

If it is blocked it might create anough syphon affect to drain the bowl. The other option is that the bowl is leaking at the base joint or outlet hose.

 

[PaulRainbow]

If it is blocked it might create anough syphon affect to drain the bowl. The other option is that the bowl is leaking at the base joint or outlet hose.

That is not possible. The pump empties the bowl, the seat/lid are sealed, so this creates a vacuum in the bowl, which causes the flush water to be drawn in. The vent hole breaks the syphon after a while, when the pumping has stopped. If the vent hole blocks the flush water can (not will) syphon into the bowl, filling it and possibly overflowing.

If the bowl is emptying after the seat is lifted there is either a leak, or it is syphoning. There is no syphon break on the outlet side, syphoning here is usually prevented by having a high loop in the outlet pipe.

 

[LittleSister]

If the outlet pipe from Kallesin's toilet does not rise before it enters the first diverter valve, and the diverter valve (perhaps a little leaky?) is at all open to both the toilet and the holding tank, then the toilet would simply drain to the tank.

. . .

If the bowl is emptying after the seat is lifted there is either a leak, or it is syphoning. There is no syphon break on the outlet side, syphoning here is usually prevented by having a high loop in the outlet pipe.

I read it suggested on this forum years ago (by VicS, perhaps?) that in the Lavac enough air is pumped into the outlet pipe with the waste to create a syphon break in the raised loop and this was why no anti-syphon valve was required. I don't know whether this is true, but do know I never had any syphoning problems when I had a Lavac.

That is correct, but it has nothing to do with why the bowl would empty. The syphon break is on the inlet not the discharge pipe.

I am not disagreeing, but just to clarify - an inlet pipe with a failed anti-syphon vent can siphon out water just as easily as in, depending on the relative heights of the toilet and the waterline at any moment. It cannot drain the remnants of water in the bottom of the bowl that is troubling the OP simply because the water inlet to the Lavac bowl is well above that level.

 

[kalessin]

Thank you everyone and especially @LittleSister who I think is correct. I now have a better understanding and further information. When I loosened the outlet hose on the incoming (valve) side of the Mk V pump there was hissing and gurgling, and the bowl refilled with a moderate amount of water as usual. (And two shrimps).

This pipe runs from the diverter valve which allows the same pump to be used to clear the (rarely used) holding tank. The valve is in the cockpit locker and at the moment is next to the liferaft. I think in choppy seas the liferaft may have nudged the diverter valve so it was slightly ajar. I have moved it back and forth a few times and definitely into place, and now the bowl has stayed full for 30 minutes or so. The water has presumably been siphoning into the holding tank but it was cleanish water so shouldn’t be a problem, and I can’t try pumping it out in harbour. And I will wedge the liferaft away from the valve!

 

[vyv_cox]

Siphons are quite difficult to achieve over a reasonably tall U-bend in 1.5 inch hose. Is it possible that the holding tank vent is blocked, allowing partial vacuum in the tank? That seems a possibility.

 

[kalessin]

Siphons are quite difficult to achieve over a reasonably tall U-bend in 1.5 inch hose. Is it possible that the holding tank vent is blocked, allowing partial vacuum in the tank? That seems a possibility.

Thanks Vyv, I did think that, but the vent is clear as far as I can tell. Hopefully it was just the leaking diverter valve…. and the movement of the liferaft also explains why it arose suddenly when I thought everything was ok.