Can a holding tank fill itself or is somebody living on my boat?

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Deleted User YDKXO

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Every time we leave our boat, I make sure the holding tank is completely empty. But, the last 3 times that I've visited the boat, the holding tank has been full when I've arrived on the boat. I've checked the pipework and there does'nt seem to be any obvious reason why the holding tank should be filling up on it's own. There's an adequate anti syphon loop on the discharge pipework and the fresh water flushed toilets are turned off when we're not on the boat as well as the water pump
The holding tank itself is quite large and when we're on the boat it takes 4-5 days use for the tank to fill up and there's no sign of any leakage back into the tank. We do have cleaners and engineers working on the boat from time to time when we're away but I cannot believe that occasional use by these people can fill the holding tank in our absence
So before I get completely paranoid and start accusing people of using my boat as free accomodation, can anyone think of a reason why a holding tank should fill itself up without being used?
 
We don't have a guage but indicator lights. At first I thought it was the lights playing up but I took the inspection hatch off the tank to check and the tank is actually full of liquid so the lights are not telling lies
 
Hi Mark, no, this has just started happening this season. Yes, I thought it must be seawater flowing back into the tank but I realised that a faulty seacock would'nt make any difference as, firstly, I have always left it open anyway and, secondly, there's a big anti syphon loop on the discharge pipework which would'nt allow seawater back into the tank
 
Mike, difficult to tell I know, but are the contents mainly water diluted effluent, or obviously effluent? The top layer of the tank is going to be effluent based, so maybe some way of sampling it further down tank?
 
[ QUOTE ]
We do have cleaners and engineers working on the boat from time to time when we're away but I cannot believe that occasional use by these people can fill the holding tank in our absence


[/ QUOTE ]

.......unless one of 'em's not been shown/trained properly in how to flush the bog, likes vindaloo, and keeps pressing the flush button continuously after vacating his bowels of two gallons of fizzy gravy......
 
clear sellotape over the lid when you leave will soon tell you
: /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif
 
As most tanks are below the waterline it may be syphoning through the toilet bowl, macerator pump etc so its not uncommon for this to happen, I would shut the relevant Y valve if its on that system when you leave the boat.

Dont forget to turn it back on before you have a S??T as the back pressure may have you cleaning up for the rest of your holiday!
 
First you have to decide wether its filling with salt sea water or fresh water, simply put your finger and taste it!

Sound like your seacock it faulty and evan though the handle turns the ball in side does not. Ball valves can fail after a few years) With the tank low down the water with capirally and cause a syphon back into the tank
 
Mmmm, lovely.

Out of interest, we have 4 flexible diesel tanks and a manifold then pump with 2nd manifold to each main fuel tank. While sitting at anchor, overnight, the slight rocking managed to empty our main tank of 80 gallons of fuel. It was quite disconcerting in the morning till we saw that the valves were still open!
 
Is the tank purely a black waste tank?
Does any grey go ito it; shower for example?
If the shower (or basin whatever) is dripping it may be the cause. An intermediate 'mini-tank' with a float switch is designed to empty automatically if you have a drip / leak to stop the shower itself overflowing.
A check of freah-water level would give a clue as previously suggested.
 
Brendan, it is difficult to tell. I've had the inspection hatch off the top of the tank and the top layer is definitely effluent. I'm also fairly sure that the top of the tank is above the waterline so I can't see how seawater can be leaking back into the tank and filling it to near the top
 
No deck pump out fitted so it can't be that. We leave the water tank as near empty as possible when we leave to avoid the water festering for weeks/months
 
volvopaul, I reckon the top of the holding tank is above the waterline so it could'nt fill with seawater to the top IMHO
 
jotheboat, we have a separate greywater tank on our boat
 
You might have an anti syphon loop....But is the syphon break at the top of the loop unblocked. If this is blocked abd the tank is below sea level within your boat then the water will have no problem syphoning back in. /forums/images/graemlins/laugh.gif

Check your air vent and filter is clear as this could help start the syphon process if partially blocked and causing a partial vacuum after draining the tank.
 
Not a lot to add to the previous replies. Baffling.

Happened on our boat when new but it was the tank backfilling with sea through the discharge pipe. We don't have an antisyphon loop (deliberately, I specced it that way) and the whole tank is under waterline and folks forgot to close the discharge seacock. Cured this by fitting an elec seacock and warning system

If you have an antisyphon loop in the discharge pipe have you checked the little air-admittance valve at top of loop is working? These are highly prone to seizing, cos obviously gungy macerated [--word removed--] flows along the pipe.

What type/make of toilets have you got?

You could eliminate the backfill scenario by closing the dump seacock when you leave the boat, and eliminate the "folks using it" scenario by turning the heads diverter valves to sea not tank when you leave the boat
 
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