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  1. #11
    Join Date
    Feb 2005
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    193

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

    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

  2. #12
    Join Date
    Jun 2002
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    57,426

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

    Putting your finger in amongst floating turds is hardly an option. Needs sampling further down tank, rather than surface.

  3. #13
    markdj is offline Registered User
    Location : Strangford Lough, Northern Ireland
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    Jul 2002
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    1,223

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

    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!

  4. #14
    jotheboat is offline Registered User
    Location : Barge in Belgium
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    Feb 2007
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    495

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

    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.

  5. #15
    mikef's Avatar
    mikef is offline Registered User
    Location : Chesham, Bucks
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    May 2001
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    27,235

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

    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

  6. #16
    mikef's Avatar
    mikef is offline Registered User
    Location : Chesham, Bucks
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    May 2001
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    Default Re: Can a holding tank fill itself or is somebody living on my boat?

    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

  7. #17
    mikef's Avatar
    mikef is offline Registered User
    Location : Chesham, Bucks
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    May 2001
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    Default Re: Can a holding tank fill itself or is somebody living on my boat?

    volvopaul, I reckon the top of the holding tank is above the waterline so it could'nt fill with seawater to the top IMHO

  8. #18
    mikef's Avatar
    mikef is offline Registered User
    Location : Chesham, Bucks
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    May 2001
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    Default Re: Can a holding tank fill itself or is somebody living on my boat?

    jotheboat, we have a separate greywater tank on our boat

  9. #19
    Join Date
    Jun 2001
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    5,935

    Default Re: Can a holding tank fill itself or is somebody living on my 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. [img]/forums/images/graemlins/laugh.gif[/img]

    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.
    Eastern Scotland and beyond.

  10. #20
    jfm's Avatar
    jfm is offline Registered User
    Location : London/Antibes
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    May 2001
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    Default Re: Can a holding tank fill itself or is somebody living on my boat?

    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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