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  1. #1
    Join Date
    Jul 2013
    Posts
    57

    Default Holding tank vent pipe

    I am just about to fit a Holding tank in my Southerly and am planning the route of the vent pipe which I will fit below the toe rail outboard of the deck. As the diameter of the pipe is 1.25 inch to match the input/output of the tank I was wondering whether I could get away with using a standard re-enforced water hose which is much more maleable than the conventional white heads pipe. Can anyone forsee this arrangement causing a problem. Waste will never enter this pipe, it will be simply be to allow a free passage for the input/output of air. The pipe run will be less than 6 feet.

  2. #2
    Join Date
    Oct 2006
    Location
    Bradwell and Leigh-on-Sea
    Posts
    14,367

    Default Re: Holding tank vent pipe

    The only problem might be that it absorbs odour.
    Not all who wander are lost

  3. #3
    Join Date
    Nov 2008
    Posts
    6,790

    Default Re: Holding tank vent pipe

    I know next to nothing about holding tanks, but I can't see why the vent pipe needs to be 1.25". As a vent pipe, it should only ever have air in it, so 0.5" pipe should be plenty, and much easier to route.

  4. #4
    Join Date
    Oct 2006
    Location
    Bradwell and Leigh-on-Sea
    Posts
    14,367

    Default Re: Holding tank vent pipe

    Quote Originally Posted by NormanS View Post
    I know next to nothing about holding tanks, but I can't see why the vent pipe needs to be 1.25". As a vent pipe, it should only ever have air in it, so 0.5" pipe should be plenty, and much easier to route.
    If the pumped outlet is 1.25" then the vent pipe has to let in air at a reasonable rate. Ta
    Ektamks recommend all pipe work at the same size. I thought it was 38mm.
    Not all who wander are lost

  5. #5
    Join Date
    Nov 2008
    Posts
    6,790

    Default Re: Holding tank vent pipe

    Quote Originally Posted by MoodySabre View Post
    If the pumped outlet is 1.25" then the vent pipe has to let in air at a reasonable rate. Ta
    Ektamks recommend all pipe work at the same size. I thought it was 38mm.
    In my experience, water (and shit) is thicker than air.

  6. #6
    Join Date
    Jul 2002
    Location
    Home: Kent. Boat: Chichester
    Posts
    43,075

    Default Re: Holding tank vent pipe

    Quote Originally Posted by NormanS View Post
    I know next to nothing about holding tanks, but I can't see why the vent pipe needs to be 1.25". As a vent pipe, it should only ever have air in it, so 0.5" pipe should be plenty, and much easier to route.
    It needs to able to ventilate the tank freely to prevent anaerobic conditions developing .......... which is what leads to nasty smells.

    As large as possible / sensible and as short as possible

  7. #7
    Join Date
    Jan 2004
    Location
    Bricks & mortar: Italy. Boat: Aegean
    Posts
    10,419

    Default Re: Holding tank vent pipe

    The purpose of the vent pipe is not simply the allow material to enter and leave the tank, but to allow a free exchange of air between the tank and atmosphere. (Hence they're also called breather pipes.) This reduces the likelihood of anaerobic decomposition, the type that gives rotten eggs smell. A convoluted 0.5" pipe six feet long would vent OK, but is unlikely to be very good as a breather. Mind you, neither is a 1.25" pipe. Ideally, they should be short and straight. (Ours is about 200mm.)

    ...as VicS wrote whilst I was typing.
    Last edited by macd; 20-04-19 at 07:58.
    All epigrams are false

  8. #8
    Join Date
    Nov 2008
    Posts
    6,790

    Default Re: Holding tank vent pipe

    Wow! 200mm.
    OK, I do know what you mean.
    My boat actually has a holding tank, and its vent pipe is only 0.5", fitted by the builders, but as the tank is never used, I am in no position to say whether it is adequate or not.

  9. #9
    Join Date
    Feb 2019
    Location
    The Known Universe
    Posts
    626

    Default Re: Holding tank vent pipe

    Quote Originally Posted by VicS View Post
    It needs to able to ventilate the tank freely to prevent anaerobic conditions developing .......... which is what leads to nasty smells.

    As large as possible / sensible and as short as possible
    This is the best solution and from the Head Mistress herself Peggy who wrote the book on odors in boats
    the Largest hose you can fit 38mm in recommended for large tanks
    Mine is 38 mm
    But like anchor , Nav lights and Batteries there is always an opinion
    https://www.amazon.co.uk/Get-Boat-Od.../dp/1892399156
    Flying birds have no master

  10. #10
    Join Date
    Jan 2004
    Location
    Bricks & mortar: Italy. Boat: Aegean
    Posts
    10,419

    Default Re: Holding tank vent pipe

    Quote Originally Posted by NormanS View Post
    Wow! 200mm. .
    It's metric where the boat is. Metric food, metric holding tank. Much more efficient.
    I'm not actually sure you can buy 1/2" hose in the UK these days, only 25.4mm. If I remember correctly, it costs something like a guinea a foot
    All epigrams are false

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