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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    I have a couple of downpipes, one from our rainwater harvesting in the attic, and one from our gutter. I need to join them together. I have these rather inelegant solutions (see attached). I'm sure there's a better way. It seems to me that I may have misunderstood the purpose of a hopper.

    See: http://www.gutter.co.uk/index.php/what-we-have-forgotten-about-victorian-gutters-and-drain-pipes/ which suggests 'A hopper head fitted just below a gutter was the Victorian way of enabling their drain pipes in heavy rain to carry 66% more water. How I hear you cry? Water flowing from a gutter down a drain pipe only runs a third full. However, if you put a hopper head in the system with the outlet pipe from the gutter at least 100mm from the base of the hopper head in heavy rain fall the hopper head fills up with water and hey presto it becomes a syphonic system, causing the drain pipe to run full of water.'

    So do I need a hopper where the rainwater collection comes out of our attic? Or possibly one there and another where it meets the swan neck from the gutter?

    Any illumination as to how hoppers work would be appreciated!
      20180112 Outlet to Hopper.png
    Here's another slight alternative.
      20180112a Outlet to Hopper.png
    Pipes flow more volume when under full siphon
    Thanks @victorianeco. I've had a look at some of the old houses near me and it seems that:
    (1) the hopper is invariably right at the top; and
    (2) the pipes run almost vertically.
    So here's a revised design with 2 hoppers.
      20180112d Outlet to Hopper.png
    Just think of a hopper as a self flushing urinal. Works on the same principal.
    The Victorians had very heavy but narrow cast downpipes with very chunky supports, that can withstand the hammering forces due to surging at incipient syphon flow.

    In syphoning flow, the pipe fills itself with kgs of water, the weight of which pulls a vacuum and sucks itself out with a rush, giving the extra capacity that was mentioned in the link, and shaking the supports. the cycle repeats unless the rainfall rate exactly balances the head height to the hopper. Only at that rainfall, can a stable vacuum be established. Unfortunately if the rainfall increases just a little more, the hopper will overflow.

    Modern downpipes are not intended to flow full of water, the water needs to entrain a lot of air to break the vacuum and avoid heavy weight. However they are cheaply available in bigger sizes than the Victorians could make.

    If you need extra capacity, increasing the pipe by one size is better than adding a hopper. They do look nice though. If you just use one like a tee piece to join your two pipes, it'll do no harm, if the downpipe is big enough to avoid syphoning flow.

    CoP's previous thread: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15380
    Thanks Will, that's really useful. I hadn't appreciated that old Victorian drainpipes were much narrower.
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