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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    We are collecting rain water in a large header tank. It's very satisfying! One issue is manually filling it when the weather is dry for a long time. I'd like some kind of ball valve that only fills up the bottom 10% of the tank to prevent it running dry. Normally ball valves keep the tank topped right up.

    Any ideas on how to achieve this?

    Comments gratefully received.
    • CommentAuthortony
    • CommentTimeOct 14th 2018
    What I did was to fit a second mains fed ball valve 150mm above the rainwater one with a bespoke longer arm, the float sits below the normal water level of the rain water controlled by its own ball valve, the cistern refills once a day from my underground rain tank pump. When rainwater is exhausted the mains fills the cistern about half full til it rains again then does nothing more till the next drought
    Thanks Tony, that's asking the lines I was thinking. There's quite a big difference between the levels though, so a bespoke arm might be a bit contorted.
    • CommentAuthormike7
    • CommentTimeOct 15th 2018 edited
    Assuming the 2nd ball valve has to be at the top of the tank so you can get at it, I'd remove the ball from the valve arm, fix it to one end of a rod with the other end pinned to the arm, the rod being of such a length that the ball dangles down close to the bottom of the tank. The rod would need a running guide of some sort near the ball to keep it roughly vertical.
    As the ball will spend most of its life completely immersed its buoyancy will exert much more force on the valve lever than would normally be the case, so it might be a good idea to pin the vertical rod to the valve arm nearer to the valve body to reduce the leverage on the valve. Doing so would also make the valve open and close with a smaller range of level change, which might be useful.
    • CommentTimeOct 15th 2018
    Posted By: mike7Assuming the 2nd ball valve has to be at the top of the tank so you can get at it

    The mains ball-valve has to be 150 mm above the rainwater one as Tony said for reasons of the water regs say there has to be an airgap. Very reasonable in this case, IMHO.

    A cranked arm, perhaps bespoke as Tony suggests, sounds simpler to my mind than a pin joint.
    • CommentAuthorCWatters
    • CommentTimeOct 21st 2018
    +1 to cranked arm.

    If that doesn't work out.. I suppose you could fit a submerged level sensor connected to a motorised valve higher up?
    • CommentAuthorbxman
    • CommentTimeOct 22nd 2018
    Assuming you are pumping into the header tank you could use a diaphram switch as used on most washing machines to switch the pump on and off when required
    Posted By: bxmanAssuming you are pumping into the header tank you could use a diaphram switch as used on most washing machines to switch the pump on and off when required

    Except the OP is talking about a back up mains water connection filling only the bottom 10% of the tank in the event of prolonged dry weather.
    I've been doing some thinking on this, and it occurs to me that as the tank is at atmospheric pressure, I could have the top up in a different place, it need not be filling the tank, as long as it is at the same level. My thought was say a length of vertical drainpipe next to the tank with a ball valve at the end of a long arm (as per earlier suggestion) that could be kept in place by the drainpipe. Obviously the drainpipe would be connected to the tank at the bottom using a pipe, and at the same level as the tank.
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