I am using a large trashcan as a greywater reservoir using a few bulkhead fittings. I need a valve or float combination that will empty it when full. It needs to hold the drain open until the water level is very low then close again to wait until it fills up (laundry, showers, bathtub etc...) I want to avoid pumps and float switches and use a simple valve but with a long enough float "throw" to work properly. I've never seeen a float valve used to EMPTY or drain a tank\cistern. They always seem to be fill valves.

I don't think a toilet style ballcock will work because the throw is too short and the size is too small. Is there some kind of sliding valve that will work? Thanks!

  • Good luck. There's a reason floats are generally use for fill, not drain purposes. You might look at an "autosiphon" as something closer in function to what you seem to want, though you'll need to ignore the beer type (which are somewhat of a misnomer, but they pollute search results just fine.) You want one with an "air bell" and normally seen in septic applications - you might be able to cobble one up when you see a diagram. Not affiliated;look here for how they work: siphons.com/how-siphons-work.html
    – Ecnerwal
    Apr 6, 2015 at 2:10
  • thanks so much! Don't know why I didn't think of it. I am going to have to size it properly with my discharge rate and distribution lines, but I have plenty of elevation to work with (house on hill with orchard below) Even a simple double tube-style siphon may work fine. Ill have to experiment with some pvc of different diameters and see what works best. Thanks!
    – Tob
    Apr 6, 2015 at 4:22
  • You'll need the air bell if you want it to start itself as a siphon and draw the tank down - otherwise it will just act as an overflow, trickling out as it trickles in.
    – Ecnerwal
    Apr 6, 2015 at 16:25

1 Answer 1


I would try a toilet tank system, just without the assembly. Attach the flap stop over a hole in the bottom and use the floating ball on a chain to lift it up when the tank is full. You just would have to support the ball in such a way that the chain doesn't get sucked down the drain, preventing the flap from closing completely when done. Good luck

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