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REPOSTING - rainwater management system (drawn a blank)

Postby omnivorist » Thu Jun 10, 2010 8:13 am

I have a large cistern in my back garden that collects the rainwater run-off from the house roof and yard. I mainly want to use the water for watering plants but I'd also like the cistern to syphon to a pond in a lower part of the garden - and to self-syphon when the cistern becomes full. I already have the pipe-runs in place to be able to do this but no way of controlling the flows.

I'm keen to experiment to find-out the different options I can set up with a bit of design effort, one or two gate valves and a one-way valve to release air from the syphon. My pipework is 40mm plastic (type used with solvent-weld).

Installation is outside. Very modest pressure - 1 metre head.

I'd be grateful for any info on the availability the sorts of valve that would be suitable. I imagine there are fish-farms, etc that use this kind of stuff.

Also - is it possible to use an air-admittance valve (say the kind used on waste traps) back to front, so as to make an air-release valve? I'm pretty sure this is what I need to start my self-syphon. I have experimented using by hand as an air release-valve and it seems to work. I can see how I might have done it all a lot better if I was starting at the beginning but there are certain critical levels that are impossible to change now.

Thanks in advance

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Simply Build It

Postby htg engineer » Thu Jun 10, 2010 7:20 pm

Firstly do you need it to empty ? if yes, why ?

Is it just a case of not wanting it to overflow ? if so why can't you just run a overflow pipe from the tank - to the pond.

Can the pond take the extra water ? if we had heavy rainfall, which filled the pond, then the tank emptied into the pond - the pond could overflow - possibly along with fish !!!

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Postby plumbbob » Thu Jun 10, 2010 7:53 pm

Sorry for ignoring your post, but have to admit to being baffled about what you are trying to do.

It's like Htg says, why not run an overflow from the tank? Surely you don't want the tanks to fully empty each time they fill?? Or do you?? What are you hoping the syphon would achieve over a simple overflow?

If you do want to use a syphon, where do the gate valves and AAV enter into the equation anyway? The principle of a syphon is simple and requires no special parts.

How a syphon process could be made automatic needs careful thought though as it is may not be easy.
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Postby omnivorist » Fri Jun 11, 2010 7:50 am

Many thanks for replies to my query on my rainwater system.

Yes, I could arrange a simple overflow. It's just that I'd like the option of simulating a running stream into the pond on a hot day (a syphon does this very effectively). The pond *does* overflow but it's a natural pond (no fish - just newts, frogs etc) and it overflows down to a deliberately boggy area with bulrushes etc so that's OK.

I'd also like the option of draining off some of the rainwater for watering the garden - hence the need for valves etc.

But if I'm really honest, I also like the idea of a bit of a control panel from which I can start the syphon, run off rainwater to the veg garden etc. all from the comfort of my deck chair (didn't you ever have a train set?)

So it really is down to:

Can I get simple, low-pressure gate valves, one-way valves etc. that are compatible with 40mm welded plastic waste pipe and, if so, where?

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Postby brithebuilder » Fri Jun 11, 2010 9:40 pm

Hi, you can find the gate 40mm valve here (Google ""
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