January 19th, 2011 by Tim Leave a reply »

I was listing to a panel guest on SAFM  – think it was Jeremy Taylor, talking about rain water harvesting. I agreed with much that was said but one of his solutions struck me as a bit odd and it got me thinking that there has to be a better way.

He was talking about the problem of how to handle the tanks running dry when the house is setup to run off an automatic pump. Obviously if you have a Municiple supply the idea would be to connect back to this supply. His solution was to install a ball valve near the bottom of the tank running off the mains. Now whilst this will certainly solve the problem of the tanks running dry, it seems silly to waste the inherent  pressure of the municipal supply by filling up your tank, which you then have to pump out again to make usable. Not only does this waste electricity, but also will wear your pump out quicker. Also during a power cut you will be a bit stuck.

So this begs the question is there a better way? Well a less elegant system would be a set of two valves to enable a switch over between tank and municipal supplies, but that would be too easy and requires user intervention – so this is what I am going to do:

I am going to install 2 solenoid valves (1 ” irrigation type) – one on the municipal supply, the other on the tank. These I will connect via a transformer to a float switch. The float switch I have has 3 wires which effectively gives you 2 states, “tank empty” or “tank has water”. So by simply using this float switch as a kind of relay you have an intelligent system which will automatically switch between the 2 supplies without the need for any fancy control panel.

Another advantage of this system is that the pump can’t run dry and un-prime itself, disadvantage I suppose is the transformer is always pulling electricity. I doubt this is going to be a large cost factor as the solenoid valves are designed to very efficient, so I would be surprised if they use more than Jeremy’s system above. Also in a power-cut you would need to manually operate the municipal solenoid valve to get water – but this is easy enough as most solenoid valves have a manual overide.

Here some pics and a a diagram of what a mean (note non-return valve on municipal supply as precaution):

I am about halfway though my installation so I’ll keep you posted on how well it works, as always if you have a suggestion or comment post away.



  1. Al says:

    Tim, as always, you blow me away.

    LOL at the comment “but that would be too easy “…

  2. Paul says:

    Hi Tim, William PJ introduced me to your site. It is packed with great info, thank you!

    Rain water harvesting is great but how do you look after the water? I was thinking of 3x 5000L tanks connected to the house. The tanks will be above ground and would get full sun, unless they are covered. I have received conflicting info. Some pro the system others say the water cannot be managed. How well has your system worked for your household?

  3. Tim says:

    Hi Paul, to be honest I havent gone into too much depth with managing the quality as I use the water for the garden, filling the pool and some of the toilets. My tanks are under the deck in full shade and even so there have been times when there has been a Sulphurous smell (this would probably be related to anaerobatic bacteria breaking down plat matter). They have seemed to settle with time though and the quality now is fine.

    A fair portion of the country relies on rainwater for drinking so I cant believe its that problematic though, also the tanks all seem to have a black liner to keep light out – although in full sun they might warm up ?
    You might want to think about partly burying them though as 5000 l tanks are big ! Sorry I cant be of more help.

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