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The requirement here is something related with emptying two water tanks at different levels giving priority to one over the other. Real life scenario hers is about usage of harvested rain water which is stored in one tank and fresh water stored at a higher level.

The building in consideration would have 2 levels and upper level has a small utility area for washing / terrace gardening etc. Roof of upper level catches the rain water and is collected in a small tank installed in upper level.

This water so collected has to be flown to a pipe line connecting to outdoor hose etc which is primarily sourced from fresh water tank. A non returning valve safeguards the fresh water source from being mixed with rain water.

As the intention is to consume the rain water first, would this setup work if same thickness pipes are used or would it require bigger pipe for rain water tank as the pressures would differ due to tanks being in different levels, of different sizes (RW tank is smaller) and quantity of water varies(RW level reduces with consumption and not refilled unless there is a rain).

To simplify the building configuration:

  • No of levels: 3
  • Fresh water tank level: 3 (top most)
  • Rain water tank level: 2
  • Consumption taps level: 1 (base/ground level)

Question: FW tank and RW tank are connected to a common pipe feeding to a tap at level 1. How to achieve RW tank to get empty first when both tanks have water?

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I can't comment (I need 50 points, that forum sucks), but if I could, I will: what is your question ? Could you draw something to understand the configuration ?

Ok, I see. You can use one way valve, called I think Check valve:

https://en.wikipedia.org/wiki/Check_valve

Or:

enter image description here

Connect the rain tank at left, the fresh water tank at right and the common pipe to the base/ground level at right.

If the fresh water tank is empty, the pressure at left will be greater than at right and the valve will be opened from the rain water to the base/ground. Sure, the spring must be well choosed. Don't forget to add filter before the valve, to prevent failure.

Use a float valve:

enter image description here

To cut the upper tank with the lower

enter image description here

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  • $\begingroup$ Thanks for pitching in. Question: Do we need a pipe with wider dimension from RW tank so as to counter the pressure from FW tank at a higher level? If so, how exactly to figure out the dimension of RW pipe. I have updated the question to give a better understanding of the configuration. $\endgroup$ – Pavan Kumar May 14 '18 at 9:53
  • $\begingroup$ I got what you are trying to convey, making use of a check valve ensures that the water from FW tank does not enter the RW tank. But there would be pressure difference and to my knowledge pressure from FW tank being at higher level would be more. This might result in a higher percentage of water in common pipe to be from FW tank which is an opposite behavior of my requirement. For a precise understanding, this is the actual situation - diy.stackexchange.com/questions/138829/… $\endgroup$ – Pavan Kumar May 14 '18 at 14:43
  • $\begingroup$ The solution using a float valve actually solves the problem, but at a higher expense and a few practical limitations. Float valve installation puts me into a requirement of either the tank has to exist next to the pipe (and should have enough vents) or a smaller tank has to be built next to pipe as shown in the picture. This is an approach that solves the problem and I accept this answer, but will look forward for more solutions that are easy and economical. $\endgroup$ – Pavan Kumar May 16 '18 at 4:39

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