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I'm installing a 500 gallon water tank next to an existing water tank... The existing tank holds 300 gallons and is fed by well water. This tank has a float shut off and a pump that pumps into a pressure tank and then the house. We will connect at the bottom w/ 2" PVC.

I've read that the two tanks need to be set at the same heights. Does this mean that the new 500 gallon tank will only store 300 gallons since the existing tank will shut off at that point via the float switch?

Can I raise the level of the 300 gallon tank so that it is higher than the 500 gal tank (I was thinking the 500 would store more water but now realizing that the water in this tank wont flow back to the 300). Thank you!

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    Well, yes, the float will shut things off regardless of what's in the new tank, but that assumes a simple connection with free crossflow and no check valves or anything. Is that the case here?
    – isherwood
    Commented Jan 29, 2021 at 21:34
  • If they are just connected together with a piece of pipe, and properly vented, then the water level will be the same in both tanks. (note: not a plumber; I don't know the requirements for venting)
    – Criticizing Israel not allowed
    Commented Jan 29, 2021 at 22:20
  • It appears you've created two accounts. Please merge them so you have access to edit, comment, and accept answers on your own question. Posting multiple responses as answers can result in an automated lock that can prevent others from assisting you.
    – BMitch
    Commented Feb 1, 2021 at 0:40

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With proper valve and control setup, you can set it at whatever height you like. Level control in each tank would be independent of the other tank.

There are multiple ways to do this. One would be to have the float switch on the 500 gallon tank, and a float valve (fed from the 500 gallon tank) on the 300 gallon tank, as well as a check valve on the output of the 300 gallon tank to the pressure pump before it joined the output of the 500 gallon tank to the pressure pump.

If it's just open pipes, sharing one float valve in one tank, you can't.

Of course, if the 500 gallon tank was larger diameter but the same height, that could work - you have not given specifics about those details.

If you want to elevate the 300 gallon tank, you'd move the pressure pump input to the bottom of the 500 gallon tank, so that water from the 300 would flow into the 500, with the tops of both tanks at the same level.

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  • that all makes sense. To answer your question the new 500 gal tank is rectangular Width 74” Depth 31” yep fits thru the doorway Height 68”
    – Dave Edwards
    Commented Jan 31, 2021 at 22:58
  • ...and the height of the 300 gallon?
    – Ecnerwal
    Commented Feb 1, 2021 at 1:50
  • Currently about 60" but we have room to raise
    – Dave Edwards
    Commented Feb 1, 2021 at 16:55

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