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Thread: Help! Where to install pump pressure switch?

  1. #1
    DIY Junior Member HawaiiBob's Avatar
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    Default Help! Where to install pump pressure switch?

    Aloha & help! We are off municipal water, and I want to use the garage as a rainwater catchment system to feed the house water supply. Thats very typical here in Hawaii, except that the garage water tank & pump is 200 feet from and 6' below the house entrance, where the 85 gal pressure tank is located. The 10-12 GPM pump is appropriately sized (200+ Hft) for PSI and line/fitting losses. My question is: Where do I install the (40/60) pump pressure switch - at the pump, the pressure tank, the farthest/highest fixture, or does it make any difference? I understand the static (no-load) pressure is the same no matter what the pipe diameter or where horizontally in the system it is located. I am less clear on the effects of the appox 9 psi loss (1-1/4" PEX 200' pipe friction & 6' rise) from the pump to the pressure tank, the appox 24 psi loss (3/4" PEX 60' pipe friction, connecting to 1/2" fixtures w/nominal run, & 15' total rise) from the pressure tank to the last fixture, and the flow rate and pressure changes as the pressure tank cycles. Said differently, I want to be sure I have a minimum of appox 40 psi for up to 10 GPM use anywhere in the line, at any time.

    Thanks so much for your thoughts. [I can attach a diagram & pump performance curve chart if thats helpful]

  2. #2
    Moderator valveman's Avatar
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    The pressure switch must be located very close to the pressure tank. If not, the pressure switch will bounce the pump on and off rapidly. At what flow rate where you figuring the friction loss? The pressure switch just needs to be set high enough to overcome the friction loss and still have 40 left. Also you won't see the flow rate and pressure change as the pump cycles if the pump is not cycling. A Cycle Stop Valve will hold the pressure constant instead of letting the pump just cycle on and off and the pressure keep going up and down.

  3. #3
    DIY Junior Member HawaiiBob's Avatar
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    Thanks so much for your fast response. The flow rate was calculated at 10-12 GPM for a Grundfos CRI 1-9. You've brought me full circle - if the setting s/b 40 + the 9 psi loss (6.6 frict/2.6 rise) from the pump to the tank/switch, that one thing, but if it also should include the 24 psi loss (17.3 frict/6.5 rise) from the pressure tank to the last fixture, we're talking some serious pressure. Removing the pressure tank from the equation, if my very lay calculations are correct, the pump should deliver @9+GPM at 50psi to the last fixture, (with a corresponding increase in volume & pressure to those fixtures closest to the pressure tank in actual use - in this case, principally the kitchen & laundry). So have I completely missed the boat here? Again, thanks. Sorry - I can't seem to get my diagrams small enough to upload, unless you have another vehicle of receiving them. [the cycle stop valve looks interesting - I'll definitely want to do some more investigation]
    Last edited by HawaiiBob; 10-05-2012 at 01:50 PM.

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    Moderator valveman's Avatar
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    All friction loss from the pump to the faucet is added together. So if you have 24 + 9 PSI friction loss and want minimum 40 PSI at the end, you need a 73/93 pressure switch setting. The problem with that is if you are using 10-12 GPM, the friction loss eats up 33 PSI, so you only have 40 at the faucet. But when you are only using 2-5 GPM the pressure at the faucet will be between 73 and 93, which may be enough to rip off skin.

    Are you sure you are figuring PSI and not Head in feet? Makes a big difference.

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