Well Gallons Per Minute

Discussion in 'Open Loop' started by Bostonceltics, Oct 20, 2016.

  1. Bostonceltics

    Bostonceltics New Member


    I would like to install an open loop geothermal system using my existing 50gpm water well, which I also currently use for domestic water. My well was drilled 30 yrs ago so I want to do a drawdown test to verify the gpm it will still produce. How is this test done? I am having difficulty finding a local well driller that does this test.
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

  3. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Well function is explained in gallons per foot of production. Specific capacity.
    Remove existing pumping equipment.
    Measure standing water level.
    Pump the well off at a known gpm.
    Measure the pumping level.

    simple math will yield the wells capacity expressed in gallons per foot.

  4. Bostonceltics

    Bostonceltics New Member


    I did my own test a couple weeks ago. I kept my garden hose running for an hour. Initially the well level dropped 3' with the hose running but after that initial drop the well level did not change. I an thinking that initial 3' has to do with a static level.

    I hope the well produces at least 20gpm. I am thinking if I pump water out of the well at 20gpm and the water level doesnt drop after 3' then the well in fact produces at least 20gpm.

    If the well level drops past 3' than it seems like the recovery rate of the well would give me my gpm.

    Does this sound right?
  5. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Why do you need 20 GPM?
  6. Bostonceltics

    Bostonceltics New Member

    15 for a 5 ton heat pump and 5 so ill have enough for domestic water.

    Does that sound like it would work to you?
  7. arkie6

    arkie6 Active Member Forum Leader

    I doubt you are getting 20 gpm out of a garden hose unless it is a very short fat garden hose and you have very high supply pressure. Probably closer to 10 gpm considering the pressure drop across the supply valve.
    Last edited: Oct 21, 2016
  8. arkie6

    arkie6 Active Member Forum Leader

    Also, you don't need 3 gpm/ton water flow if you are on open loop. You can probably get by with closer to 2 gpm/ton or 10 gpm for a 5 ton unit.
  9. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    You need a little more data to find the answer, here is me example from my con ed class.

    well is 100' deep
    static water table is 20'
    pumping rate 10gpm
    pumping level is 30'

    10gpm at a drawdown of 10' means the wells specific capacity is 1gallon per foot. That well is poor in production. How far you choose to drawdown a well based on output is another discussion.

  10. Bostonceltics

    Bostonceltics New Member

    I appreciate your assistance everyone.

    Based on the little test I did myself, how many gallons per foot did my well produce?

    The water level dropped 3' while pumping, I am guessing around 15gpm because the garden hose and kitchen faucet were really running together most of the time. I have an 80' deep well.
  11. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Typical garden hose is 5 GPM. Kitchen faucet maybe 2 GPM, but don't take my word for it...clock the fill time of a 5 gallon bucket using the garden hose and maybe a 1 gal milk jug at the kitchen sink. Then you'll have your actual GPM.

    Ditto on needing only 2 GPM per ton.
  12. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Most manufacturers specify that they would like 1.5 gpm/ton if your EWT is above 50F, and 2 gpm if your EWT is below 50F (in the continental USA). The point is that you need enough gpm that your leaving water temperature is always above 38F otherwise you coil will start to freeze.

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