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Well - minimun GPM

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  • Well - minimun GPM

    I hope this is the right place for this question, I did see a heading for low water pressure.

    I am responsible for the flooring int his case, but I was asked about the owners water situation. I expected to be able to get it out of a manual. but can't find anything.

    Is there any standard for the rate at which water shouuld be returning to a well for a residence to be occupied by a familly of 4. Theire well is rated at 1-2 gallons per minute.


  • #2
    Re: Well - minimun GPM

    Here in CT water flow from a well varies immensly and 1-2 gallons is not an ideal flow rate but a great deal depends on the depth of the well. ie the deeper the well, the more storage. My own home has a 600' well and only produces 1.5 gpm and I only run out of water if I leave my garden hose running full blast for more than a few hours.
    I believe the formula is 1.5 gallons per lin foot of well which is 6" diameter.

    If the well is not deep there are ways of adding storage with additional tanks to increase the storage.

    Your best bet if you are a flooring contractor is to tell the Owner there are too many variables to consider to answer the question properly and a well contractor should be contacted if they want a proper evaluation.


    • #3
      Re: Well - minimun GPM

      Hi Thom,

      There are an awful lot of variables to deal with when trying to decide whether a private well is supplying water at a suitable rate. For instance, at 50 psi. constant pressure, supply volume is greatly affected by size and length of the water supply pipe and the depth of the well.

      Some general rules of thumb. As long as there is nothing peculiar about the particular home to differentiate the well from others in the immediate neighborhood, a private water system should be able to supply 3 gpm of water for an hour, without distressing the well. General rule by home type: 2BR w/1BA = 3gpm; 3BR w/1-1/2BA = 5gpm; 4BR w/2BA = 7 gpm.

      This can require you to do some interpolation. For instance, a 3 bedroom home with 2-1/2 baths would probably need about 6 gpm.

      I recommend you check with your local water department or see if they have a website where you can determine the type of aquafir in your area. Depending on the type of aquafir, these rules of thumb could be higher or lower for your area.




      • #4
        Re: Well - minimun GPM

        Here are some thoughts from good ole GAD:)

        FACT 1:
        The absoulute minimum a well needs to supply is the daily usage in a 24 hr period. Thus is based on the ultimate need to replace whatever has been used. Demand is managed by the tank. If you are lucky enough to have a relatively high yield well, you can make your tank smaller and basically use the water that comes out of the well to meet the instantaneous demand.

        FACT 2:
        A well yield varies all over the country. In many coastal areas of NC the yield of the well is usually limited by the size of the pump. There is a seeminly endless flow of potable water. So you will need to know what you should expect in your area and even then it can vary by the workmanship of the well, and the geology which can change from lot to lot.

        FACT 3:
        There are several sources of estimated demand. Numbers I'm familiar with are a daily demand of 160 gallons per day per bedroom, with an instanteous demand of about twice that. You can do the math and come up with a maximum instantatous demand or use 5-7 gpm. The tank size can be determined by constructing a hydrograph. The maximum acculated difference between the demand and the supply (or supply/2 for a 2.0 service factor) will be the size of the tank.

        FACT 4: With regard to the performance of a well, you are at the complet mercy of God and the well driller. There are methods availble to measure the effiecienccy of a well to determine whether it is God's fault or the Well drillers for anyone wondering if the well driller did his job. but unless it is a large well, its cheaper to drill another well than run the tests.

        Now you can take all thatup to this point, to the bank. If anyone wants to square off with me, start here :)

        Probably the best design (not taking into account costs) is to drill a well and determine its continuous yield. The yield of a well will dimish with time (usually minutes) and will ultimately level off at some specific GPM. That is the continous yield. Conservatively speaking, it would be good practice to size the pump to pump slighly less than the yield of the well so that you aren't likely to run the well dry. (Someone mentioned the storage capacity of the well pipe as a source of demand capacity. I'd like to think of that as a bonus and not count on it for my design, its isn't going to change the tank size that much.) Then buy the biggest tank you can justify up to a size equal to the total daily demand, and then double it. For a residence, that would be 700 gallons although I've heard of folks using up to 1500 gallons.. I have a 30 gallon tank but my well yields more than my instaneous demand so I only need the tank to iron out the pressure spikes. The effective size of the tank is less than the actual size since your pressure will change as the tank level lowers.

        Now, if the yield is low and the tank gets large, it then becomes feasible to use secondary pumping. That is, dump the well in a gravity storage tank and use a second pump size for amximum instanteous demand and then use the smallest pressure tank availble. This makes a lot more sense for a low yield well but I doubt it is often considered. You basically size the well pump for the well yield and size the demand pump for the instanteous demand using something like a 20 or 30 gallong pressure tank. A gravity storage tank is much cheaper than a pressure tank.

        In short, the size of the well doesn't have anything to do with the need for water as long as it can yield at least the total daily flow in less than 24 hours. (conservatively about 1 gpm)

        Hope this is food for thought.


        P.S. I didn't really understand the original question.


        • #5
          Re: Well - minimun GPM

          Mike-just a side note- I recently had a procedure called "hydro-fracture" done to my existing well and increased the GPM by 2 1/2 gallons. Has'nt changed in six months- were keeping our fingers crossed. Not a cheap but came with a two year warranty to come back if it stops producing and price was cut in half if it did not work. TIM