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How Can I Find Out What My Well Pump Flow Rate Is?



Hello Sam,

Thank you for your email. And regarding your question:

"I need to get an iron filter for my lake house, which is on a private well.  I have a question about how to check the well Pump flow rate.

The water Pressure seems good, but how can I make sure it will be enough to backwash the iron filter?  I read on your site that the backwash flow rate is critical for iron filters and sometimes bigger is not better. 

The water pressure seems good, but how can I make sure it will be enough to backwash the iron filter?  I read on your site that the backwash flow rate is critical for iron filters and sometimes bigger is not better."


Thanks for emailing.  This is very important, and it is easy to figure out and get a good approximate figure on what the flow rate is.   You are correct, it is not accurate to simply turn on a spigot near the well and time the water running into a bucket.  This is not accurate because a hose bib (or spigot) restricts the flow rate to 5 to 8 gallons per minute. 

You can check the flow rate though more accurately however yourself, and all you need is a 5 gallon bucket and a watch or timer on your phone.

It is easy! All you need is a 1 or 5 gallon bucket and a watch or clock! It takes just a few minutes. Just follow these steps:


 What IS Flow Rate?

Flow rate is defined as simply how many gallons in one minute can be pumped from the well. This is determined generally by the size of your well pump, and to an extent, the capacity of your well.  If your well pump can pump 10 gallons of water in one minute, it has a "10 GPM Flow Rate".
Most homeowners on their private well have a standard single-speed submersible pump with a pressure tank.  If you have a ‘constant pressure’ or variable-speed pump, this test described below is not useful or won’t work for your application.  
Constant pressure systems often have very small pressure tanks, or expansion tank, about the size of a beach ball or less, whereas standard pressure tank systems have (typically blue color) pressure tanks that are larger in size.
With the standard submersible pump systems, the pump typically turns on when the pressure drops to 30 to 40 PSI, and then turns off when the water pressure in the pipes and pressure tank build up to 50 to 60 PSI.   
As the high pressure (Cut-Off) point is reached, water flows into the pressure tank and builds up pressure until high pressure level is reached and the pressure switch turns off the well pump.


Why Measuring Flow Rate is So Important
The flow rate of the water at the pressure tank should be measured because many iron filters and other well water treatment systems require approximately twice the backwash flow rate as the service flow rate. 
If you want to install an iron filter that requires 8 gallons per minute to properly backwash and flush out accumulated iron in the filter, but your well pump can only pump 7 GPM, then you either need to get a smaller iron filter that can backwash at 7 GPM, OR increase the size or output of the well pump by replacing it.  
Another option is to install a small storage tank, so the water flows from the well into a storage tank first, then a booster pump with adequate flow rate can boost up the pressure after the tank.
Simple Steps To Take
Timing how long it takes to fill up a 5 gallon through an outside faucet is an inaccurate method of attaining flow rates as the faucet restricts the flow and gives you a false number.
1.     The proper way to determine it is by counting the gallons drawn down from the pressure tank and the time between cut in and cut off cycle of the well pump.  
2.     To do this, one must allow the well pump first to build up to full pressure. Close the main shut off valve to the house to assure that no water is being used. Then, open a hose bib (AKA ‘spigot’ or ‘faucet’) below the pressure tank, run the water into a 5 gallon bucket, and measure the number of gallons drawn down from the pressure tank until the well pump turns on. 
3.     When the pump turns on, immediately close the hose bib and time the period it takes for the well pump to build back up to pressure that is, between the cut in and cut out.
4.     The formula for determining the flow rate is gallons drawn down that were measured above, divided by the seconds required for recovery, then multiplied by 60:
5.     (Gallons / Seconds) x 60 = Gallons per Minute (GPM) flow.
6.     For example, if 20 gallons are drawn down and it takes 120 seconds to build pressure back up, then: 20 divided by 120 = .166. Therefore, 166 x 60 = 10  gallons per minute flow rate.  So you have 10 GPM flow rate.


Here is some more information on this:

Flow rate is tested when a well is initially drilled and can be tested again to determine if there are problems with the well. It can also be important to determine your flow rate if you are looking to install new pumps or filters on your well water system.

If you have a standard well with a pressure tank and pressure switch, you can use the steps below to determine your well’s flow rate in gallons per minute. This type of system turns on the well pump at one pressure and off at a higher pressure, usually about a 20 PSI difference.

Standard Well with Pressure Tank and Pressure Switch


Pressure Tank and Well Tank Setup Similar to Diagram Above

If you have a variable speed pump, or ‘constant pressure’ type pump, that keeps the pressure in the house the same all the time, then this type of method won’t work. For a constant-pressure type system, consult your well contractor or send us the specifications on your system and we can help you find out what it might be.
Well Diagram For A Typical Household


Well Diagram For A Typical Household

Step By Step Guide

1. Open any hose bib or faucet until well pump turns on.
2. Close hose bib or faucet and let pump fill up pressure tank until well turns off.
3. Using a 1 or 5 gal. bucket, open faucet, collect and measure all water discharged until pump turns on.
You do not have to fill the buckets as fast as you can, simply measure the water.
4. When pump turns on, immediately close faucet and start timing pump cycle in seconds until well shuts off.
5. Divide the number of gallons collected in Step 3 by the number of seconds in Step 4.
6. Multiply the answer from Step 5 by 60.
7. The answer in Step 6 is the average pumping capacity of the pump in gallons per minute (GPM).
See our ON-LINE CALCULATOR to make it faster and easier.

If you need assistance or have any questions about this, email our support staff at [email protected] or call us anytime at 888-600-5426. 

Let us know how it goes!




This post first appeared on Clean Well Water Report, please read the originial post: here

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How Can I Find Out What My Well Pump Flow Rate Is?

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