There are many reasons why someone would want to know the flow rate of their pump: they want to order a pump before the installer pulls the current one that has failed, they are trying to size a filter system, they want to upgrade/add an irrigation system, etc..
We regularly have customers that want to know the flow rate of their well pump before purchasing a replacement. The failed pump has been in place for many years and they would like to replace it with something that performs equally. They may have bought the house with the system in place and do not have any documentation that describes the make, model, horsepower, and/or flow rate of the pump that is down the well. If you have a traditional system with a pressure switch and pressure tank, there is a method to help estimate the flow rate of your pump.
The steps below will work for both submersible and above ground jet pumps.
What you need:
- Pressure tank
- Pressure switch
- Timing device (stopwatch or phone)
- Table that shows the drawdown capacity of your pressure tank
Step 1: Make sure that water is not being used in the house. Open a faucet close to the pressure switch. The water will empty from the pressure tank as shown in Figure A. You want to be close enough so that you can hear the sound of the pressure switch contacts opening and closing.
Figure A
Step 2: Listen for the sound of contacts on the pressure switch closing. The pressure switch contacts close completing the circuit that will turn on the pump. This is commonly referred to as the cut on pressure.
Step 3: Start your stopwatch or your phone’s timer when you hear the contacts close. Close the faucet immediately.
Step 4: Listen for the sound of contacts opening on the pressure switch. This is commonly referred to as the cut off pressure.
Step 5: When the contacts open, note the time elapsed. Your tank should be filled with water as shown Figure B.
Figure B
Step 6: Look up the drawdown capacity of your pressure tank. The table in Figure C is for Well-X-Trol brand pressure tanks. For example, if you have your pressure switch set at 40/60 (cut on/cut off) and you have an 81 gallon pressure tank, your drawdown capacity is 21.7 gallons.
Figure C
Step 7: Calculate Flow Rate GPM (gallons per minute)
GPM = Drawdown in gallons from Step 6 Time in minutes from Step 5
Example:
81 gallon pressure tank
40/60 pressure switch setting
21.7 drawdown
4 minutes: time elapsed between pump on and off recorded in Step 5
GPM = 21.7 gallons 4 minutes
= 5.4 gallons per minute
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