How to Measure Flow Rate in Gallons Per Minute/GPM

For an Open Loop (well water)

You can use a flow meter measuring device. If you don't have one, then you need a 5-gallon bucket (always check the bucket to make sure it holds 5 gallons, because there are a lot of 4 and 4.5 gallon buckets out there) and a watch or clock with a second hand.

  1. Count how many seconds it takes to fill the bucket with the water leaving the geothermal heat pump's water coil.

  2. Divide 5 (gallons) by the number of seconds it took to fill the bucket, and then multiply by 60 (seconds). This gives you the number of gallons of water per minute going through your geothermal heat pump's water coil.

For example: If you measure 36 seconds to fill the 5-gallon bucket, your equation is: 5 ÷ 36 = 0.14. Next, 0.14 x 60 = 8.4 gallons per minute.

For a Closed Loop (vertical bore loop, horizontal loop, or pond loop)

If you do not have a flow meter measuring device installed in the closed loop, then you must measure the gallons per minute by measuring the pressure drop across the geothermal heat pump's water coil heat exchanger. To be able to do this there must be P/T ports (pressure and temperature access port) installed into the earth loop piping at the geothermal heat pump's water coil.

  1. Measure the pressures at each P/T port with an accurate pressure gauge. The difference between the two measurements is the pressure drop in PSI through the water coil.

  2. If there is antifreeze in your loop, multiply the pressure drop by the antifreeze correction number. This antifreeze correction number chart is found in your installation manual. For example, if you have a 15 percent solution of methanol you would multiply your number by 1.16. If you have a 25 percent solution of propylene glycol you would multiply your pressure drop by 1.36.

  3. Using your geothermal heat pump manufacturer's water coil pressure drop chart, you can convert your (corrected) PSI measurement into gallons per minute.

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