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How to Determine GPM in Hot Water Coils

Hot water coils are used for a variety of purposes, primarily in air-heating applications. The gallons per minute, or GPM, is a useful calculation to know. It represents how much water passes through the coil every minute, and is important for determining the coil's efficiency and knowing what kind of coil to purchase in order to replace a hot water coil. GPM can be easily calculated if you know a little about the coil.

Hot water coils are often used in heating systems.

Step 1

Determine the BUTH of your coil, or British thermal units per hour (BTU/hr).  This is the amount of energy that is either transferred to or from the fluid in the coilThis information is usually provided by the manufacturer of the unit.

Step 2

Calculate the leaving water temperature, or LWT, and entering water temperature, or EWT.  This can be measured using the coil itself. The entering water temperature is measured from the coil marked "water in" and the leaving water temperature is measured from the coil marked "water out" If you know the temperature difference (TD), this can also be used in your final equation. 

Step 3

Put your numbers into this simple equation: GPM = Q / 500 * (LWT - EWT).  So, subtract the EWT from the LWT, or use the TD, which is the same number. Multiply by 500.  Q is your BUTH. Divide Q by the result of your earlier calculation.  This is the GPM.

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