How to Figure GPM Water Flow on an Existing Chiller

A chiller pumps water or another fluid to heat or cool a system.
••• pump image by Aleksander Nordaas from

Technicians calculate a chiller's volumetric flow rate similarly to how they find the flow rate in other pump systems. As with other systems, a chiller's flow rate depends on the chiller's pressure and the system's overall efficiency. This pressure is typically measured in terms of total dynamic head, a figure that considers the fluid's static pressure, the pressure the pump adds and pressure losses due to friction.

    Multiply the fluid's horsepower by 3960. If the chiller works at 25 horsepower: 25 x 3960 = 99,000.

    Multiply your answer by the pump's efficiency. If the pump works at 80 percent efficiency: 99,000 x 0.80 = 79,200.

    Divide your answer by the total dynamic head, measured in feet. If the head equals 130 feet: 79,200 / 130 = 609.2. This answer is the chiller's flow rate, measured in gallons per minute.

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