When pool water flows through the recirculation system, resistance to that flow is created in the pipes, the valves, the fittings, the pool filter and basically anything in the flow path. That resistance is called Total Dynamic Head, TDH, and it has a large impact on the determining the size of your pool pump and filter. Your pool's TDH was estimated when your pool are first installed. This guide shows you how to accurately measure the TDH of an existing pool system to verify that your pool pump and filter are correctly configured.
REMOVE PUMP - Since the drain ports are too close to the ground to insert the gauges, you will have to take the pump out of the circulation line. This pump was installed with unions on either end so extraction is fairly simple. If your pump does not have unions, see if you can lift it slightly to get enough clearance for turning the gauges. Otherwise, you will have to cut the motor out and replace it later with couplings or add unions for the next time.
DETERMINE SYSTEM FLOW - With your measured TDH and your pump make and model you can now determine your system water flow in Gallons per Minute (GPM). Our pump is a Hayward 1 HP Super II EE pump. The chart at the left is an abbreviated version of the Performance data for this pump. For a 1 HP Super II EE pump and a TDH of 70, our pool system water flow is 60 GPM.
DETERMINE IF GPM IS WITHIN ALLOWABLE LIMITS - The maximum recommended water velocity in swimming pool lines is eight feet per second for main suction lines (before the pump) and 10 feet per second for return lines (after the pump). See local regulations for your area. The table at the left shows the maximum flow in GPM based on pipe size and water velocity. The pipes for this pool are 1 1/2" so the maximim allowable flow on the suction side is 51 GPM; for the discharge side, 63 GPM. Our measured flow of 60 GPM is somewhat higher than the recommended suction side GPM but is within the recommended value for the return side. So this 1 HP pump is on the edge of being too big for this pool. However, if we considered installing the next smaller size motor, 3/4 HP, for a 38 GPM flow rate, it might not provide sufficient flow for the heater and suction cleaner. The better answer would have been to design this pool with 2" piping which is the way most pools are being built today.