Measurement of Total Dynamic Head With Gauges

This guide explains you how to accurately measure the TDH of an existing pool system to verify that your pool pump and filter are correctly configured. 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 determining the size of your pool pump and filter. Your pool's TDH was estimated when your pool was first installed. 

1. You will need to purchase two gauges: a pressure gauge to measure pressure in pounds per square inch (PSI), and a vacuum gauge to measure suction in inches of mercury (inches Hg). Both should have the standard 1/4" threaded end.

2. These two gauges will be inserted into the two drain plug port at the base of your pump. The PSI gauge will be inserted on the pressure side of the pump usually under the impeller. The Vacuum gauge will be inserted on the suction side of the pump usually below the Strainer Basket.

3. Before inserting these gauges be sure to TURN OFF ALL POWER TO THE POOL PUMP at the breaker box.

4. RELIEVE THE PRESSURE in the system. To relieve pressure, screw the relief valve on your filter counter-clockwise. The reading on the pressure valve should drop to zero.

5. 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.

6. REMOVE THE SIDE PLUG AND THREAD THE PSI GAUGE on the pressure side of the pump under the impeller. Gently tighten with an open end wrench.

7. REMOVE THE FRONT PLUG AND THREAD THE VACUUM GAUGE on the suction side of the pump under the basket. Gently tighten with an open end wrench.

8. REPLACE PUMP - Reinsert the pump. If you have unions, wrap 3 layers of Teflon tape around the threads before screwing them together.

9. FILL THE STRAINER BASKET with water for a couple of minutes to reprime the pool pump and water lines.

10. CLOSE THE COVER ON THE STRAINER BASKET. Use hand pressure to screw the cover tight.

11. TURN THE POWER ON to the pool pump.

12. CLOSE THE RELIEF VALVE - close the relief valve on the filter when water starts to spray out.

13. READ GAUGES - Record the PSI and In Hg from the two gauges.

14. CALCULATE TDH - Multiple the Vacuum gauge reading x 1.13. Multiply the Pressure gauge reading x 2.31. Add the two products together to determine TDH for your system. Our readings were 15 and 23 respectively So (15 x 1.13) + (23 x 2.31) = 17 + 53 or 70 TDH.

15. 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.

16. 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 maximum 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.

If you have any other questions about pool and spa products please do let us know - we are here to help!

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