How To Downsize a Pool Pump Impeller


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If your pump is cavitating and significant bubbles are seen at the pool return ports, your pool pump HP may be too high for your pool system. Your pump may be trying to pull too much water through your suction piping than the system can handle. For example your pump is set up for 2" piping and your system has only 1 1/2" piping. You might need a smaller pump. One way to accomplish this reduction without having to buy a new motor, is to downsize you pump's impeller. This guide shows you how to reduce the HP of a pump by reducing the size of its impeller. Although this guide is based on replacing a Hayward motor impeller, many of the step are applicable to other makes of motors.

Step by Step


Step 1

REVIEW IMPELLER OPERATION - For a quick introduction, this picture shows the operation of the impeller. Water enters the center of the impeller. As the impeller is spun by the motor, a series of blades, like a fan, pushes the water to the outside edges of the impeller where it exits through slots to be collected by the diffuser. For an expanded explanation of how a pump operates click How A Pool Pump Works.

Step 2

REVIEW ISSUES [1] - First, understand that a motor's HP can only be reduced. You can only downsize a motor by substituting a smaller impeller. You cannot increase a motor's HP by substituting a larger impeller. The motor would not take the extra torque generated with a larger impeller and will burn out.

Step 3

REVIEW ISSUES [2] - Second, it's easy to pick the wrong impeller. You need to know your pump motor's HP and Service Factor (SF) which should be printed on the motor's label. A motor with a low SF, around 1.0 to 1.2, is referred to as an uprated motor. A motor with a high SF is called a full rated motor. In this guide we are reducing the impeller for a Hayward Energy Efficient (EE) Super II Pump. According to its motor label, its HP is 1 and its SF is 1.5 so it is a FULL rated motor 1 HP motor. We want to change the impeller for a FULL rated 1 HP motor to an impeller for a FULL 3/4 HP rated motor. So according to the Hayward Super II Pump parts list, Key #8, we will be replacing part # 5110-10J with part # 5110-10H

Step 4

REVIEW ISSUES [3] - Third, you may also have to replace the motor's diffuser. See Hayward Super II Pump parts list, Key #6. In our case, the diffuser is the same for FULL rated motors with HP from 1/2 to 1 so we can use the existing diffuser. On the other hand, if we were reducing the impeller size from a 1-1/2 HP motor, we would have had to also use the smaller diffuser for a 1 HP FULL rated motor.

Step 5

BUY PARTS- Buy your replacement parts ahead of time so they are on hand. These parts will include the smaller impeller, a smaller diffuser (if required), and gasket lubrication.

Step 6

TURN OFF PUMP - The first step in preparing to remove your pump's impeller is to turn off all power to the motor. For maximum safety turn off the power at the circuit breaker to the motor - usually located next to the timer box.

Step 7

RELIEVE PRESSURE - Next you will have to relieve the pressure that builds up in the system during operation. To relieve pressure turn the relief valve on top of the filter counter clockwise. Water will spray out initially as you see the pressure on the pressure gauge go to 0 psi.

Step 8

UNBOLT PUMP MOTOR - Now you will want to remove the motor pool pump assembly from the wet end housing. Remove the six 9/16" bolts and washers that hold the motor pool pump assembly to the housing. Note: some models have four bolts.

Step 9

REMOVE MOTOR - Slide the motor assembly out of the pool pump housing. After the assembly is out, you should see the diffuser and diffuser gasket on the end of the assembly. If you do not see the diffuser or its gasket, check inside the housing.

Step 10

REMOVE DIFFUSER - Pull the diffuser off the assembly. It should snap off easily and expose the impeller.

Step 11

REMOVE IMPELLER RING - Lift the impeller ring off the impeller. Note how the ring was placed on the impeller with the broader side up. This is marked on this ring but may not be marked on all rings. Note: Some motors do not have this impeller ring so don't panic if you can't find one.

Step 12

REMOVE IMPELLER - To remove the impeller you will twist the impeller off in a counter clockwise motion. However, the impeller cannot be freed from the motor shaft without first securing the motor shaft.

Step 13

REMOVE BACK COVER - Move to the other end of the motor shaft in the back of the motor assembly. Take off the two screws that hold the back cover onto the assembly.

Step 14

SHORT OUT CAPACITOR - You will see the motor wiring and capacitor. To gain access to the end of the motor shaft you will have to remove the capacitor. Capacitors store power. To avoid a possible shock, lay a screwdriver across the two leads to short out the capacitor. Wear protective eye glasses and cover the capacitor with a cloth in case the capacitor bursts.

Step 15

MOVE CAPACITOR - Unscrew the single screw that hold the capacitor in place and push it out of the way. No need to disconnect the capacitor from its leads.

Step 16

SECURE SHAFT - Place a 7/16" wrench on the flat end of the motor shaft to secure the shaft from rotating. Note: Securing the motor shaft on other non-Hayward type pumps varies. Some pumps provide a slot on the end of the shaft to secure with a screwdriver. This slot is often accessible by simply taking off the end cap of the motor.

Step 17

TWIST OFF IMPELLER - At the impeller end, twist off the impeller counter clockwise. You may have to use a strap wrench to gently free up the impeller.

Step 18

VIEW SHAFT SEAL - With the impeller off you will see the motor shaft seal. One half on the impeller and the other half in the seal plate on the shaft. You will want to remove the shaft seal half that is on the old impeller to put it on the new impeller

Step 19

REMOVE IMPELLER SHAFT SEAL HALF - Grab the seal on the impeller and twist it off. Make a note of how the seal is positioned on the impeller so you know how to replace the seal on the new impeller. It is possible to place this seal upside down. Note: You may have to also have to remove the metal collar if the new shaft seal also has one. Several people have written in saying that the new seal won't fit because they were trying to place the new collar on top of the old one. DO NOT touch the bearing (top) surface of the seal. It can corrode the surface.

Step 20

PLACE IMPELLER SHAFT SEAL HALF ON NEW IMPELLER - Push the half of the seal that you just removed onto the new smaller impeller. Make sure the new seal on in the same position noted above. Again DO NOT touch the bearing surface of the seal.

Step 21

SCREW ON IMPELLER - With the wrench securing the back end of the motor shaft, screw the impeller back onto the shaft (clockwise). Hand tight is sufficient.

Step 22

REPLACE CAPACITOR - Replace the capacitor on the backside of the assembly. Make sure the plastic strip is in place between the clip and capacitor so that the capacitor does not short out.

Step 23

REPLACE BACK COVER - Replace electrical cover and secure with two screws.

Step 24

REPLACE IMPELLER RING - Place the impeller ring onto the impeller with the wider base up. The correct side is labeled on this pump's ring.

Step 25

SNAP ON DIFFUSER - Snap on the diffuser - no screws are required. Ensure that the part of the diffuser labeled "TOP" is aligned with the "TOP" labeled on the seal plate.

Step 26

LUBRICATE DIFFUSER GASKET - Inspect the diffuser gasket for wear. Replace if required. Put a thin coat of lubrication on the gasket.

Step 27

LUBRICATE HOUSING GASKET - Lubricate the housing gasket with a thin coat of silicon lubricant.

Step 28

REPLACE MOTOR ASSEMBLY - Clean the sealing edge of the seal plate and slip the motor assembly back into the housing.

Step 29

REBOLT MOTOR - Replace the six bolts evenly. If bolts were numbered 1 through 6, tighten bolts in this sequence -1, 3, 5, 2, 4, 6 - to ensure that the motor is set properly inside the housing. Loosely tighten bolts in this sequence, then go back and tighten them down again in this sequence. If pump has 4 bolts, tighten bolts in this sequence - 1, 3, 2, 4.

Step 30

PRIME MOTOR - Prime pump by removing strainer lid and filling strainer with water. Replace lid.

Step 31

RESET PUMP CIRCUIT BREAKER - Reset circuit breakers and start pump.

Step 32

CLOSE RELIEF VALVE - Close the relief valve on top of the filter, after the air is purged and water starts to spray out of the valve.


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InyoPools Product Specialist  Posted: 6/9/2017 

Alex - When you downsize a pump's impeller, you are reducing the amount of water that the pump has to move, which reduces the work it has to perform, which should equate to a corresponding reduction in operating costs.

 Posted: 6/7/2017 

Hi, thanks for your informative guides. Could you tell me that if I downsize the capacity of my pump, would the motor use less power? I ask this as I've recently replaced my 1.5hp pump (Sachi winner 150m) which is correctly sized for the installation, but have since found a pipe restriction which I could rectify and therefore reduce the dynamic head. If successful it would make my new pump oversized, I can fit the impeller and diffuser off the smaller model of pump (winner 100m) but would it actually save on any running costs of operated for the same length of time per day?