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.
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.
REVIEW ISSUES  - 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.
REVIEW ISSUES  - 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
REVIEW ISSUES  - 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.
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.
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.
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.
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.
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.
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.