The mechanical shaft seal in a spa pump is what keeps water from leaking along the shaft, as the shaft enters through the seal plate. A shaft seal is a two part design, which when compressed, prevents leakage, even though it is not touching the shaft. It can’t touch the shaft, whirling at 3500 RPM, and would quickly burn up, if the shaft were rubbing or touching the seal.
What causes a spa shaft seal to fail? Bad water chemistry or high sanitizer levels can damage the soft rubber and steel parts of a shaft seal. A water hammer effect, caused by suddenly closing a valve while the spa pump is running, can blow out a seal. Using a pool pump shaft seal, made of Buna or another soft rubber will not stand up to high spa temperatures or the degradation caused by spa ozonators. Finally, running a spa pump dry, due to an air lock or low water level can cause a shaft seal to overheat and fail.
Replacing a leaking shaft seal begins with shutting down the power and disassembling the wet end of the pump. Four bolts are removed to open up the wet end and expose the impeller. If your spa pump has an open bracket, where you can see the shaft, hold the shaft stationary while you unthread the impeller from the shaft (counter clockwise). When the impeller is removed, you can pull the seal plate off of the motor.
Remove the old shaft seal halves – the spring half is pulled off of the back of the impeller, and the ring half is pried out of the seal plate with a screwdriver or just your fingers. After removing the old shaft seal, wipe clean the impeller and seal plate, removing any grease or deposits.
With clean hands, press the ring half of the seal into the seal plate. Be careful to keep the ceramic face clean, and press it all the way into the seal plate, as shown below.
Next, slide the spring half over the shaft collar on the back of the impeller. The soft rubber side faces the impeller, and the hard plastic side faces the ceramic seal ring half. It’s very common to install the spring half backwards, so be careful to install it in the right direction.
Now you can reassemble the wet end, bolting the seal plate to the motor, and threading on the impeller. If the motor has a lot of white crusty deposits, or if you notice rust on the motor shaft, clean before reassembly with sandpaper or emory cloth. Lubricate any seal plate o-rings, but don’t lubricate the shaft seal.
Replacing a shaft seal can be accomplished in less than a half hour. If you want to complete the job in half the time, replace the entire wet end assembly for your spa pump.
The wet end assembly includes everything but the motor, and in addition to the new shaft seal, it comes with the impeller and complete impeller housing or volute. In short, everything that is in contact with the water (the wet end of the pump), shown here in the picture.
If you have any questions about buying the correct spa shaft seals, or which spa pump wet end to use – or if you run into some unexpected complications, see Danny’s excellent post on replacing spa shaft seals.