If you are seeing a significant number of bubbles coming out of your return lines into the pool, you probably have an air leak in your filtration system. Possible sources of this problem are low pool water levels, leaks around the strainer lid, leaks in the unions or leaks in the pump seals. For optimum pool pump operation, the source of the leak must be identified and corrected.
Your swimming pool filtration system has two sections: the suction side (from the pool to the pump) and the discharge side (from the pump to the pool). Check the strainer pot of your pump. If you see a significant number of air bubbles moving through the strainer pot, you know that the air leak is somewhere on the suction side of the system plumbing before or at the pump.
Go over to the pool and check the water level. Ideally the pool water level should be at least halfway up the skimmer intake. If the water level is below this level, the skimmer is probably sucking air into the skimmer pipe to the pump. This is a simple problem to resolve. Simply fill the pool to the halfway point of the skimmer. Also check that nothing is holding up the flap of the skimmer opening to the pool. This could also let air into the skimmer suction pipe.
Remove the pool pump strainer lid. These are generally screwed on or are secured with toggle bolts. Pull the O-ring out of its groove. Clean out any debris that might be in the groove. Check the O-ring for any cracks or wear. Sometimes an older O-ring is stretched so that it bulges when you try to replace it in its groove. This is a common source of leaks. When you screw on the lid, the O-ring will bulge out and create a leak. Replace a worn or stretched O-ring. Also, it's a good practice to lubricate the O-ring lightly before replacing the lid. Use a good pool seal lubricant (Teflon or silicon). DO NOT use petroleum jelly.
If these steps have not corrected your air bubbles, and you have an older pool pump, you may have a leak in the motor shaft seal. The picture (left) shows the location of the motor shaft seal in a cutaway version of a Hayward pump. For instructions on replacing a motor shaft seal go to our guide, How To Replace A Motor Shaft Seal.
Another source of air leaks is the motor's drain plug on the suction side of the motor. If this plug is loose, tighten it to stop the leak.
If you still have a significant number of air bubbles coming out of your return lines into your swimming pool, you may have a bigger problem like leaks in your underground lines. Call a pool maintenance professional to help you isolate your problem.