We receive a lot of phone calls asking the same question. In most cases, a pool or spa owner has checked all of his or her pool equipment and even had a pool service technician do the same. Even after tightening every visible screw and lubricating all of the obvious seals, the bubbles continue to flow through the return lines. So what might be happening?
In most cases, air is getting into the system at some point that just might not be discernible to most homeowners or even pool service men. For example, there may be a leak on an underground suction line. As the pump draws water from the pool through this line, it also draws air through the leak. Without x-ray vision, homeowners and most pool service companies cannot detect this kind of leak. It takes specialized testing with modern leak detection equipment, something a good leak detection company can perform.
Besides underground leaks that allow air intrusion into the system, there may even be leaks on above ground equipment that cannot be detected without modern leak detection methods.
There is one other possibility that is not leak related. Sometimes the impeller inside the pump becomes so worn that it actually spins too fast. As a result, the impeller can end up spinning fast enough to break apart water molecules and form air bubbles to be sent through the return lines.