I have a conventional in-ground pool and spa with a solar system on a second story roof with a vacuum breaker on roof but below the solar panels.  After using the solar and the pump turns off the weight of the water in the solar return line opens the one way valve (30 ~pounds) at the bottom of the 25 ft solar panels and creates a vacuum which opens the vacuum breker and lets in a lot of air into the lines.  The pump pushes the air out of the system the next day when solar turns on.  The air is pumped out of the system in about 2 minutes.  But during the 2 minutes the pipes water hammer a lot.  I have tried to give added support to the pipes but not good enough.  There is not enough room to rework the piping.  Does anyone know of a good air vent to fix this problem.