No, the air gets purged as you run the system, there isn't any particular purging cycle that has to be done, the air is purged into the pool everytime the system turns on. Didn't mean to jump on you, but ground/pool level mats and homebrew systems are quite different from tacking big panels onto the roof and you did mention the part about purging and vacuum pressure in 2 different threads....

Although the website is a bit dramatic, www.powermat.com (not an endorsement, I did get a quote from him, but didn't get my system from there) has some really good diagrams on how these systems work. I think when it was originally put together, drain back systems were somewhat new. Now, if you go to any of the manufacturer's sites (Heliocol, FAFCo, SolarIndustries) they all show rooftop systems installed with vacuum breakers and valves that allow full system drain back. That is also the reason rooftop systems are installed at a slight angle across the roof, it allows for full drain back (especially crucial for parts of the country that freeze, like New York).

I am just trying to help others jump start their understanding of how roof top solar works. As I said, I did months of study and when I finally thought I understood how the systems were supposed to work, THEN I called contractors. It really helped me figure out the ones that understood the systems and those that were doing it as a sideline.

Just a minor example: because you want the pressure side to drain back, the actuator valve has to be either a special "non-sealing" type, or it has to have a small hole drilled in the valve ball by the installer (right in the instructions for the SolarIndustries system I finally had installed). They don't make the "non-sealing" valves anymore (or they aren't widely available), so it is up to the installer to drill that hole! (Remember, this is right in the instructions for the system from the manufacturer). At least one installer I spoke to had NO idea what I was talking about when I asked about this, ALL of the others knew all about it (needless to say, that one didn't get my business).