Its an air cooled unit, not sure on the model. Three times in the last 5-6 years we have found the same circuit empty upon spring start up. After taking the proper steps each time (pressure check, leak check, standing vacuum) everything checks out fine. Unit runs fine all summer long and magically loses its charge during the winter. The maintenance guy at the school called york direct to try and get some answers. The york tech said that if the unit has barrel heaters they could be heating enough to bleed the charge out the relief valve. My boss thought about that, but didnt think it was very likely. But after the york tech brought it up it seems that it has happened to them before. My question is have any of you guys ran into this before? We are out of ideas. We even pulled a 300 micron vacuum on it yesterday, kinda hard to do with a leak in the system.