Venting Pressurized Solar Thermal Systems
Since a pressurized system has a solar loop which is completely filled with fluid (the industry standard being a mix of water and propylene glycol), it is very important to get all the air out of the system when you're setting up. This is doubly important because the solar fluid only needs to be checked every 10 or 15 years, so it's going to be in there a long time before you look at it again.