I have to air eliminator a on my boiler, 1 high near the expansion tank and 1 right at the top of the boiler. The have a small bleeder screw on them so when I open it I hear the air come out and then stops so I know the air is out. Last time I was down there I left them both open so they would purge the self automatically but I went down today and looked at it there was a bit of water on top of the boiler from where the air would bleed out. They have both done that from day 1 so I usually just leave them closed and go down once a week and open them up and close them.
Are they meant to be left open or closed all the time
They're designed to be left open and will purge when air is present. However, it seems like they get deposits built up in the valve seat over time and begin to leak. Then you'll need to either replace the air vent or manually bleed the air like you're doing now.