Air vent screw help !!!

[Gibbo1471]

Morning all I have main boiler could you tell me from the photo is the black screw suppose to be open or tightly closed don’t know what is best

 

[ShaunCorbs]

Closed unless you drain and refill the system

Tighten it until it stops don’t use much force as it’s plastic

 

[Roy_66]

When you open it, as Shaun says, just a turn or so, listen carefully to it. Close very gently when the warbling stops. Don't rush. You may well get a few drops of fluid. They can dribble so close when you are done. There is a float inside operating a valve, so how well the valve seats depends on the pressure in the system. It's an AAV.

Cheers, Roy (Amateur plumber)