Leaking from the top red valve?

[StuW]

Can somebody tell me what this part is as it's leaking from the red cap. Is it easy to replace?

 

[ShaunCorbs]

Auto air vent needs replacing

 

[SJB060685]

Common problem with those AAV's. Needs replacing.

 

[Last Plumber]

Replace it.
Personally, I would replace it with a manual type and get your system water checked for quality.