water leaking from pressure vessel

[jimmygods]

hi i have only seen that there is water leaking from what i think is a pressure vessel , it is on top of the cylinder in my hot press and is the shape of a ball . there is a small outlet valve at the bottom of it which i thought was for air as it wasnt piped in to any overflow pipe, but everytime i turn on my oil heating water leaks from this and it has destroyed my ceiling in my sitting room with water marks . i have attached a garden hose to it and put it into a bottle for when i turn the heating on . the plumbing in my house is poor to say the least. i would like to know is water meant to come out of this and should it have been piped in to an over flow . or should i get some other system ? any advice is appreciated thanks.