Hi Guys, My son has just had a his plumbing done about a year ago and one of the air vents seems to be bubbling/leaking up. I have attached some pictures, I can see a total of 3 air vents, flow and return near the boiler hve one each and there is one on the return on the return pipe near the cylinder. does this sound about right or has he guessed the work and stuck one every where? I'm thinking of closing it off if not required... In my house i only see 2. Thanks!