Viesmann 100 leaking water

Hi,

Just about to have a gas smart meter fitted and as part of his checks, the engineer noticed the boiler has a small leak. Engineer had a quick look for me and said I need to get a central heating engineer out.

I've been burnt before by central heating plumber, so just want to make sure I'm not going to get burnt again.

It's a Viessmann Vitodens 100 WB1C and there seems to be a leak from the grey cap with the small hole on the pic. It look like to be a slow leak, and the hole seems to be there for a reason.

Pressure seems fine - it was about 1 bar and I've topped it to 1.3 bar. Any idea on what could be causing it? I'd like to get it repaired as soon as possible, and would like to make sure that I'm not going to get taken for a ride.

Thanks for any advice.

That’s the automatic air vent dribbling water, some say close them off, but it’s designed to dissipate any trapped air in the system. Shouldn't get burned for that.

Close / screw down the grey cap done no leak