GB20 dripping water

Hi,

I have been down this road before where I have found water dripping from under the condensing boiler.

It runs down the gap in the base of the unit and down the gas supply pipe and drips on my worktop.

I previously removed the covers to find that the boiler was oozing water from the top left of the unit.

There is a small vessel on the left hand side which has a white screw cap on.

The water seems to be sleeping out of the join below (probably because we live in a hard water area)

The white screw cap sits on a nut, which looks like it has a small hole (perhaps for an Allen key)

This then sits on a cylindrical bit of metal that sits on the small vessel.

I have tightened the nut and so far it has stopped dripping, but I am sure I did this before.

Remove the white cap, and water starts spurting out (I can only assume I am draining the system !)

Previously I had topped up the water but instead of putting 1.5 bar in the expansion tank upstairs, I did something like 2.5

That is when I first to 1.5 bar and not noticed the drip until today.

Does anyone know how to solve it.

Is it possible to remove the little white cap and not drain the system and perhaps fit some prefer tape or sealant round the area.

If anyone can advise I would appreciate it.

The manual does not state what this part is and does not mention it as part of the servicing

Update

I have identified the part that is mounted on the vessel

It is an air vent

I guess it releases trapped air

Maybe it is somehow clogged, or leaking around the seal due to scale ?

Picture of it on the below website

I still need to know how to remove it without draining the system, if there is any maintenance on it etc

http://www.gledhill-spares.net/spare_parts.asp?SP_ID=33

Item is GT454 Manual Air Vent