External wall above boiler vent soaking wet!

[LateLunch]

I have a standard boiler (Potterton Kingfisher) on the ground floor of my house. The vent is situated on the front wall, and a large area of the wall directly above the vent is soaking wet within one hour of running the heating system.
I don't think this has been happening throughout the winter, though perhaps I didn't notice it. Could something have suddenly changed, causing this?
I have noticed elsewhere that the advice is that the control should always be turned to Max - I'm currently on 3; could this make a difference?
Would be grateful for any advice. Thanks

 

[bengasman]

Photo would not go amiss.
Which Kingfisher is it?

 

[LateLunch]

Hi, it's RS50 : the wet wall is directly above the vent, and extends for about a metre up the wall - it's really wet, ie when I put my hand on it, it comes away wet.
Thanks