how can i fix my vaillant combi boiler?

my vaillant vcw boiler has a leak on the top of a black box located on the left of the boiler does anyone know how to fix this is it just a seal and what is this black box type part called. thanx.
mel, March 2005


done
Replacement Domestic Hot Water Vessel required.
Not repairable.

R, July 2005
link Click here to see other fixes for Vaillant.