you can upgrade to 8.5kw shower no problem,how ever do not need to change fuse size 30 amp rewireable fuse will take it , and if it is an M.C.B it will takeup to 45amps for about 4 hours befoe triping
I have a grant combi oil fired boiler,on which the air vent has started to leak water intermitantly.When this occurs the central heating pressure drops and i have to fill system back up to 0.8 bar.Any thougths please