In message , Nick
writes
My central heating is controlled by a combi boiler. Occasionally the
hot water doesn't come on when the hot water tap is turned on. Having
spoken to a plumber he says that the problem a valve in the boiler
getting stuck because of a build up of limescale. The boiler also
€˜buzzes' a bit when the heating is on. He has advised me to descale
the system using descaler fluid.

Can anyone advise me how to descale the system €“ the plumber advised
on draining a radiator and topping it up with the fluid €“ how do I do
this ?

Does it come on for a short period ?
This would indicate limescale

I think you would be wise to post more details to the group in order
that a proper diagnosis might be made

what boiler for example

--
geoff