What do you mean by closing hot water circuit? Do you mean only central heating on? Also I presume that the boiler is working fine if there is no problem at all with hot water, therefore what do you mean when you say that the boiler is ok?
I have a very old Ideal E type boiler. Last month the central heating radiators were only getting lukewarm when I turned the heating on the radiators remained lukewarm and the hot water stopped getting very hot. The hot water without the heating is working perfectly fine.
I had a gas...