I have a combi boiler and have had a downstairs radiator replaced for a new one and an extra radiator added after that. All radiators heat up perfectly and after 2 hours at full heat the new radiator (not the extra one) goes cold. When the radiators were put in the whole system was flushed and protected and bleeded. The TRV has been replaced on the radiator in case it was faulty but to no avail.Is it a balancing problem? A radiator problem or a pump problem? Or is there anyway i can prove where the problem lies. Any ideas please, thanks, Clare