Just got in from looking at a problem with a colleagues radiators. She recently had a new Hot water storage vessel fitted. The problem she has now is one of the downstairs radiators heats up ok, but after a short time suddenly goes cold. I have checked a couple of the basics like bleeding and that the lockshield was open. The system is approx 17 years old and never been flushed. But why would that 'one' radiator heat up, but then go suddenly cold? Thanks