We have a gravity fed ch system approx 15 years old.w Three rads on a loop downstairs have suddenly stopped working whilst all upstairs and another rad downstairs (on a separate loop) are working. I have checked the stop cock in the loft which is working. I have checked downstairs rads to see if they need bleeding and they are all full. I have opened up a downstairs drain plug and run water through till it went hot but it didn't circulate beyond drain valve, it goes into a rad after drain valve so I guess there could be a blockage in ad or trv. The expansion tank in loft is full of hot water but I'm guessing this is because the upstairs water is overheating. Pump is noisy but it does sound as though water is flowing, I have also turned the bleed screw to ensure there is no air in pump, and I have bled upstairs rads to ensure no airlock. I'm guessing it is either a blocked rad or a failed pump, my question is, how do you know for sure it is pump before replacing? Likewise with rads, is there an easy way of ruling these out? Is there anything else I should try such as flushing?