Hi folks, got what I think is called a system boiler. It fired up for hot water and heating but then cut out within seconds. Changed the pump ( a lot of water came out even though I had tightened both isolation valves above and below pump ) All is good with heating and hot water now. Only problem is before the initial pump problem all radiators worked. After the new pump was fitted all the radiators work apart from two downstairs (the other four downstairs ones work). One pipe leading to the radiators is really hot but the other is cold. I`ve bled them and there appears to be no air. One radiator is old but the other one is not all that old.Any help would be great, cheers.