Boilers regulate water flow temperature, and when the chosen setting is reached they will switch off and after some time will come back on again dependinging on the temperature of the return flow, so either increase the flow temperature and also check that all your rads are fully bleeded or vented i.e. no air trapped in them, and then also ensure that there is no flow restriction in the rad valves, a faulty diverter valev can cause similar symtoms but I wouldn't have thought this would be the case in your situation as you said it comes on for almost 15 minutes, a faulty diverter valve will shut the heatinmg down very rapidly withing a few seconds.
Mike, March 2007
