Check that the back boiler(if it is one)hasnt started to soot up causing the lack of heat to the water when the heating is also on.If that is ok if the system is gravity fed the pump will be taking water to the rads when the heating is switched on causing the gravity circs to virtually pack in.Leaving it on for DHW only until you get the water hot enough is the best thing to do for now,but the system may need altering to a fully pumped one.
Tony S, November 2008
