I've a Harcol 430 80,000 btu oil fired boiler approx 35 yr old, the boiler heats the water fine and cuts out when temp is achieved - but when I put the central heating on the boiler struggles to maintain its temperature, all radiators heat up ok but the boiler continues to run and fails to reach the temperature to cut out - I have a copper water cylinder in an airing cupboard and its temp drops when the heating is on.
Its an old installation there are no motorised valves and no cylinder stat
