faulty diverter valve, classic fault heating cming on with no call, water pre-heat (green light on front panel) causing rads to come on with no call, boiler thinks its heating small amount of water in secondary heat exchanger but because valve is sticking is heating rads instead, turn pre-heat off by turning hot water dial down min, this will stop heating coming on when it shoulnt until valve is renewed
tojo
February 2009