Or perhaps the DHW microswitch is stuck in the 'demand' position. To check see if the burners are modulated by turning the hot water temperature setting dial up and down, also if microswitch is faulty CH will stay on when in 'summer' mode. Hard to say more without the model.
c
October 2008
your boiler main thermostat is knackered causing the boiler overheat thermostat to operate.there are also other reasons which could cause this ie bypass radiator shut off/boiler bypass closed/ corrosion in main heat exchanger and radiators
stevie w
October 2008
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.