I have elderly boiler above which seems to have a life of its own. For some reason it ignores the thermostat and fires up the boiler despite the wall thermostat saying it is off. I have replaced wall thermostat to no avail and have also replaced the thermostat PCB in the control box again without any improvement.
Tonight with the wall thermostat set for 18.5 C the boiler went to 19C and continued to fire up despite there being no flame displayed in the wall thermostat a new Honeywell Digital

Any ideas what may be at fault
Thanks

David, December 2009

Thanks Ian will look into your suggestion

David, December 2009

hot water flow switch may be stuck on

Ian, December 2009