The 12 year old Potterton gas boiler in the loft remains 'on' (heating water and radiators) despite the livingroom thermostat being turned to zero and the airing cupboard (acl lifestyle) controller switching the heating and water off. Water temperature and radiators become extremely hot (far more than in normal daily operation). This is intermittant but appears to coincide with the heating and water being 'on' at the same time while the thermostat is demanding more than 22 degrees. The only way to stop this is to turn off the electric switch to the boiler.