there is nothing wrong with the stat if you set it at 20 the boiler can't come on when it reads 20 only when the display changes from 20 to 19 same when it is calling for heat if you set it at 20 it will switch off at 21 not 20 (think about it 19.99 isn't 20 and 20.99 is still 20 )it only works when the display is not 20 the temp overshoot can be caused by the stat being cold part of the room and also by the heat still in the rads when the boiler switches off
brian
March 2011