The F5 fault means the overheat stat has tripped out. To reset the overheat first you need to remove the front cover 2 torx screws underneath. Then remove the main metal cover another 2 torx screws about 1/3 of the way up on both sides. Then on the top left hand corner attached to a copper pipe is the overheat stat it has 2 wires connected to it and a little red reset button sticking out press this down to reset but make sure power is off as the wires have 240v across. If the overheat stat continues to trip there could be further problems poor circulation or possible pump failure.
SE gas man, February 2009
