back boiler heating water but not rads?

my back boiler in my kitchin trips out after about five mins when trying to heat rads. It does not trip rcd just its own switch! It works fine when just heating the water. Water is a gravity fed system and dhw is pump fed. The pump sounds to be running and i cant find air in the system. Theres burn marks when i take the cover off of the timer on a metal plate marked combined. The system has been running fine in recent weeks. Gratefull for any help!