The Heating Unit may be faulty.
I have the same System and in Newcastle I would recommend Koziwarm.
Anywhere else, look for a Bosch Worcester specialist. I could same time and costs.
new_home
February 2010
maybe faulty pump, this boiler has separate pumps for ch and hw
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.