F5 is an overheat fault, due to maximum temperature being exceeded
Check both the overheat stat and ch thermisters
Possible air in the system, make sure system pressure is 1-1.5 bar and all air has been bled from the system
Check pump is working correctly and does over run when boiler reaches temp.
If you want the manual email me, tony@oilburnerservices.com
T, April 2008
