The installation and service manual could be sourced from Vaillant. May have to be a service engineer. Otherwise could photo mine.
How old is your boiler? I found cleaning out the control lines, the water, differential, and divertor valves will help. Lime scale (HARDWATER) doesn't help .
For small cost can photocopy my manual 76 pages?
email me on kaliso17@hotmail.com
Kali
December 2006