This fault according to Vaillant Instructions and Servicing manual is a Heating Flow NTC Thermistor Fault or Heating Return NTC short circuit
Cause of F11
- There Is A Short Circuit On The Heating Return NTC Thermistor (Should Read 12Kohms @ 20C)
- Wiring Harness Fault Or Connection Error Between The Heating Return NTC Thermistor And The Main PCB.
Solutions to repair this fault
Check the NTC sensor. Check the wiring using a continuity tester and resistance tester if necessary. Replace NTC, harness or PCB as necessary dependent on checks.