Glowworm flexicombi 30cx. Constantly having to reset either flashing code F11 or F25 and raise pressure. Boiler is 8 years old. Pressure valve and expansion valve has been replaced. Once reset it is o.k. But after it shuts off on auto at night, it does not come on in morning - same routine flashing code & then reset.

Elsiemary, October 2018

F11 flow/return NTC thermistor fault

F25 CH temperature difference between flow & return too high during operation

BOTH these faults indictate a circulation problem during CH operation

Boiler is OVERHEATING when CH ON

Why is boiler OVERHEATING when CH ON?

System sludge?

Pump defective?

Excessive air in system?

Poor system layout leading to poor system circulation?

Etc etc

Could be any number of reasons

I'm afraid you will need a gas engineer on site to
INVESTIGATE FAULT
DIAGNOSE FAULT
& REPAIR FAULT

Anon, December 2018