Help with Glowworm 15HXI Boiler

Posted 7th Jul 2021
Hello all,

So I have an issue with my brothers boiler, it keeps showing a fault code F5 - which means a "Overheat Fault"
The possible causes as listed in the manual are as below, in italics I have put what I have done so far:
Overheat stat operated - I have reset the overheat stat by pressing on the reset button located on the overheat stat - within an hour it the boiler turns off, tries to restart again and shows same fault code F5 - Also I have replaced this component, still no success.
Maximum Temperature met - I reduced the temperature down to the bare minimal.
Check thermistor connections - checked both and all are fine.
Air in system with thermistor at maximum temperature - As above, I reduced the temperature down to bare minimal, also I checked all rads and taps for air, all were clear.
Faulty overheat stat - as above, I have replaced this.
Check pump is wired into the appliance and not the programmer - Checked and all is fine.

I have proceeded with draining the system, and I am going to replace the condense syphon reservoir because when I removed the old one, it was completely filthy and jampacked with dirt. I intend on changing this part tomorrow afternoon, and that's when I will then be able to run the system again after flush and above component is replaced, in hope it will work after that.

Side note: When the boiler does fire for the limited time before the fault shows, pilot ignites, pump turns on. However, one thing that has just clicked in my head, the temperature shown on the digital gauge rapidly increases(10-15seconds) from 20c odd to 78c. Then the fault message shows.

What would your opinion, ideas or advice be on this situation?

PS: I am a competent plumber/electrician and understand risks involved, I make sure to ensure all precautions are in place.
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