Back Combi 105 HE - Fault on Fan or Flue

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Hi all!

I'm having an odd problem with my Back Combi 105 He, and I'm wondering if anyone can offer suggestions.

The problem is that when we are using the shower, the water will sometimes run cold for a minute or so, before going back to warm. This has happened on and off for a few years but has never been bad enough to motivate me to fix it, however. It now seems to be getting worse - it's pretty much every single shower now.

I can recreate the fault reliably. The boiler is in my utility room, next to a sink. If I turn on the hot tap, the boiler will start up; the hot water demand light comes on and the temperature lights start coming on. The hot water keeps flowing until fine. However, if I quickly turn the hot water off and on again within a second or so, then the hot water demand light goes off (despite hot tap still running) and the temp lights start to drop out, and then finally one of the fault LEDs starts blinking, indicating 'Fault with Fan or Flue'.

If I don't do this trick of turning the hot tap off and on again, then I can run the hot tap in the utility room all day without seeing the fault once. However when I run hot water in the shower, upstairs, the fault will happen within a few minutes. This makes me think that the problem is somehow related to the pressure or flow rate of the water: The hot water in the sink next to the boiler comes out like a nuclear fire hose, and never faults, while the upstairs shower is more of a tepid trickle and the fault happens every time.

I'm happy to do some investigatory work if anyone could offer pointers. I do a lot of practical electrical and electronic work in my job, as well as some light mechanical assembly. Though I'm not super keen to play with any gas containing components.

Thanks in advance.

Tom

(NB I recorded a video of the me forcing the fault, but I can't upload as the file is apparently too big (about 40mb). If there's another way to share, and if it would be helpful, I'll happily upload to wherever)
 
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shower dropping hotwater out then its more than likely the water venturi has fell apart and needs changing very common fault which is unrelated to what your getting it to do when turning it on and off quickly
 
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now that is a weird one i would be looking at the APS sticking and boiler thinking that its still made which would stop it firing again till it unsticks .
 
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