Baxi 105E working but not right

[macsco]

Hello, my Baxi 105E has a problem with the hot water side.
From cold, I turn on the hot tap and everthing works as it should.
But if the water has been on, and say lights 70 or 80 are lit, then when the hot tap is turned on everything starts, call light -fan- pump but the burner does not light.
Instead the lights start to go out 80 then 70 then 60 etc. (as the flow is cooling the sensor - I guess) then 50 starts flashing on its own. It does this for 5-30 seconds (variable) then the boiler lights and the hot water is back on. (and stays on as long as required).
In the last month new pcb (old one died), new heating and DHW sensors, new overheat thermostat, new 3 way valve (old one started leaking).
Fault light suggests Fan or Flue.
Yet from cold is fine? I was thinking more flow rate issue?
This problem was the same with the old pcb as it is now, and I suspect it is the reason the old pcb gave up.
Anyone come accross this? Any ideas?
Thanks in advance
Macsco

 

[Agile]

Whose decision was it to replace all those parts?

Normally a boiler has one fault and just one part is required to repair.

If the light shows fan or flue fault then we dont advise on those faults on an open forum because they are gas related.

The advice is then to call a competent gas registered engineer.

Tony

 

[macsco]

Hi Tony
The parts were changed as they failed, (and not in an attempt to solve the problem) that is the pcb failed first, then within days a leak developed on the 3 way valve. The sensors and thermostat were changed ad hoc, but at £12 worth of bits, seemed sensible.
If its the fan or flue, then thats what it is. But why would it fire perfectly from cold?
Mac
20 years Corgi, now retired.

NOW SOLVED
It was the hot water heat exchanger, flushed it, and now working OK.