Vaillant turbomax Plus 824E (approx 7years old)

[Coolhelp]

Any advice much appreciated.
Unlike other problems noted on the site about this boiler, this one produces DHW fine but will after a day or so will cut out the central heating and show F22. After switching off for awhile and back on usually works fine for a day or so(Both heating and dhw) then back to F22. Have thoroughly flushed with cleaner and water is clear no sludge, changed diverter valve, but same problem.
The pump works fine when running domestic hot water for ages, I have noted when it has gone to F22 and you try to restart it without waiting it attempts 3 times, almost as if the pump or something is trying but then cuts out. Pressure in system is fine 1.5 no leaks. Could replace pump or could the APS be better bet first?

 

[Agile]

I dont remember fault codes as I dont want to risk mistakes.

Is it temperature differential or ignition failure?

If temp differential then it could be a lack of flow caused by TRVs all closing. That can still happen even if you have one rad without a TRV as the boiler is very sensitive to flow rate.

Either fault could be caused by an incorrectly set up gas valve but that would need a competent CORGI to check the min/max.

Tony