Vaillant Turbomax plus F22 fault 10 mins after HW demand

Patience gasguru. I am an engineer but boilers are not my speciality. I do understand heat exchangers. What should the temperatures be on the plate heat exchanger? I think we must have been at cross purposes.

Top left is the output of the diverter valve so should be the outlet of the main heat exchanger above the burner.
Top right is to the pump and back up to the main heat exchanger
Bottom left is out to the taps
Bottom right is in from the water main.

With the DHW temp dial cranked up to max and the hot taps fully open what are the temperatures across the primary heat exchanger (the one bone the burner)

To the upper heat exchanger 61 deg C in - 54 deg C out

Below to the hot water 42 deg C out - 20 deg C in

I bought a new differential thermocouple meter.

Here is an update: No changes to the boiler and it still gives an occasional F22 and very occasional F29. Seems like the coming of the cold weather has had some effect and made F22's less likely. F29 is a 'no gas' error. Not sure why that happens but it happens very rarely. There does not seem to be any connection between F22 and F29.
Re the heat exchanger I strongly recommend everyone, pro or not, to watch John Wilson's Technical Corner on YouTube. He has one video 'Plate heat exchangers' in which he clearly explains the significance of the temperatures in and out. Basically the in and out temperatures for both the input and output sides should be substantially different confirming that heat is being transferred. JW says that as the unit becomes blocked the temperatures come together. He really takes the trade to task saying many heat exchangers are removed unnecessarily and he gives other reasons for poor boiler performance.

In fact F22 and F29 are possibly linked and in some cases are burner pressure related...
You'll need to get a Gas-Safe eng round to check the minimum burner pressure and up it a little...whatever the manual says raise it by a mbar say.
The boiler software (especially the earlier models) is too sensitive when monitoring the flow and return temps across the primary heat exchanger versus burner heat input.
If it's not happy it can throw an F22 eg. if the temperature rise is too slow (say burner pressure a little too low) it faults. The F29 failure to ignite also points to the minimum burner pressure being a little low for reliable ignition.
In their arrogant wisdon Vaillant launched the Turbomax Plus with several design faults...including the failure to have a higher ignition burner pressure.
Make sure the manual is followed precisely regarding burner pressures...AFAIR the chamber pressure must also be connected to the gauge on these models (as it has a small effect on measurement).

Thanks Gasguru. I will do that. I have been wondering if there is nothing wrong but the setup.