This post is for future end users looking for solutions to this problem (diverter banging/replacement).
Viessmann is a German made product and Germans have very high standards with regards to water purity/quality i.e. VDI 2035. Unfortunately, similar standards are only a recent thing in the UK and the UK industry is far behind.
The boilers are exceptional IF installed properly - This includes a full system power flush before installation - and that includes flushing the radiators individually too. The water needs to be clean - getting 80% out is not enough, it needs to be 98-99%. Most plumbers do a half baked job and this can cause issues. When powerflushing, make sure the plumber sets the diverter valve to mid-position - This means the boiler is not in heating-only or hot water-only, both "heating circuits" are accessible so the water can clean the system properly. This is usually done by setting the boiler into the "filling program" which is setting C.1. to ON in the commissioning assistant (
page 33).
The second component is limescale. These boilers should NOT be filled with tap water, they must be filled with distilled or dimineralised water. The limescale build up and/or sludge can cause the diverter to "stick" hence the banging when switching from heating to hot water. Viessmann do sell rebranded Purotap filters for this:
https://viessmanndirect.co.uk/Catalogue/Water-Treatment/Technical-Water/PUROTAP-micro
The limescale issue is both a blessing and curse. Limescale in general is not good for boilers as it can significantly reduce a boiler's efficiency by creating a thin insulating layer between the water and the heat exchanger.
The other great thing about using demineralised/distilled water is you do not need any inhibitor and the system shouldn't have any corrosion issues so sludge will not be a problem for the duration of the system's lifetime.
If you currently have diverter banging issues, get your system power flushed
thoroughly, then replace the diverter valve and re-fill the system with dimineralised water.
There is a chance the boiler itself still has small amounts of gunk in it and in such cases, it will need to be cleaned out manually by removing the hydraulic block, and the exhanger. Viessmann will not cover this since the installation was done incorrectly and you will have to pocket it yourself.
If the system water is clean the valve will not stick and bang - if it does, that means the water still has impurities.