You can send water sample away to be analysed ,
Do vaillant carry out water sampling to prove they are right when they say not flushed properly ???
Seen problems in past with too much inhibitor where it caused bubbles and frothing up in the system but not with any of the modern stuff
I think ive seen a small kit you can get in the merchant to test if yourself you know mate.
can you tell me what to ask for please?