Controller of underfloor heating control valves "clogged"?

In my perception, it is disputed whether the system is functionally impaired or if it is just the appearance. Otherwise, the solution would be to empty and flush the system.
Do you have a sludge separator in the system? Flush!

One still has a warranty and could at least have the builder of the system perform the flushing and refilling with VE water. Possibly also have a sludge separator installed (if not already in place) and have components made of plain steel replaced with ones made of precious metals or stainless steel.

Do you have any tips on how I should best approach this with the heating engineer? Sure, it is first disputed whether there will be a functional limitation or not, but the heating engineer will almost certainly downplay everything.

Do you have any tips on how I should best approach this with the heating engineer? Sure, for now it is disputed whether there is a functional impairment or not, but the heating engineer will almost certainly downplay everything.

In the sense of a complaint about defects, the sober phrasing of your observation would be the first step. Water discolored, control regulator no longer readable. Something like that. No interpretation or suggestions for solutions. Then wait and see what happens.

If they say "that's normal," ask why that is normal. Announce that you will present the technical justification to a third party if it is implausible.

Thank you, I'll try my luck