X200 is for scale and 800 for sludge, as a rule kettling is scale so I would try X200 first. It may take several weeks to work but the nice thing is you just leave it in there. If the problem persists use 800 but that needs flushing repeatedly afterwards.
X200 is a good product but often you think it is not working then suddenly realise it's gone quiet
Edit sorry Made a typo with X100 now rectified hope it didn't cause confusion