I tend to use one shot drain cleaner or spirits of salt. I find they'll inevitably block up again. The crud/sludge comes in from the system and primary heat ex.
Fx2 is suitable for this process and is not a risk for stainless steel. It is a dilution of Phosphoric acid I believe. If you are interested Remco van Triest runs a forum for water treatment science. All the answers are there.
