Cleaning a tankless coil

Steve_152

i would try a sparco mixing valve & a flow restrictor on inlet side to coil, use restrictor rated 1/2 gallon smaller than coil, that should make big improvement.

jrc2905

The other day I installed a new tankless coil on a hot water boiler, after the old coil was failing to produce enough hot water. When I removed the old coil I noticed that the outside of the coil was covered with black crud that I was able to wipe away. I aways hear coils fail due to scale buildup on the inside, this coil looks like the problem was the outside of the coil.
I have other customers whose coils are failing and removing them is not a viable option. I am wondering if a boiler cleaner would remove this type of buildup and help restore the performance of the tankless coil.