Hi Guys,
Let jump in here,
I have ran into this before, It is calcium build up. Some of the water we use is really bad
A lot of people never use a filter, especially week-enders.
Sediment settles in the bottom of the coils most times, build up and slowly plug coils,also cause Tempering valves to seize. ( another reason to exercize your tempering valve,) (John I am supprise your valve is still working)
I have found sediment plugging the manifold systems ( I have the Manifold system for display)