Debris in domestic heating system

[bharesc]

I have a WB Greenstar 8000 Life feeding an underfloor heating system and domestic hot water through a low loss header. There has been debris, granular in form, circulating around the system the source of which is unknown. A picture of the debris is attached, the maximum particle size is about 3mm and the debris at the top of the picture has been ground to a powder. I have dosed the system with X100 and a biocide. The underfloor heating achieves a dangerous temperature of 65C at times.
I have fitted strainers to (1) the return as it enters the boiler, to (2) the flow to the underfloor heating circuits out of the low loss header and also to (3) it's return before it enters the low loss header. Recent checks showed that (1) was almost blocked, (2) & (3) were clear. Also the flow out of the boiler which descends before rising also contained some debris but not enough to block the 28mm pipe.
The debris can't be being produced in the underfloor circuits as it would be seen in its return strainer. It's not entering the underfloor circuits. The only explanation I can think of is that it's being generated in the boiler, dropping down the low loss header and being fed into the return. I've checked the bottom of the header and this seems clear of debris.
I've tried dissolving the debris in caustic soda, oxalic acid, ferric chloride and vinegar to no avail. Does this indicate it's organic?
Questions.
1. What is the debris?
2. How and where is it being generated?, and
3. What is the cure?

 

[Harry Bloomfield]

There has been debris, granular in form, circulating around the system the source of which is unknown.

There is another similar thread recently, with no conclusion found.

 

[JohnD]

The colour suggest to me a cement base. But acids would attack it, as they would limescale.

Sometimes when plumbers push open pipes through cavities they scrape up rubble. Have you got a system filter, and is the amount of debris reducing as you trap it?

The Spirovent filter, which has a sort of centrifugal trap, is quite good at catching non-magnetic particles.

You might pick up a used one on eBay, as they aren't as good with black sludge they are often replaced with more modern magnetic filters.