@old_scientist 

That sounds like text book biofilm (I used to work on very high purity water systems for IV use and experienced it several times). If you still have the pipework you cut out, can you swab it out to see if it’s slimy. If it is, then that’s the cause of you colouration and corrosion.

You are better finding someone who knows more than me about glycol, but I believe that concentration and the right mix of additives (including biocides I’d guess) is critical. I’d have thought your installer owns at least part of the remediation. 

@papahuhu 

It's definitely not slimy. It's dry and powdery.

When I swabbed some on a dry cotton bud, I could see extremely fine orange dust in the air in the sunlight streaming in through the window as I dislodged it.

Some pictures, as best my phone camera will manage:

That’s not biofilm. Looks like a very fine particulate rust, but fine enough that I doubt it will do too much harm. If the cleaner doesn’t shift it you might have to live with it, as long as the inhibitor stops it getting worse. 

Thanks @papahuhu, I appreciate your help.

@old_scientist You will probably find that most of it has settled out in the rads or the volumiser. Anywhere where there isn’t laminar flow. 

I had a somewhat similar situation, I had just over 300L system volume but the installer only added enough inhibitor for 100L. I found a little black sludge when taking off a couple of rads for decorating. But I just rinsed them and added the correct amount of sentinel inhibitor. I know that BG has switched from ferrnox to sentinel, as they did it for me as part of service. If you are going to be servicing it in the future, might be worth getting some test strips, but they have to match the inhibitory system you are using. 

Yes, my 50L volumiser uses the two lower ports, so there's probably around 5-10L of dead space in the bottom for any debris to settle. Only niggle is the installers never fitted a drain valve to the bottom so I have no way of fully draining it or flushing any collected debris/sludge out of the bottom.

Top Tip - if you are having a volumiser fitted, make sure a drain valve is fitted to the bottom and an automatic air vent fitted to the top (with a compression fitting isolation valve so you can easily replace the air vent if it fails). Also, pipework in at the bottom and out at the top would make more sense to me, assuming it's a 4 port tank.

Flush and fill all complete.

Octopus kindly gave us an hour of free electricity today, so after heating the DHW to 57C (ASHP and immersion on full whack), we cranked the heating up to 50C LWT to give the cleaner a bit of a run. After the free electricity ended, I turned the heat off but continued to circulate the warm water in the radiators at full flow for another hour.

Full system drain followed by a good flush and another drain. Refilled with 2L of Sentinel X100 inhibitor and bled all the radiators repeatedly. I've circulated the inhibitor for around 30mins and will set the pumps going again this evening to make sure it's well mixed and circulated.

Hopefully that's me done for another year! 

While skim reading a manual for our Vaillant HW tank, I saw they are quite funny about not using glicol on the system, I'd expect similar or even more prescriptive rules for the Arotherm. Not sure if this is the same with your  equipment.

Surely you would be aware of this but just for any future forum readers!