I=92ve just drained & refilled my CH system (first time I=92ve done it myself) & it went fine. The water that came out was surprisingly clear, with no signs of black sludge. (It was last drained 2 yrs ago & Fernox was added.)
1 small thing I=92ve noticed is that 1 downstairs rad has more of a difference between top & bottom than other rads (eg when I measured it the radiator was about 44 deg C at the top & about 38 at the bottom). I think the rad was also a couple of degrees cooler than some of the others. The radiator bleeds fine.From a google it looks like sludge might be the cause ... except the water that came out was pretty clear. Not sure if I just need to give it a bit longer, or if it=92s maybe something to do with the rad valves needing to be adjusted?
Michael