I check the rads for cold spots, if I find anything obvious I powerflush, if not a simple normal flush will do the job, though I tend to leave the cleaner in for a couple of weeks if possible. I have also found on more than one occasion that the conversion from open vent to sealed system was carried out without cleaning the system. Finding a sealed system does not mean you find a cleaned system. I guess that at least 50 percent of breakdowns and replacements I do is due to corrosion and limescale problems. Pumps wear out, valves get stuck and heatexchangers block up, the list goes on. Must admit though that in some cases the trick with mains pressure can do the trick as well if it is only sludge. If there is a lot of limescale, it will not come anywhere near a proper powerflush with fx2 at 50 degrees.
