I am currently mini flushing my boiler i e : drain boiler into bucket or sink (an absolute ton of gunge and black water filled my sink) then fill up boiler again via refill loop, this needs to be done numerous times I'm up to 7 times as i write this. each time you do it put on hot tap or heating to heat water before draining again. The water is starting to now run clearer. My heating is now back on and water is getting warmer. (Heat exchanger getting clogged up with crap seems to be problem if the above doesn't help you'll need to get someone to clean it out properly or get a new one)
paul kinsella
February 2010