Thanx captain_pugwash for the info. I have a sneaking suspicion my CH is full of cr@p. When bleeding the rads, the water is pitch black, a couple of them are cold at the bottom and the header tank above the thermal store is sludgy when I checked earlier.
Think I'll drain and treat similar to...
My CH system is driving me potty! :( It is a Thermal Store with standard condensing boiler supplying CH and HW to a detached 3-bed house with 6 single and 2 double radiators.
When the central heating/hot water turns on the whole system, including the CH pump, is very quiet to start with...