Surprised that the muck possibility has not reared its head before, so I need to clear that up. Fact is there was some rust evident on the sides of the header tank when I first started looking at this and rust in all the circulating water, which would have contained some Fernox, but it would have been there since 2006ish. All cleared out but not considered relevant to flow rates. Before any pump changes, I added 2 pots of cleaner stuff I happened to get from Wicks and got that hot round everywhere by shutting most of the system off in stages. That was drained out. Then a refill with just water, a hot flush as before, and another drain. That was all before I found the capacitor (which happens to be buried on the unseen side in the airing cupboard), and I have an in-circuit capacitance meter from work days. Just for good measure the whole system was drained again to change the pump and be sure I got all the cleaner out. The pump that came out was not anything like or even approaching charlie275's one. Since then I have drained again to change the CH control valve, effectively on a whim, knowing already that the replacement pump was not proving effective on HW circulation. Just to give you a laugh; I was fooled at one stage because a demand from me for CH was not happening and I though I had a control valve or timer fault. Both upstairs and downstairs thermostats had opened on a warm day.
A few fill-in facts, and just to be clear; all controls, pump and valves are in the airing cupboard upstairs. Only the boiler is downstairs with the header tank in the loft:
1. Since the radiator circulation split-off is under the upstairs landing, draining the system involves draining all the downstairs rads in turn which have their own drain outlets. Long job on carpets but all done.
2. No one. not even the likes of RS and Farnell have correct spec capacitors in stock. That is what I considered first. Variously due in July, August, September. There are some far less expensive ones but the spec is inferior and I have no desire for controller output faults or blown fuses a little later. Has there been a run on faulty capacitors?
3. Water comes out at the boiler drain (system low point) at a fair rate considering the hose involved. It does that with or without the CH & HW valves open or shut, thus effectively proving both legs to and from the boiler in the 3/4"pipework.
If a blockage remains, since the fault is common to both circuits, then it would have to be in that 3/4" pipe run. but I suppose there could be a partial airlock. Not a very serious comment.
4. I can do the same sort of thing with each downstairs rad. Close one end off and prove a good drain cock flow from both legs to each rad.
4. With CH on (HW off) and all rad valves open, the upstairs rads get hot eventually. The downstairs ones (longer pipe runs) are just aired and don't get hot with the upstairs ones closed. This is just about how things were at winters end before the pump change.
So., Kidgreen61, fair comment, but no sludge, serious or otherwise. I have done my homework. perhaps I should try and get at the capacitor in the replacement pump. Too many other things on and this is not high priority at this time of year. However I'm not leaving it until it is.
Hope all that clears the sludge/blockage possibility. This is getting longer.