screeching pump

Can somebody advise? A few weeks ago I had to turn off my heating system (pumped, open vented) as the pump was screeching. I assumed that this was because there was no water/air trapped in the system. I was advised to drain the system down and refill. This I've done and have bled the radiators. On turning on the heating one side of the pump gets very hot but the other side is cold. The pump is spinning but no water dripped out when I opened the vent. The motorised valve I've opened to manual although it's only 2 way so I assume the hot water should flow through to the hot tank anyway. Could it be air in the system still? Could the pump not be pumping even though it's spinning? Could the cold feed be blocked somehow even though the feed & expansion tank was backfilling when I refilled the system? Is the boiler blocked?

If the radiators, boiler, etc. filled up OK from the F&E tank when you 'refilled the system', then obviously the cold feed is not blocked.

If the pump is noisy and no water comes out when you open the centre bleed screw, then there's little or no water in it. The pump depends on water for lubrication and cooling and will very quickly die if you run it with no water in it!

You probably have an airlock near the top of the system above the pump (if that's where it's fitted).

Thanks for the reply. With regards to the pump, from what you say it seems more than likely that that too is at fault as it was left on with possibly no water running through it for an hour or so whilst my mate went down the pub (I'd forgotten to tell him that I had a problem with the heating and not to turn it on!). I read somewhere that the pump could still be spinning but not pumping. Is that likely? I thought it was the spinning of the impellers that did the pumping?

IMpellers come off, sludge blocks pumps.... Turn off the pump isolation valves and remove the 4 (or 2) allen screws holding the motor bit to the wet bit. Have a look inside. I've known some pumps run dry for a long time and be ok - did you say your prayers the night before?

If you have 1/4 turn ball valve type isolators they will leak water out.

Thanks ChrisR, I guess being down in Antartica you'll know a fair bit about heating!