"Julian" wrote in message
...
It's a grundfos selectric.

Can't see any screw except the big one in the middle which I have removed
and replaced to no avail.

It spurted out a little water and the pump rotor is visible. Seemed to
be
spinning happily.

Jules

With my pump there is a big black plastic screw in the middle of the cover.
If I unscrew this a few turns then jiggle it water comes out.
This effectively bleeds the pump.
Had to do this last time I drained and refilled because of symptoms like
yours - pump running but no heat to radiators.
After some jiggling and spurting (plumbing can be fun) the note of the pump
changed, there was a 'shoosh' noise, and everything started working again.
If your setup is like mine (pump in airing cupboard by hot water tank,
higher than highest radiator) then you probably got into the situation where
the upstairs radiators had water but the pump was running on air.
This can cause an airlock in the feed down from the header tank to the
system, and/or in the pump.

(1) Try jiggling that central screw a bit more - from what you said you
didn't get a lot of water out so you may still have mainly air in the pump.
If the pump was full of water and pumping at presssure I would expect water
to come out quite forcefully. Also, if you have a speed switch, try
increasing the speed of the pump.

(2) Try to clear out the feed from your header tank to the system. You can
do this from the top or the bottom.

(i) from the top:
tie up the stop cock and then drain the header tank using whatever drain you
have at the bottom of the system.
If your header tank will not drain because of an airlock or blockage, then
siphon it out into the bath using a hosepipe.
This also cleans any crud out of the header tank.
Disconnect the feed pipe from the bottom of the tank, and connect a hose
pipe to it with a jubilee clip.
Other end of the hosepipe on a mains cold tap. This will pressurise the
system and then you can bleed all the radiators and hopefully clear all the
air out.
This is especially effective when gunge has been sucked out of the header
tank into the pipework because you let it empty.

(ii) from the bottom:
attach a hose pipe between a cold mains tap and your drain tap at the bottom
of the central heating.
Open the drain tap and turn on the cold tap.
This will pressurise your system and should drive water up the filler pipe
to the header tank, then out the overflow (check carefully first that your
overflow is working).
This should shift any air out of the top end of the system above the level
of the upstairs radiators.

Do this slowly and carefully!
Too much oomph with the cold mains, or a poorly tightened jubilee clip, can
rain on your parade.

I would be mildly surprised if you boiler had overheated because the
thermostat should shut it off as soon as it is up to temperature, and you
haven't been running it dry as far as I can tell because your upstairs
radiators were still warm, therfore at least partly full of water.

My (virtual) money is still on an airlock in the pump.

Cheers

Dave R