"Colin Wilson" o.uk wrote
in message ...
If your system is open vented then check the feed & expansion tank
(probably
in the loft). The float valve may have stuck causing the tank to run
dry, a
few yanks up and down usually frees it and gets the water running again.
Alternatively if the tank is full then check that the cold feed is not
blocked where it tees into the system.

Thanks for that - we've already drained and refilled it a few times
recently while decorating, and everything seems to work as normal
though :-}

It certainly seems like an air problem then if it has been drained &
refilled several times.

You should be able to vent most of the air through the pump vent screw.

Loosen the screw and run the boiler. Put a container or piece of rag under
the pump to catch the drips.