As the gentleman said, check the valves are open and the ballvalve is operating freely in the feed tank (loft?)
If they are oK then you may presume it is airlocked. Take a piece of hose and attach a hot tap to a cold tap (any where in the house that is convenient eg. Kitchen sink, Bath or basin) and turn them both on (hot first) for 2 minutes. You may need to hold the hose onto the taps. This should force the air out of the hot pipe. Turn off the taps and remove the hose and then turn on the hot. It will probably gurgle and splutter which is normal. Then it should run fine. If not, repeat again for a further minute or so..
ps. important to open only the pair of taps where the hose is connected to maintain pressure when flushing the air.
Bucket head, December 2010
