sorry new to this forum and couldnt see where to post own question..
got a main combi 24he..
runs for a little while then goes out.
if i disconnect aps tube and pinch closed its ok and stays on.
the fan is fine,venturi has not melted,the flue is ok.
the condenser is to big to block up on...
the side of the rad you cannot turn by hand is the lockshield.
if you turn them all down(close them then just crack them open half a turn)on the rads that are hot this willcreate more flow to the rest of the system allowing the further rads to heat.
this is called balancing,and isnt always...