Not sure what you mean by that Can you elaborate?IS it possible the pipework has been piped Flow-Flow by mistake or vise-versa?
I simply put a 'T' in the existing flow and return pipes (just after the pump) and ran these through the loft to the extension where I then 'T'd each of the new rads off in turn.
If I 'drain' the system at the end of the new flow pipe (with pump off) the hot water flows through, but there is no circulation when the pump is running.