Originally Posted by Al Bags
So how else do you explain the change of heat transfer between individual rads so suddenly .. where, in this case .. there is not even any head required?
Is the circulator deciding for itself which rad to heat at what time? This was the clue all along, wasn't it?
Needs a system study, doesn't it?
if the pump is wearing down it will go down the easist route possible - if one circuit has lots of bends and it is struggling to push water round the system then it will take the easist route.
if 4 out of the 7 rads are shut off then the 3 rads will heat easily - but once you open up the other 4 it will struggle to push water round all 7 - thus taking the less restrictive routes back to the return circuit
i admit someone needs to be stood in front of the boiler - but its not rocket science.
if he drains the system down and refills then this should clear the air lock if there is one.