my worcester boiler 28i junior greenstar is firing for a few mins only and then switching off . this is happening continuously. the bgas engineer changed the heat exchange, the fan and he said the flue is faulty. another engineer came today he said he reckons there is something wrong with the boiler itself. please advise

ASHA, March 2011

If the brithish gas engineer says the flue is faulty then he should have certified the boiler as 'immediately dangerous' and disconnected the boiler so it is not able to function.
Sounds more like an air lock since he has changed the heat exchanger hence the boiler is going out on thermostat so quickly.
Get on the phone and get british gas to put it right

boiler bill, March 2011