whats the boiler make/model?
sounds to me like, whit the cover on, its not getting enough air,
therefore the venturi is not making the aps......with the cover off its taking the air from the room.....
You sure there is no obstruction in the flue, both exaust and outer ring...for combustion air.......
Look at your terminal carefully, could it be recycling the flue gases when the cover is on...? (ie...gases out the centre of the flue being take back in the freah air port)
With it running witht he cover off ok, youd have to be looking at flue outle/intake possiblilities, as its working with cover off you can effectively rule out most of the other devices...
Let me know how you get on.......
T
March 2008