Chimney venting requirements after fireplace removal

[Moneypit42]

I’m removing a fireplace & understand the chimney should be vented by a grille. I’ve also heard of chimney balloons to stop draughts.
Is the venting mandatory? Would the house structure be compromised by fitting a balloon and blocking the flue?
Many thanks.

 

[joe-90]

Either block it completely or vent it totally. In between those two poles you get condensation.