When a non-ventilated ceiling support is used (safer than using a ventilated one) then 4 ventilation grilles must be used to ventilate the boxing in. This stops the heat within the boxing from building up and as an added bonus recovers heat that would otherwise be wasted.
Two grilles must be placed opposite each other at the bottom of the boxing in with the grille vents pointing downwards. Two more must be placed opposite each other at the top of the boxing in. If the boxing in is in a corner of a room then the grilles may be placed on adjacent sides of the boxing in.