not sure of the exact size, but most fire & back boiler units require between 65 & 100 sq cm. the preferred method now is to use a core vent directly to outside through the wall of the room the appliance is installed in. a standard 5" core vent provides 100 sq cm so should do it. as there is such a high focus on safety as far as ventilation goes i'd give glow worm a ring to check the exact requirements and seek the advice of a qualified engineer to advise you on suitable ventilation methods and installation.
Ian, September 2009
