Baxi Back Boiler Room Vent?

I have just been told that the vent in my room where the Baxi is is too small. It apparently needs 90 cm2 of vent through to the outside. I have bought a 4" (98 cm2) plastic tube and two Gas vent grilles. Is there anything special I need to be aware of when installing it?

Help would be appreciated as it is very cold and my heating is off.

Stevie