In the re planning of our kitchen we have found a vent through the wall ( behind a cupboard and thus of somewhat limited use in the last 10 years or so we guess) Our builder says this can and should be closed off but I have a query about why it would have been there. There is a modern oilfired stanley range cooker which also serves the central heating in this room. The cooker has a chimney flue as you might expect. Should I not retain this vent? It is huge, size of a cat flap. If i should retain it does it have to be so big?