Removing a fireplace to make more space on a flat wall

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We plan to remove a fireplace and gas fire from a flat wall (i.e. no chimney breast). The house is twelve years old and the gas fire fumes must be exhausted by flue I guess. There is a mysterious vent halfway up the same wall to the right of the fireplace, in the lounge. I intend to block off the fireplace as if it was never there. But, do building regulations require me to put in a vent or anything where the fireplace once stood? Please advise!