Size of flues (other than balanced flues and flues designed to discharge through or adjacent to walls)

5.5 A flue should be sized to suit the intended appliance such that it ensures an adequate discharge velocity to prevent flow reversal problems but does not impose excessive flow resistance. 5.6 The flue pipe should be the same size as the appliance flue outlet and the flue should be the same cross-sectional area as the appliance flue outlet unless the appliance manufacturer specifies otherwise. 5.7 The flue in a masonry or flue block chimney should be the same size as the appliance flue outlet or of a larger size that should allow for the later insertion of a suitable flexible flue liner matching the appliance to be installed.