The flue is the part that takes the burnt gasses produced by the boiler to the outside, just like a chimney would. On new boilers they are assisted by a fan, old boilers can have open flues and rely on a draught like a gas fire would to remove the spent gasses.
Old style flues were either round or square. Square flues are normally larger as there is no fan to aid the removal of gas. All new boilers now have the standard round shape offering the least amount of resistance for the gasses to escape.< back