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.
Flues can be either round or square, horizontal or vertical.
Old style 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.
When a boiler is installed on an outside wall the flue is installed horizontally from the back of the boiler to outside. But when a boiler is installed in a loft or less frequently on an inside wall it will be vertical out of the roof.
They will have a cage if they are lower than 2m from the ground for safety purposes.