Some fireboxes were equipped with a so-called combustion chamber which placed additional space between the fire and the rear flue sheet.

The "front flue sheet" is in the front of the boiler and at the rear of the smokebox.

The sheets on the left and right are called "side sheets" while the sheet in the front of the firebox is the flue sheet.

This results in the draft being forced through the fire bed in the thinner areas towards the flue sheet, which usually is hindered by the lack of draft between the grates and the arch brick.

Firing while the engine is working hard is done with a large "heel" pattern, generally with as little coal on the flue sheet as possible, and gradually sloping the fire bed towards the door sheet to the height or higher than the firedoors.

Firetubes serve the additional purpose of staying the flat tube (flue) sheets (front and rear) so that only the top of the front flue sheet and the bottom of the rear flue sheet must be separately braced.