A necessary adjunct to the boiler will be the chimney, which is required for two purposes: to produce the necessary draught for the proper combustion of fuel and to furnish a means of discharging noxious products of combustion high in the air. In other words, the chimney is the lungs of the building. In countries like France and Germany, where waste is a social or commercial crime, no building may be erected without the height and area of the chimney being passed upon by an official board. There is no more important feature in a house, nor one that will cause greater loss than an improperly built chimney, or effect greater saving than a correctly built one - no matter what kind of heating device or fuel be used. The burning coal must get the correct amount of air it decomposes in order to throw off the proper amount of heat.
The value of a chimney flue depends on its area and height. It is better to have a chimney flue generous in area and height, than to build it too small, for. a flue can be choked down by dampers if the draught be too strong, but if it be too small it is always a failure. Improper draughts often may be corrected by a change of coal; large or coarse coal being employed to strengthen the draught and small coal to retard it.
All chimneys should extend above the highest part of the roof and be topped with a shifting cowl that will turn the outlet away from the adverse air currents and thereby promote better draught. A round chimney is a better form than a square one, and a straight flue better than a tapering one. Most chimneys are built of brick, lined with vitrified flues. In constructing a chimney four feet or more in diameter it is cheaper to build it circular with a straight batter on the outside.
Chimneys of great height are not built uniform in size from top to bottom nor with a uniformly varying thickness of wall. Instead, the wall, heaviest at the base, is reduced by a series of steps as it ascends. Large chimneys are built with an outer stack and an inner tube or core, independent of the outer one, with an air space between.
Many engineers extend the inside core, which is designed for fire safety, only to a height of forty or fifty feet. All chimney flues should be ample in size, should start at a point three or four feet below the smoke-pipe entrance, and should have clean-out doors at the bottom for the removal of dust and soot.
The size of flue required may be calculated from the following table:
Total contents of building in cu. ft.
Aver. of direct radiation in sq. ft.
Size of flues in sq. ft.
Diam of round
. Brick flues 1 inside
10,000 to 20,000
200 to 400
8 1/2 x 8 1/2
25,000 to 50,000
450 to 900
8 1/2 x 13
60,000 to 100,000
1000 to 1600
100,000 to 150,000
1600 to 3000