This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The different heating surfaces may be described as follows: Fire pot surface; surfaces acted upon by direct rays of heat from the fire, such as the dome or combustion chamber; gas or smoke heated surfaces, such as flues or radiators and extended surfaces, such as pins or ribs. Surfaces unlike in character and location, vary greatly in heating power, so that in making comparisons of different furnaces we must know the kind, form and location of the heating surfaces as well as the area.
American Radiator Company.
In some furnaces having an unusually large amount of surface, it will be found on inspection that a large part would soon become practically useless from the accumulation of soot. In others a large portion of the surface is lined with fire brick, or is so situated that the air currents are not likely to strike it.
One of the first items to be determined in estimating the heating capacity of a furnace is its efficiency, that is, the proportion of the heat in the coal that may be utilized for warming. The efficiency depends chiefly on the area of the heating surface as compared with the grate, on its character and arrangement, and on the rate of combustion. The usual proportions between grate and heating surface have been stated. The rate of combustion required to maintain a temperature of 70° in the house depends of course on the outside temperature. In very cold weather a rate of 4 to 5 pounds of coal per square foot of grate per hour must be maintained.
One pound of good anthracite coal will give off about 13000 B. T. U. and a good furnace should utilize 70 per cent. of this heat. The efficiency of an ordinary furnace is often much less, sometimes as low as 50 per cent.
In estimating the required size of a first-class furnace with good chimney draft we may safely count upon a maximum combustion of 5 pounds of coal per square foot of grate per hour, and may assume that 8000 B. T. U. will be utilized for warming purposes from each pound burned. This quantity corresponds to an efficiency of 60 per cent.