Tubular boilers are largely used for heating purposes, and are adapted to all classes of buildings except dwelling houses and the special cases mentioned for which sectional boilers are preferable. The capacity of this type of boiler is usually stated as so many horse-power, and the method of determining the size is different from that just described. A boiler horse-power has been defined as the evaporation of 34 1/2 pounds of water from and at a temperature of 212 degrees, and in doing this 83,317 B. T. U. are absorbed, which are again given out when the steam is condensed in the radiators. Hence to find the boiler H. P. required for warming any given building we have only to compute the heat loss per hour by the methods already given and divide the result by 33,330. It is more common to divide by the number 33,000, which gives a slightly larger boiler and is on the side of safety. The ratio of heating to grate surface in this type of boiler ranges from 30 to 40 and therefore allows a combustion of from 8 to 10 pounds of coal per square foot of grate. This is easily obtained with a good chimney draft and careful firing. The larger the boiler, the more important the plant usually, and the greater the care bestowed upon it so that we may generally count on a higher rate of combustion and a greater efficiency as the size of the boiler increases. The following table will be found useful in determining the size of boiler required under different conditions. The grate area is computed for an evaporation of 8 pounds of water per pound of coal, which corresponds to an efficiency of about 60 per cent and is about the average obtained in practice for heating boilers.

The areas of uptake and smoke pipe are figured on a basis of 1 square foot to 7 square feet of grate surface and the results given in round numbers. In the smaller sizes the relative size of smoke pipe is greater. The rate of combustion runs from 6 pounds in the smaller sizes to 11 1/2 in the larger. Boilers of the proportions given in the table, correspond well with those used in actual practice and may be relied upon to give good results under all ordinary conditions.

Water-tube boilers are often used for heating purposes but more especially in connection with power plants. The method of computing the required H. P. is the same as for tubular boilers.