The quantity of heat given off by radiators or steam pipes, in the ordinary methods of heating buildings by direct radiation, will vary from 1 3/4 to 3 heat units per hour per square foot of radiating surface for each degree of difference in temperature. An average of from 2 to 2 1/4 heat units is a fair estimate.
One pound of steam at about atmospheric pressure contains 1146 heat units. If the temperature in the room is to be kept at 70°, while the temperature of the pipes is 212°, the difference in temperature will be 142°. Multiplying this by 2 1/4, the emission of heat will be 319 1/2 heat units per hour per square foot of radiating surface. A boiler must always be capable of generating as much steam as the radiators are condensing. There should be 1 sq. ft. of heating surface in the boiler for every 8 to 10 sq. it. of radiating surface.