In the early days of steam heating, radiating surface was generally figured by various rule-of-thumb methods, based chiefly upon the cubic contents of the room to be heated. These varied all the way from one square foot of radiation for 30 cubic feet of space, up to one square foot to 100 cubic feet, according to the building considered. Mr. Baldwin, in the earlier editions of the work mentioned, gives a rule which only takes into account the exposed surface of the building. According to this rule it is first necessary to figure what may be called the "glass equivalent surface." This is the actual glass surface in a room added to the wall surface reduced to its equivalent in glass. Mr. Baldwin refers to his table of relative heat transmitting powers, previously given, and his rule is as follows:
Exposure Diagram.
"In figuring wall surface, etc., multiply the superficial [exposed] area of the wall in square feet by the number opposite to the substance in the table, and divide by 1,000 (the value of glass), the product is the equivalent of so many square feet of glass in cooling power, and may be added to the window surface." Mr. Baldwin then gives a rule for finding the number of square feet of radiating surface for each square foot of glass, or the equivalent of other building substances in glass, which may be expressed by the following formula:
where is the required temperature of the room, is the temperature of the outside air, the temperature of steam in the radiator, the radiation surface, and , the glass equivalent surface.
With an outside temperature of -5 degrees, an inside temperature of 70 and steam at temperature of 220, this formula would allow ½ square foot of radiating surface for each square foot of glass or its equivalent. Mr. Baldwin further adds: "It must be distinctly understood that [this] . . . offsets only the windows and other cooling surfaces it is figured against and does not provide for cold air admitted around loose windows, or [through walls] of poorly constructed . . . houses. These latter conditions, when they exist, must be provided for separately, and usually require as much as 50 per cent. additional; a good com-
