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 varying exposures of the rooms of a school or other building similarly occupied require that more heat shall be supplied to some than to others. Rooms that are on the south side of the building and exposed to the sun may perhaps be kept perfectly comfortable with a supply of heat that will maintain a temperature of only 50 or 60 degrees in rooms on the opposite side of the building which are exposed to high winds and shut off from the warmth of the sun. With a constant and equal air supply to each room it is evident that the temperature must be directly proportional to the cooling surfaces and exposure, and that no building of this character can be properly heated and ventilated if the temperature cannot be varied without affecting the air supply. There are two methods of overcoming this difficulty: The older arrangement consists in heating the air by means of a primary coil at or near the fan to about 60 degrees, or to the minimum temperature required within the building. From the coil it passes to the bases of the various flues and is there still further heated as required, by secondary or supplementary heaters placed at the base of each flue.
With the second and more recent method a single heater is employed and all of the air is heated to the maximum required to maintain the desired temperature in the most exposed rooms, while the temperature of the other rooms is regulated by mixing with the hot air a sufficient volume of cold air at the bases of the different flues. This result is best accomplished by designing a hot blast apparatus so that the air shall be forced, rather than drawn through the heater, and by providing a by-pass through which it may be discharged without passing across the heated pipes. The passage for the cold air is usually made above and separate from the heater pipes (see Fig. 19, Part I.). Extending from the apparatus is a double system of ducts, usually of galvanized iron, and suspended from the ceiling. At the base of each flue is placed a mixing damper which is controlled by a chain from the room above and so designed as to admit either a full volume of hot air, a full volume of cold air or to mix them in any desired proportion without affecting the resulting total volume delivered to the room. A damper of this form is shown in Fig. 22. Fig. 23 shows an arrangement of disc fan and heater where the air is first drawn through a tempering coil, then a portion of it forced through a second heater and into the warm-air pipes while the remainder is by-passed under the heater into the cold-air pipes. Mixing dampers are placed at the bases of the flues as already described.