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.
This system of heating combines the advantages of both the furnace and direct steam but is more expensive to install. The amount of fuel required is about the same as in the case of furnace heating. Instead of placing the radiators in the rooms, a special form of heater is placed beneath the floor and encased in galvanized iron or brickwork. A cold-air box is connected with the space beneath the heater and warm-air pipes at the top are connected with registers in the floors or walls as already described for furnaces. A separate heater may be provided for each register if the rooms are large, or two or more registers may be connected with the same heater if the horizontal runs of pipe are short. Fig. 6 shows a section through a heater arranged for introducing hot air into a room through a floor register and Fig. 7 shows the same type of heater connected with a wall register. The cold-air box is seen at the bottom of the casing, and the air in passing through the spaces between the sections of the heater, becomes warmed and rises to the rooms above. Different forms of indirect heaters are shown in Fig.. 8 and 9. Several sections connected in a single group are called a "Stack." Sometimes the stacks are encased in brickwork built up from the basement floor instead of galvanized iron as shown in the cuts. This method of heating provides fresh air for ventilation, and for this reason is especially adapted for schoolhouses, hospitals, churches, etc. As compared with furnace heating it has the advantage of being less affected by outside wind pressure, as long runs of horizontal pipe are avoided and the heaters can be placed near the registers. In a large building where several furnaces would be required, a single boiler can be used and the number of stacks increased to suit the existing conditions, thus making it necessary to run but a single fire. Another advantage is the large ratio between the heating and grate surface as compared with a furnace, and as a result a large quantity of air is warmed to a moderate temperature in place of a smaller quantity heated to a much higher temperature. This gives a more agreeable quality to the air and renders it less dry. Direct and indirect systems are often combined, thus providing the living rooms with ventilation while the hallways, corridors, etc., have only direct radiators for warming.
Direct-Indirect Radiators. A direct-indirect radiator is similar in form to a direct radiator and is placed in a room in the same manner. Fig. 10 shows the general form of this type of radiator and Fig. 11 shows a section through the same. The shape of the sections is such, that when in place, small flues are formed between them. Air is admitted through an opening in the outside wall and in passing upward through these flues becomes heated before entering the room. A switch damper is placed in the duct at the base of the radiator so that the air may be taken from the room itself instead of from out of doors if so desired.