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and for lessening the fire risk as well as preventing loss of heat should be made with double walls separated by an intervening air-space. The register boxes should also in every case have double walls. The general form of a stack in position in a partition, with boot attached at bottom for leader-pipe and with round connection for register box, is shown in Fig. 209.

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FIG. 209.-REGULAR STACK WITH COLLAR AT TOP AND FLAT BACK BOOT AT BOTTOM.

The leader-pipes and stacks, boots, and register boxes are now a standard article of manufacture by several firms. I am indebted to the Excelsior Steel Furnace Company of Chicago for the table of capacity and dimensions of various forms of stacks and leader-pipes, given on page

278.

It will be found profitable in nearly every case to wrap the leader-pipes with two or more thicknesses of asbestos paper

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and mineral wool in order to prevent loss of heat. It is desirable to locate the stacks in the inside partition-walls of the building, or where they will be protected as much as possible from loss of heat, since any loss affects the rapidity of circulation. It is generally necessary to have the leaderpipes not over 15 feet in length, otherwise the circulation will be uncertain in amount and character.

Size of Stack as Listed. (In Inches.)

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144. The Areas of Registers or Openings into Various Rooms. Registers are made regularly in various forms, square or round, and arranged for use either in the floor or side walls.

* TABLE OF SIZES AND DIMENSIONS OF SAFETY DOUBLE HOT

AIR STACKS.

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*This table is copyrighted by Excelsior Steel Furnace Co.

as required. These registers are usually supplied with a series of valves which may be readily opened or closed.

The

FIG. 211.-REGISTER BOXES SHOWN IN POSITION.

space taken by the screen and valves is usually about of that of the register, so that the effective or net area is about of

Bars deducted.

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the nominal size of opening. These registers may be obtained finished in black or white japan, or electroplated with nickel, brass, bronze, or copper. The table on page 280 gives the various sizes of registers which are regularly on the market, their effective area in square inches, and diameters of round pipe having the same capacity.

The areas of stacks may be considerably less than those of the registers, since it is generally required that the velocity of air entering the room shall not exceed 3 or 4 feet per second, while that passing through pipes and stacks may have the highest velocity possible, which for the different floors will not differ greatly from 4 to 6 feet per second, as already explained. For methods of proportioning ventilating flues see page 233.

FIG. 212.-SIDE-WALL REGISTER HEAD OR FLANGE.

Considerable difference of opinion exists as to the relative merit of floor and wall registers for heating purposes. It is the common practice to use floor registers for most rooms on the first floor, and wall registers for rooms on the second and higher floors. The floor register, from its general form and position, can be supplied with hot air somewhat more readily than the wall register, and for that reason may induce somewhat stronger circulation, but it is a receptacle for dust and sweepings of the room and in a position to materially interfere with the carpets. It will be found that the experiments made by Briggs (see page 46) as to diffusion of air hold in the case of furnace heating the same as in that of any other

system. From these experiments it would seem that the highest efficiency would be attained when the inlet for the heated air was at the side near the top of the room and the outlet for ventilation near the floor. This distribution is one that, so far as the writer knows, has never been practised in furnace heating of residences, although it is the commonly accepted method in school-house heating, whether with a furnace or an indirect system of steam or hot-water heating.

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145. Circulating Systems of Hot Air.-By connecting the cold-air box with the hall floor or the lower portion of a passage communicating with all rooms of the building and closing outside connections a downward current of air will pass from the rooms to the furnace, which, being warmer than the outside air, will aid materially in heating. Such a connection

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if properly made and used with judgment may be of great service in reducing the cost of operation without seriously affecting the ventilation. Such a system if erected, however, should be supplied with devices to prevent overheating and arranged so that cold air can be drawn from outside of the building whenever desired. There is so much danger that ventilation will be poor with this system that it is not recommended.

146. Combination Heaters.-A combination heater consisting of a hot-air furnace, with the addition of a boiler for hot water or steam, is meeting with somewhat extensive use and has been described on page 189 so far as relates to the construction of the steam and hot-water appliances. In case a combination heater is used the area of the grate and heating surfaces will need to be proportioned for both systems. A combination heater is better suited to large buildings than a hot-air furnace. In practice, however it will be found, difficult to so proportion the amount of heating and radiating surface, as to give a perfect distribution of heat in rooms some of which are heated with hot water or steam, and some with hot air, but this difficulty will no doubt be largely overcome by experience.

147. Heating with Stoves and Fireplaces.-The manufacture of stoves for heating purposes is a very great industry in the United States and they are extensively used in the cheaper classes of dwellings. In every case the stove is located directly in the room to be heated and is connected with a chimney by means of several lengths of sheet-iron pipe. Stoves are built in many forms, some of which are very elaborate and highly ornamented, and in many cases they are provided with magazines from which the coal feeds itself automatically as required. The heat, given off from a stove, is generally nearly all utilized in warming, perhaps not over 10 or 15 per cent being carried off by the chimney. Stoves do not, however, present an economical mode of heating, largely because the wastes which occur from the operation of small fires are very great and cannot be avoided. It is doubtful if the efficiency averages much above 25 per cent. In addition, the stove occupies useful room, is the source of very much dirt and litter, and requires a great deal of attention.

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