272. In this system, all of the foul-air flues are brought together and connected into a single large chimney or shaft, so that there is practically only one outlet. There are three methods in vogue of arranging the flues and the movement of the air:

1. To carry a separate flue from each room to the attic, where they converge into a few large ones and finally enter the base of the aspirating shaft. The shaft, in this case, starts from the attic and does not extend down through the lower stories.

2. To carry each foul-air flue horizontally and connect it directly into the aspirating shaft, which in this case must extend through the entire building.

3. To carry the flues downwards into the basement, and connect them to the stack at the lowest practicable level.

In the first and third methods, the number of flues increases with the height of the building, being most numerous in the upper story in the former case, and in the first story in the latter case.

An aspirating chimney which extends to the basement is necessarily much more expensive than one which starts from the attic. The space required for a brick chimney of this kind is considerable, not only on account of the thickness of the walls required in the lower stories, but also because the sectional area necessary to carry the foul air and allow for frictional resistance is so large.

In practice, the velocity of the air will seldom exceed 6 feet per second, and the area of the shaft should be calculated upon that basis. In cases where an exhaust fan or steam jet exhauster can be used, the estimate of velocity may be increased to 8 or possibly 10 feet per second.

273. The principal advantage possessed by the third method over the first is the facility which it affords for using heaters at the base of the stack to aid the draft. A part of the increase in draft pressure gained in that way is spent in overcoming the resistance offered by the foul air while descending the flues to the basement; consequently, the net gain is not very great.

When an aspirating fan or a steam jet exhauster is employed to increase the draft, instead of a heater of some kind, the advantage of cost and convenience is largely in favor of the first or upward method.

274. During the cold season, the difference in temperature between the foul air and the outer atmosphere is usually sufficient to create a satisfactory draft in the aspirating chimney. In addition, the hot gases from the heating apparatus may be turned into the stack to increase the temperature and aid the draft But during mild weather, the temperature difference diminishes, and in summer time it dwindles to almost nothing, on some occasions being actually reversed. The chimney thus becomes impotent and inoperative as the weather becomes warmer, and auxiliary apparatus, such as steam coils, stoves, grates, and gas burners, located at the base of the stack, must then be employed to aid in maintaining the ventilation.

275. Steam coils, in order to be effective, should be placed crosswise of the foul-air current, and the pipes should be spaced wide apart, so as to impede the current as little as possible. The requisite number can be used by placing them in several tiers; 1 1/4-inch pipes should be spaced not less than 4 inches apart between centers.

If the coils are located in the stack, they should be placed horizontally, about as shown in Fig. 88, care being taken to insure perfect drainage. It is poor practice to arrange steam coils vertically and around the sides of the stack, because only a small part of the air will come into actual contact with them and be properly heated. The coils should be placed as low down in the stack as practicable, in order to secure the greatest available height for the column of warm air, and every foul-air inlet should enter below, never above them.

Fig. 88.