In the case of tenement houses, also, whose roofs in the summer season are occupied by the inmates, the escape of a constant stream of sewer gas would seem to be a thing to be dreaded.

Another, and a very strong point against the employment of plumbing systems having no main trap, is the fact that under such conditions air contaminated with disease germs coming from the human excreta of any infected house on a line of sewers, may find its way through defects in the plumbing systems of other houses on that line, and thus gain entrance into the living apartments of the inmates. Plumbing systems should always be so installed that there may be no opportunity for such occurrences as this, whether in the manner just mentioned or through local vent systems, which have been known to carry infection from one to another apartment in the same building.

For this reason, as well as for other reasons, it is always poor practice to connect the drainage system of one house into that of a neighboring house. Such practices were more or less common years ago, but since the matter of sanitary conditions has begun to receive its proper attention, the connection of two or more houses to the same house drain or sewer has been strictly prohibited.

It would seem that there is an opportunity for the display of good judgment in the employment of the main trap. In sections of a city where the houses are detached, as in the residential sections, it would be wiser to do without the main trap than in the more densely populated sections.

The use of the main trap makes necessary the use of the fresh-air inlet, which, as shown in Plate 15, must be connected on the house side of the trap. The purpose of this pipe is to bring into the plumbing system a supply of fresh air, and to create a circulation of this air through the system and out through the roof pipe. It also serves to prevent air lock between heavy bodies of waste flowing down the house drain and the seal of the main trap.

If the fresh-air inlet were connected on the sewer side of the main trap, it would not only fail of its purpose of supplying air to the system, but would form a direct vent for the sewer at a particularly bad point.

The fresh-air inlet should under no conditions receive drainage of any sort. Formerly the fresh-air inlet was connected to the trap itself, as shown in Fig. A, which method allowed but one cleanout to be used on the trap, whereas two should always be used. Experience proved, however, that this connection had another disadvantage, from the fact that it brought in a current of cold air directly upon the trap seal, which resulted in the chilling and sometimes in the freezing of the water in the trap. Even though not frozen, the chilling of the waste caused the grease to separate from the sewage and cling to the inner surface of the trap, making ultimate stoppage more possible.

The freezing and the stoppage of the trap are two of the arguments against its use, but by the employment of proper means these results may be largely overcome. The fresh-air inlet, when properly constructed, is taken out of a fitting placed next to the trap, on the house side of it. This fitting may be either a tee or a Y, as shown in Figs. B and C. The more bends there are in the pipe, and the more indirect its course, the less will be the possibility of chilling and freezing.

The fresh-air inlet should never end at a point within 15 ft. of any door, window, or cold-air box supplying heating systems. The reason for this is that when heavy volumes of sewage pass through the house drain, a discharge of foul air passes through the inlet. This same trouble also occurs sometimes owing to a heavy atmosphere. When the fresh-air iniet ends at a distance greater than 15 ft. from any opening into the house, it may terminate at the outer face of the foundation, as seen in Fig. B. In this case its end must be provided with a perforated cap, or with a bend looking down, in order to prevent different articles, such as stones, etc., from being thrown into it. It must usually be carried out into the lawn or yard to cover the requirement, in which case it is often constructed, as shown in Fig. A, with a ventilating cap covering its end, or ending in a return bend, this bend ending at least one foot above the ground. In business districts, where such devices as the return bend and ventilating cap could not be used, the fresh-air inlet should open into a box, 18 in. square, located below the level of the sidewalk, and at the curb. The bottom of this box should be at least 18 in. below the under side of the end of the inlet pipe.

The box may be constructed of brick or flagging, or of cast iron, and covered with a flagstone provided with a removable iron grating leaded into the flag. The grating should have small perforations in order that refuse may not pass through, and the total area of the perforations should at least equal the area of the fresh-air inlet.

Another method of running the fresh-air inlet is to carry it through the roof, as seen in Fig. C.

In general, this adds considerable expense without giving much added value. An objection to it, especially in the case of the ordinary house where there is but one 4-in. stack, is that the weight of air in the stack and in the fresh-air inlet about balances, with the result that there is but little circulation. This method, however, is but seldom used.

As to size, the fresh-air inlet for traps up to 4 in. in size should be of the same size as the trap.

For traps larger than 4 in. it may be less than the size of the trap.

For 5- and 6-in. traps the fresh-air inlet should be 4 in. in diameter.

For 7- and 8-in. traps, the fresh-air inlet should be 6 in. in diameter; and for traps larger than 8 in. it should be 8 in. in diameter.

Care should be taken that the main trap is set level, in order that none of its seal may be lost. When located below the cellar bottom, it should be made accessible either by setting it in a brick manhole provided with a removable cover, or by making depressions in the cement bottom so that the cleanouts may be easily reached.

The connection shown in Fig. B, whereby it is made possible to use an end cleanout, is an excellent one.

With two cleanouts on the main trap, and this end cleanout, the house drain at this point is well guarded against any possible stoppage. The connection referred to is now demanded by the ordinances of a number of different cities. All connections into the drainage system must be made on the house side of the main trap.

An exception to this rule is made in the case of rain leaders, which are sometimes run outside the foundation walls, in which case they may be connected into the house sewer on the sewer side of the main trap. Such rain leaders must be properly trapped. The main trap is sometimes located underground, outside the foundation walls, in which case it must be made frost proof and accessible. This is done by setting it below the freezing level, in a brick or stone manhole, covered with a flagstone. When so located, the fresh-air inlet should never be taken off the trap, as the passage of cold air would be so direct as to cause trouble.