Assuming that the method of disposing of sewage and drainage is decided upon, the problem of how to pipe the house safely may be considered as presenting about the same conditions, whether the house drain enters a branch from the city sewer or terminates in some other means of disposal.

Granted that sewer air is a thing to be guarded against, the safest plan is to pursue that course which offers the surest means of keeping the house free of it. We know that through contamination of water supply by filtration from vaults, etc., the human system may suffer pollution, and may develop specific disease of a serious, even fatal nature. It is no less certain that polluted air will affect the lungs similarly, according to the nature of the pollution. On this ground, notwithstanding any argument to the contrary, we should proceed to exclude sewer air entirely, and to make the air of the house drain-pipes as pure as possible.

It must be remembered that where a whole system of plumbing is designed with certain ends in view, and all the details worked out accordingly, a house system may be satisfactory which under slight disturbance of conditions would be abominable. Therefore no departure from a certain means of positively accomplishing a desired result should be accepted without unanimous endorsement of those in position to know what is safe. People, however, have been at all times too ready to accept any plan that promised the immediate saving of a dollar. Certain plumbing accessories may be admirably adapted to use in one place, yet wholly unfit for service in another; but the makers cannot be expected to discriminate; they are prejudiced, and are not on the ground. It is the business of the public, through architects and plumbers, to select suitable means to the end.

With the fresh-air inlet and proper installation throughout the building, an intercepting trap is likely to exclude sewer air from the house, and t6 keep the drains in the house filled with fresh air from the open atmosphere (see page 163). With these conditions, a possible leaky joint or defective trap can permit only comparatively pure air to enter from the pipe. The intercepting trap being in the main line, all water from the house passes through it, insuring the water seal being maintained. The foul-air outlet ventilates the sewer much as would the house lines if the trap were omitted, because in it there is never any contrary rush of air or water, both of which would check or reverse the current, and the latter of which reduces the area of the pipe, even though it be assumed that no further interference occurs through discharge from fixtures. The trap may be in the yard or within the house walls, according to circumstances. Fig. 142 shows an intercepting trap in the cellar,with its fresh-air inlet terminating above the snow-level. Many jobs were formerly piped in a way permitting soil air to puff out through the inlet. Fig. 143 shows a plan that has been resorted to with the idea of carrying such discharges to a safe height without interfering with the normal action of the fresh-air inlet. It is merely a rising line with an inverted funnel over the open end of the inlet, which incidentally protects the air-pipe from lodgment of foreign matter. The foul-air outlet should not terminate near a window or door, nor be too close to the fresh-air inlet opening. It should be located so that it will be free of chance obstruction, and above the level of winter ice and snow, even though it has to be piped to above the roof-level as indicated in Fig. 144, in which A is a cone strainer with solid top, and T the main intercepting trap. The direct line of foul-air pipe to roof, and the distance between the trap and fresh-air inlet grating, provide every requisite possible to this part of the house drainage, whether a loop stack, spoken of on another page, is employed or not.

Fig. 142. Intercepting Trap in Cellar..

Fig. 143. Simple Device for Carrying Away to a Safe Height Soil Air that may be Puffed from Fresh-Air Inlet..

A very good plan of terminating air inlet and outlet pipes in situations exposed to the entrance of obstacles, is to use a single or double hub return bend above snow-level, as shown in Fig. 145. In this way, nothing can fall in by accident; sleet from any direction cannot choke the openings; nor are children likely to fill the pipe. Fig. 146 illustrates a galvanized-wire guard placed in the hub; such a guard is generally used on conductor pipe, but is equally suited to the protection of soil and vent lines in mild climates. In Northern localities, the regular cast hood and thimble made for the purpose are better. The area of the openings of the strainer should aggregate at least 12 square inches for pipe 4-inch or less; and where frost trouble is feared, the strained should be recessed several inches so that the frost will have to close the open end of the pipe instead of the grating.

The foul-air pipe should not have abrupt offsets at any point. The lodgment of foreign matter therein would be possible, and the function of the pipe perhaps thus impaired. This pipe not only ventilates the sewer, but offers egress for air when storm water is crowding the sewer, and at other times when air-pressure would otherwise drive the seal of the trap toward the house, enough ultimately, in some cases, to lose the seal by waving out when the pressure is relieved.

Fig. 144. Foul-Air Outlet from Intercepting Trap Carried to Roof..