It will be noticed that the waste pipes from the bath tub and wash basin enter the trap near the bottom and discharge at the opposite side near the top. The water will stand in the trap and pipes level with the bottom of the discharge pipe and thus form a seal that prevents the escape of sewer gas. This is a common form of non-siphoning trap. It is non-siphoning because it cannot lose its seal by reason of the siphoning effect of the water as it passes through the waste pipes on its way to the sewer. Another form of non-siphoning trap is the clean sweep trap shown in Fig. 97. Such traps as Figs. 95 and 98 are siphoning traps, since it is possible, in this form of trap, for the water to be so completely siphoned that not enough remains to form a seal. The small drawing, marked Detail L, is another method of connecting the same arrangement of fixtures. The waste pipe enters the trap as before but discharges immediately opposite. The level of the water stands in the pipes as indicated by the dotted line.
Back-venting.
—To prevent the possibility of loss of seal by siphoning and the escape of sewer gas, traps are back-vented to the main stack or to a separate vent stack. The venting is accomplished by joining a pipe to the top of the trap or to some point in its immediate neighborhood, and connecting this with the main stack or the vent stack. The water in a trap so vented will be open to the air from both sides and consequently can never be subject to siphonic action.
In the average-sized dwelling where non-siphoning traps are used, back-venting is not necessary, but in large houses and in plumbing where siphon traps are used, vent pipes must be attached to the traps to assure a satisfactory system.
Fig. 100 furnishes an example of back-venting, applied to the bathroom shown in Fig. 99. In the former figure the bath tub and wash basin are connected with the waste pipe by siphon traps. A siphon trap may lose its seal in two ways: by self-siphonage, or by aspiration caused by the discharge of the water from other fixtures. In the discharge of the siphon trap, such as B, in Fig. 100, the long leg of the siphon, formed by the discharge pipe, may carry away the water so completely that not enough remains in the trap to form a seal. Again, the discharge of the water from the bath tub through the waste pipe tends to form a vacuum above it and in some cases the seal in B is destroyed by the water being drawn into the vertical pipe. The possibility of either of these occurrences is prevented by back-venting.
Fig. 100.—An example of back-vented plumbing as applied to the bathroom.
In Fig. 100, a pipe from the main stack is connected with the bend of the trap at B and also to the waste pipe outside the trap at T. A vent is also taken from the drain C, at a point just below the trap in the closet seat. The object of all of the vents is to prevent the tendency of the formation of a vacuum from any cause that will carry away the water seal of the trap and allow sewer gas to enter the house.
The closet seat also contains a trap which will be described later. It connects with soil pipe S, leading to the sewer by a large lead pipe C.
All of the pipes under the floor, leading to the soil pipe, should be of lead. The pipes above the floor are generally of iron or nickel-plated brass. All of the connections in the lead pipes are made with wiped joints; that is, the connections are made by wiping hot solder about the joint, in a manner peculiar to this kind of work, in such a way as to solder the pipes together. The joints made in this manner are perfectly and permanently tight. Lead pipes are used under such conditions, because lead is the least affected by corrosion of any of the metals that could be used for such work.