While as a principle it has been known for ages, it is only within a comparatively few years that venting has been applied to the plumbing system as a means of overcoming such troubles as those mentioned in the preceding chapter. As a matter of fact, however, venting to-day stands second only in importance to the trap in a properly installed plumbing system.
Venting may properly be considered under two heads, soil or waste vents and trap vents. There are also two other forms of vent, the local vent and the fresh-air inlet, neither of which apply to our present subject, but which will be considered in later chapters. By the soil or waste vent is meant the continuation of vertical soil or waste-pipe lines above the highest fixtures and through the roof, terminating with an open end. It will readily be seen that the use of this vent prevents all trouble from back pressure in connection with trap seals*, the vent relieving all pressure that might be generated in the sewer. The importance of this vent is realized when the plumbing system shown in Fig. 36 is compared with the same system supplied with the soil and waste vent. Not only does this vent relieve the system of back pressure, but it also purifies it, and lengthens its life, and in addition is often an aid in installing the trap vents, as will soon be seen. The soil and waste vent should ordinarily be carried at least two feet above the roof, and always above any opening on the roof, or the windows of neighboring buildings. There is one source of danger to this vent that should always receive attention, particularly in cold climates. Steam from the plumbing system will in cold weather form a mass of frost about the vent opening, and in the case of the smaller sizes of soil pipe often close the opening entirely. All sizes smaller than 4 inches may be closed in this way. Therefore no pipe of less size than 4 inches should pass through the roof, all pipes of smaller size being increased to 4 inches below this point, as in Fig. 59. Some plumbing ordinances require an enlargement of only one size in diameter. This would mean that a 2-inch pipe need be increased only to 3 inches in passing through the roof. . The first-named provision, however, we believe to be the better. Vent covers are frequently fitted to the tops of roof pipes, which is certainly wrong in principle. It is better practice to discard even the wire cage which is so commonly used, for even this contracts the opening and allows the frost to accumulate more readily than it would in the open end of the pipe. Trap venting is the subject next to be considered, a subject both important and extensive. It has been seen in the preceding chapter that the admission of air to the crown of the siphon will destroy its action. It must necessarily be destroyed, for the admission of air destroys the vacuum upon which siphonic action depends. Briefly stated, then, the trap vent used on the plumbing system is a pipe, as shown in Figs. 39 and 40, whose purpose is to carry a supply of air to the crown of the trap, thereby preventing the formation of a vacuum in the outlet from the trap, and making the siphonage of the contents of the trap an impossibility. In order to perform its work, the trap vent must be connected with the outside atmosphere. This is accomplished by connecting the trap vents of the several fixtures into a main vent, the latter either passing through the roof to the outer air, or connecting into the soil and waste vent above the highest fixture. The trap vent, or back vent, as it is also called, should be connected to the trap at or near the crown, and on the sewer side of the trap. The connection of this vent to the trap and into the main vent may be seen in Fig. 41. The vent should always enter the main line of vent at a point higher than the fixture itself. If connected below the fixture it may easily be seen that in the event of the stoppage of (the outlet from the trap, the waste would pass out through the vent into the drainage system and thus continue until the vent in turn was stopped. This condition occurs in the work illustrated in Figs. 42 and 43, the former representing the trap vent connected directly into the main vent, and in the latter the trap vent connected into a vent from another fixture.
Fig. 39. - The Vented S-Trap.
Fig. 40. - The Vented Drum Trap.
Fig. 41. - The Trap Vent.
When properly connected, waste in the case of a stoppage would be unable to pass over through the vent, and would set back into the fixture, thus giving warning of the existence of trouble.
While in principle the proper point for the connection of the vent is at the crown of the trap, there is an objection to this course from the fact that it brings in the supply of air directly upon the seal of the trap, thereby greatly increasing the rate of evaporation. It is better, therefore, to connect the vent in such a way that this danger may be obviated as far as possible. Fig. 44 shows how this may be done in connection with the several styles of S traps. From the nature of its connections, precautions of this kind are not so essential to the drum trap.
Figs. 42, 43. - Wrong Connection of Vent.
Fig. 44. - Venting of S-Traps.
In the connection of the vent to the trap, a very important point is the making of the connection in such a way that the possibility of the stoppage of the vent opening by grease, lint, etc., in the waste may be made as slight as possible. In Fig. 44 the danger of such stoppage is present in the ease of each of the three traps, but least of all in the half-S trap, and it may be added, also, that in this trap the rate of evaporation will also be less than in the others, owing to the distance of the vent connection from the seal. The stoppage of the vent opening is in reality the greatest obstacle that the vent system has to contend with.