As previously stated, it is necessary to provide a system of vents to supply air to the fixture traps, in order that they may not suffer from the siphonage of their seals.
The one great objection to the system of trap venting as it now stands, is the fact that a vast majority of vents are found to be almost,, if not completely closed, at the end of a few years of service. This result comes about chiefly owing to the location of the opening of the trap vent into the trap. Of necessity the vent of most traps, as ordinarily installed, must be taken off at such a point that this opening readily closes up with grease, lint, etc. If the stoppage came on the waste it would quickly become apparent, but a stoppage of the vent cannot become known usually, for the fixture may be used as readily as if the vent were free, and in many cases the trap may lose its seal owing to the stoppage of the vent, and the fixture still be used, the actual conditions remaining unknown to the inmates.
Plate XXVI. Continuous Venting
Method of Plate 26.
In the use of the half-S trap, however, the vent may often be taken off the horizontal arm of the trap at such a distance from the trap itself that much less difficulty is experienced from stoppages of its vent opening.
The S-trap or other trap in which the outlet pipe is carried horizontally from the trap, or nearly so, may be used in continuous vent work, but traps of the style of full S or 3/4-S traps cannot be used, the reason for which will soon appear.
Plate 26 shows three illustrations of work in which the continuous vent principle is applied. Many attempts have been made to provide special forms of traps whose vent openings would not close up, and mechanical devices have been used for the same purpose, but without satisfactory results. The continuous vent, however, without resort to special contrivances or devices, vents the trap perfectly, and in such a way that there is little, and in fact no danger of the vent-opening closing up.
The three fixtures in Fig. C are provided with continuous vents, the half S-trap being used on each. It consists merely in connecting the outlet of the trap into a waste fitting so located that a vent may be taken off the top of the same fitting. It will be readily seen that the possibility of the stoppage of the openings of these vent pipes is very small in comparison with work of ordinary character, in which the vent is connected to the trap. Wrought iron is generally used for the waste and vent on work that is concealed, while brass is much used on exposed work. Figs. A and B show the same work installed with cast-iron pipe. The objection to the use of cast-iron pipe on this work is that it is not made smaller than of 2-in. diameter. The fittings being so large is another reason for not using it so extensively as wrought iron.
In all continuous vent work the vent is a continuation of the waste line.
As will be seen in connection with several later plates, the continuous vent finds excellent application to groups and lines of fixtures on large work, such as lines of urinals or lavatories in public toilet rooms. The fact that the vent opening is in no danger of stoppage is sufficient to recommend the continuous vent to universal use, even if no other advantages were to be gained. An additional advantage of importance, gained by the continuous vent, is a decreased rate of evaporation of the trap seal. This result is to be expected, owing to the distance of the vent connection from the seal of the trap.
Fig. B shows the continuous vent applied to two lavatories, back to back, on opposite sides of the same partition. For fixtures thus relatively located, the continuous vent is of very great value not only because of the advantages that are gained as named above, but also for the reason that a saving in cost of construction is effected by its use. As far as the waste and vent for the two fixtures are concerned, no more labor or stock is used than in constructing the waste and vent for one alone.
It may not be clear to the reader that traps with other than a horizontal outlet cannot be used on continuous vent work.
As already stated elsewhere, in order to prevent the siphon from operating, air must be brought into it at or near its crown. If air is brought into the long outlet arm of the siphon, it will not break its action. In the same way, a vent taken off the outlet at some distance down from the crown of the 3/4-S sink trap, shown in Plate 9, will not accomplish results. In order, then, that air may be admitted on the same level as the trap seal and at a distance from it, a trap of the general design of the half-S trap must be used.