In order to make sure that no vent shall act as a soil or waste pipe in the event of stoppage of any part of the waste the main branch vent should enter the vertical mainline of vent above the top of the highest of the fixtures. Very often a fixture is located at a considerable distance from the main vent to which its trap vent is to be connected. When this distance is 8 feet or more, the trap vent should either be carried independently through the roof or enter its main vent at a point above all fixtures. Another necessary provision is the increase in size of pipe used on long lines of vent. After passing 30 feet in length, the size of the vent should be increased one size. This precaution is necessary from the fact that in long lines of pipe, friction will decrease the efficiency of a pipe in delivering an adequate supply of air to the trap. In the running of main vent lines, many of the principles that apply to the trap vent are to be followed. There are, however, several points which should be considered that do not enter into the subject of the trap vent. In order to perform their work, the trap vents must be provided with a supply of air, and therefore the main vent must open into the outside air. One method of doing this is to run the main vent directly through the roof, as in Fig. 52. The ordinary method, however, is to connect the main vent into the main stack above all fixtures, as shown in Figs. 53, 54 and 55. Below the lowest waste connection into the stack, the main vent should be connected back into the stack, as in Fig. 57. This connection is made in order that all scale and condensation forming in the pipes may escape into the drainage system, to be washed away. Fig. 58 shows the use of a special fitting for making this connection, which allows the two vertical lines to be run very close together. In Fig. 59, at B, is shown a wrong method of connecting the main vent into a trap vent, instead of re-entering the main stack. The same connection may perhaps be seen more clearly in Fig. 56.
Fig. 52. - Main Vent and Main Stack Separately-through Roof.
Fig. 53. - Upper Connection of Main Vent into Main Stack.
Fig. 54. - Two Methods of Upper Connection of Main Vent into Main Stack. .
Fig. 55. - Showing Top and Bottom Vent Connections and Temple Vent Fittings.
When the lower end of the main vent stack is reduced to receive a trap vent, as in Fig. 56, there is abundant opportunity for the collection of scale, rust, and condensation in the heel of the reducing elbow, and no opportunity whatever for it to be washed away as in the correct method. The latter connection is the more rigid of the two also. For these reasons the connection of Fig. 57 is the one now generally called for by the best plumbing ordinances.
Fig. 56. - Wrong Method of Lower Connection of Main Vent.
Fig. 57. - Improved Lower Connection of Main Vent into Main Stack.
Fig. 58. - Special Main Vent Connection.
Fig. 59 shows the various connections on,the vent system, with various common fittings in use.
In Fig. 55 also are shown main vent connections, and a special form of vent fitting, which is not only excellent in principle, but allows the work to be installed very compactly.
Fig. 59. - Vent System, Showing Main and Trap Vents, Use of Fittings, etc.
A few words should be said concerning the material used on the vent system. For main vent lines cast iron or galvanized wrought iron pipe is used, and for trap vents either brass, lead, or galvanized wrought iron, the latter being the most extensively used in both cases.