The use of this trap is open for discussion, and its advantages and disadvantages have been debated more extensively probably than any other subject relating to the drainage system.
Its advocates have strong arguments in its favor and its opponents advance sound arguments against it.
When the main trap is made use of on the plumbing system it is located on the house drain and as close to the point where it leaves the building as possible. Fig. 111 shows the connections of the main trap and fresh-air inlet, the latter being a necessity wherever the main trap is used, as will be seen later. The purpose of this trap is to safeguard the building against the entrance into it from the sewer of sewer air and gases. The chief argument against the use of the main trap is that each soil and waste stack passing through the roof should have an unobstructed path throughout its course to the public sewer, in order that it may act as a vent for the sewer. In other words, the non-use of the main trap is advocated in order that every roof connection in the town or city may become a part of a great ventilating system for the public sewer system. It is not to be denied that every system of sewers needs ventilation, and that better provision is needed than is generally provided. The closed, unventilated system without ventilation of any kind is undoubtedly an exceedingly foul and filthy affair and a source of great danger to the community.
Fig. 111. - The Main Trap and Fresh-Air Inlet.
It is a question, however, whether the use of the house plumbing system as a sewer vent is desirable, and it is a question whether other adequate means cannot be provided for performing this work with as great efficiency.
At present, in most cities, the chief means of sewer ventilation is through perforated manhole covers in the streets. Needless to say, the amount of ventilation provided in this way is entirely insufficient for the amount of work to be done, and in addition it may be said that in many cases the perforations of the manhole covers become filled with dirt and in the winter time with ice and snow. Moreover, the venting of sewers into public thoroughfares is an unsanitary practice, but one which the public has become accustomed to, and therefore gives small attention to. Other means than this should be provided. One plan has been advanced which would seem to have many valuable features. It has been proposed to erect at available high points on the sewer system large ventilating stacks through which to exhaust the sewers of foul air and to draw into them a supply of fresh air. This method would entail considerable public expense, but would seem to be an object worthy of the outlay, inasmuch as it affects the public health to a large degree. Such a method seems to be far preferable to venting the sewer at all points in the community, where in heavy weather gases are bound to settle down and become a source of danger to the entire people.
It may be a point not generally appreciated, but a fact, nevertheless, that there are very few plumbing systems that have been in use for a period of years which are not defective in various ways. The settling of foundations and floors is bound to produce defective soil-pipe joints, and even result in the cracking of pipe and fittings in some instances. The action of the acids and gases from the house sewage often results injuriously to lead and iron work, and especially to wrought iron. Many other causes of defects in the plumbing system might be named, and it is safe to say that in a large majority of instances the plumbing system would fail under the tests applied to it when first installed.
The point of all this is that through the various defects in the drainage and vent system abundant opportunity is offered for the entrance of sewer air and gases from the sewer into the living rooms of the house.
The purpose of the main trap is to overcome this trouble and to prevent the occurrence of the same conditions from various other causes. For instance, the main trap prevents the entrance of sewer air through the cellar drain, floor drain, and other traps in the event of loss of seal. In the case of fixtures seldom in use the fixture trap often loses its seal through evaporation; fixture traps often lose their seals through siphonage and other causes. Danger is avoided under such conditions by the use of the house trap.
When water-closets are removed from their position, to be renewed or for the making of repairs, direct communication between the house and sewer results unless the main trap is used, as also in the case of repairs or renewals on the cast-iron drainage system, such as the inserting of fittings to receive the waste from new fixtures, etc.
These are a few of the many ways in which the house may become open to the entrance of sewer air, and the facts as presented certainly make a strong argument in favor of the use of the main trap. Some of the minor arguments against its use are the facts that it is liable to freeze when located in cold places, that it will close up with heavy substances in the sewage, and that it presents an obstruction to the free passage of sewage from the house. The two first-named arguments appear to have little weight, as practical experience shows that the trap seal seldom freezes, and that it is very seldom that a main trap becomes stopped up. The waste from the house is generally warm, and, with the exercise of good judgment, the trap can be so located or otherwise protected that it need not be in danger of freezing.
To be sure, the trap prevents the outflow of sewage as rapidly as a free passage would allow, but this fact seldom is found to be . the cause of special trouble, and the fresh-air inlet aids materially in the easy passage of the sewage.
The trap used under each fixture is liable to stoppage and presents an obstruction to the rapid outflow of waste, but the facts are never used as an argument in favor of discontinuing the use of the fixture trap. No more should the same arguments avail in discontinuing the use of the main trap.
The first point named in opposition to the use of the main trap is without doubt, then, the strongest argument against it, and really the only one, and it would seem that the weight of argument is in favor of the use of the main trap.
In connection with the main trap the fresh-air inlet is always necessary. For the ordinary house having a 4- or 5-inch house drain the fresh-air inlet should be 4 inches.
Its purpose is twofold. Its chief end is to convey into the drainage system a supply of fresh air and to create a circulation of air through the drainage system and through the roof pipes. It also makes the passage of waste through the house drain easier, as it prevents air lock between the outflowing waste and the seal of the main trap by venting the house drain at a point near the trap.
An objection to the fresh-air inlet is that, in the case of a down draught, foul odors may be driven out through the inlet. These odors, however, do not come from the sewer, and are therefore not so much to be feared. Valves of various kinds have been used to admit fresh air and to prevent the escape of foul air, but in general they are not to be depended upon and are usually looked upon as a failure. Because of the occasional escape of odors the fresh-air inlet opening should never be located within 20 feet of any door or window. This often necessitates the running of the fresh-air inlet out into the lawn, yard, or to the curb, in which case, at the point where it comes to the surface a ventilating cap should cover its end in order that nothing may enter or be thrown into it. The fresh-air inlet is strictly for purposes of ventilation, and under no circumstances should drainage of any description be allowed to enter it When carried only to the outside face of the foundation wall the fresh-air inlet should be provided at its end with a perforated cover, or, as shown in Fig. 111, with a bend pointing down. The main trap is sometimes located on the house drain or house sewer uuderground. When so located the chief precaution necessary is to make it easily accessible, and this may best be done by placing it in a well at such depth as to be protected from the frost, as shown in Fig. 112. When placed underground the fresh-air inlet for the main trap is generally carried straight to the surface, as in Fig. 112, in which case its open end should be protected by a bend.
Fig. 112. - The Main Trap when Located on House Sewer.
The best practice calls for the connection of the fresh-air inlet to a tee or Y placed next to the main trap. Until within a few years the custom was to connect it into the cleanout hub on the house side of the trap. This method not only deprived the trap of one of its cleanouts, but brought cold air so directly upon the seal of the trap that the water often became chilled.
Even though the trap seal should not become cold enough to freeze, the mere chilling of it should be avoided, for when sewage becomes chilled the grease contained in it separates out and collects on the cold surfaces exposed to it.
Fig. 113. - "Roughing" for the Plumbing System, with Main Trap and Fresh-Air Inlet.
In order to avoid this trouble the fresh-air inlet is now generally connected as mentioned above, thus making the entrance of cold air less direct. The fresh-air inlet should always be connected on the house side of the main trap. If connected on the sewer side it would act as a direct vent for the sewer and be unable to perform the duties for which it is designed. The fixture trap vent, however, should always be connected on the sewer side of the trap. There is sometimes confusion on the part of people respecting the connection of these two forms of vents, but there should not be, if due consideration is given to the different purposes of the two.
In Fig. 113 is shown an elevation of a plumbing system which is provided with a main trap and fresh-air inlet.
In this illustration is shown only the roughing, with the ends of vents and wastes sealed, preparatory to the water test. In the main trap is shown a double testing plug and in the fresh-air inlet a single testing plug, with water connection, the fresh-air inlet usually being a handy place for connecting the water to the system in testing.