The question is a debatable one, and has been since the introduction of the trap itself. Plate 15 shows three methods of installing the main trap and its accompanying fresh-air inlet. From these illustrations it will be seen that the main trap is placed on the house drain at a point as close to the place where the drain leaves the building as possible.
Plate XV. The House Or Main Trap And Fresh Air Inlet
Construction of fresh Air Inlet
The object of this trap is to prevent the entrance into the plumbing system of gases and odors from the sewer.
At first thought, the entrance of gases into the plumbing system would not appear to be harmful, especially as it has abundant opportunity to rise and escape through the roof pipes. However, although the plumbing system of to-day is subjected to rigid test after being constructed under rigid ordinances, there are numerous ways in which gases rising through the plumbing system may enter the house. The settling of floors and foundations may result in rendering soil-pipe joints defective; the soil piping is seldom properly supported, and often settles or sags through its own weight, causing the same kind of trouble. These and other conditions that might be named are of such universal occurrence that it is safe to say that only a comparatively small percentage of plumbing systems that have been in service for a term of years would be able to show perfect joints under test. Even though the plumbing system, with all its various connections, may be perfectly tight, still the danger of entrance of sewer gas is not always eliminated.
Whenever repairs are to be made on the soil piping or on branch wastes, sewer gas has a free passage until the repairs are completed.
Whenever the water closet is removed for repairs or to be renewed, sewer gas has a free entrance until it is replaced. Many other instances might be given in which the gases and odors from the sewer may find their way into the house.
The main trap is provided as a means of preventing this result.
The opponents of the main trap claim that it obstructs the flow of sewage through the house drain, that the trap will soon stop up, that in cold weather it will often freeze. These objections are not serious, and in many cases are more fancied than real. To be sure, the outflow is somewhat impeded by the trap, but the gain in providing protection to the house would much more than offset such difficulty. The strongest and practically the only real argument against the use of the main trap is that it prevents the ventilation of the public sewer through the roof pipe of the building. The weighing of the questions which arise in this connection is a very difficult matter.
In the first place it does not seem to be right to make a ventilating flue of each stack in each dwelling house, through which the sewer may throw its gases, to escape into the houses through defects and openings.
At the same time, the main drain and stacks present at present the most available means of ventilating the sewers, and are therefore often made use of. The closed sewer should not be tolerated, and the present method of venting the sewer through perforated manhole covers is open to serious objection, as it allows direct communication between the streets and the sewer. Special vent stacks should be erected at high points in the sewage system, through which the sewers might vent themselves, but such means are not provided, and therefore not to be considered.
It is claimed that where a free passage exists between the sewer and the outer air through the roof extension of the plumbing system, a circulation of air will be kept up, by means of which fresh air will be drawn into the sewer through the manhole covers, and the foul air drawn out through the roof pipes. If it were not for the matter of exposing the interior of the house to the admission of sewer gas this would unquestionably be an excellent plan.
Some go even further, and claim that enough fresh air would be drawn in through the manholes to render the gases harmless.
This does not seem reasonable when it is considered what a small area the manhole perforations really represent, and that a large percentage of these holes are closed up with dirt, ice and snow, etc.
If the house could be guaranteed against the entrance of gases, there are certainly many places in which the delivery of them into the air above the houses of the community would be followed by no harmful results.
In our towns and cities, however, with odors and gases escaping through every roof pipe, a heavy atmosphere must force them down to such points that they may often enter windows, light shafts, etc.
In our large cities, also, where low buildings adjoin high ones, it would seem very poor policy to banish the main trap, for without it the pipes through the roof of the lower building are constantly throwing their impurities out, to be drawn into the rooms on the higher floors of the high building.
That they would be drawn in in this way there is no question, as the circulation of the warmer air of the building would often create a suction sufficient to draw in the outer air.