142. All of the drains, soil pipes, and waste pipes which are wholly or partly inside of a dwelling or public building should be kept free from accumulation of foul gases or odors, and should be so freely vented that the water seals cannot be sucked out of the traps by siphonage, nor blown out by back pressure.

The necessity of ventilating all drain, soil, and waste pipes in a building is evident when we consider that no matter how well these pipes are cleansed by flushes of water passing through them, there will always be accumulations of solid matter upon their interior surfaces. These deposits decompose and evolve gases which are dangerous to a greater or less extent, according to their composition. In order to convey these gases from the pipe system, and allow them to discharge into the atmosphere, the vertical stacks are continued full size or larger up to and through the roof of the building where they terminate with open ends. A branch, known as the fresh-air inlet, taken from the lower part of the house-drainage system, is led to the atmosphere at another point for the purpose of admitting a supply of fresh air to take the place of the foul air that is ejected above the roof.

143. Fig. 56 shows a sectional elevation of a drainage system, and will be sufficient to illustrate the principles of drainage ventilation. The water closet A discharges into the soil-pipe stack B through the soil-pipe branch a. The bath C discharges into B through the waste pipe b. Drain air is prevented from entering the building by the bath trap c and the water-closet trap, which forms part of the closet, and which is molded to it. A main disconnecting trap D prevents gases generated within the sewer E from flowing through the house-drainage system. The top of the soil-pipe stack B and the orifice of the fresh-air inlet pipe F are open to the atmosphere; consequently, when the air or gases in the drainage system are lighter than an equal volume of the outer atmosphere, they will flow up through the system and discharge into the atmosphere above the roof, as shown by the arrows. They are displaced by a volume of fresh air flowing under atmospheric pressure into the system through the pipe F. This fresh air mixes with the gases, and, in turn, is soon ejected from the system. The velocity of a current of air flowing through a drainage system will vary considerably with changes in temperature, with the pressure upon the inlet or outlet orifices, etc. In many cases, the currents are reversed; that is, the drain air is forced out of the fresh-air inlet pipe. Such would be the case if the water discharged from the fixtures should form a nearly solid plug while flowing through the pipe. The effect of the flush would be to force the drain air ahead. For this reason great care should be taken in selecting the point of fresh-air inlet. The branch pipes a and b are ventilated by back-vent pipes d and e, which join the main stack at their highest points. The principal use of these, however, is to prevent the siphoning of the traps when water flows through a or b.

Fig. 56.

144. The location of the fresh-air inlet is a matter of very great importance, and many things must be considered in selecting it. If the current of air which flows through the fresh-air inlet pipe always flowed inwards, the selection of the location of the inlet would not be such an important matter, but there are times when a blow-back actually occurs at the inlet orifice, which then becomes a temporary foul-air outlet. In order to prevent the drain air so blown back from being a dangerous nuisance to the inmates of the building, many of the health department laws compel the inlet orifices to be at least 15 feet from any window or door. This, of course, is to prevent blow-backs from entering the building.

Since the main house drains from nearly all city buildings are continued through the front wall and join the street sewer, and the fresh-air inlet pipe is, consequently, run from the main house drain at the front of the building, it stands to reason that if the inlet orifice must be 15 feet or more from the windows, the inlet of fresh air must be taken from a point near the street curb, or from a point above the building. It may be done either way, but the former method is usually employed. When taken from the curb, particular care must be exercised to avoid arranging the inlet orifice in such a manner that dirt or street sweepings may at any time enter and choke the pipe, thereby cutting off the flow of fresh air to the system.

145. A very common method of running the fresh-air inlet is shown in Fig. 57. The inlet pipe a, which joins the house side of the main disconnecting trap in the cellar, is connected to a curb box b, as shown. The top of this box is sunk flush with the sidewalk and is fitted with a movable grating which can be lifted out or swung over the hinges to facilitate the cleaning of the box.

The chief objection to this method is that the box will gradually fill up with pavement sweepings, as shown, and the air will thus be choked off. It is also very much affected by snow storms. Even though the pavement is swept clean after a fall of snow, the perforations of the grating will remain clogged with snow and ice. In fact this form of fresh-air inlet, if not attended to, becomes practically useless.

To prevent any chokage by dirt, the pipe a is sometimes continued through the curbstone c, thus opening into the street gutter, and is provided with a grating over its mouth, which is set flush with the face of the stone. While this removes the objection to dirt accumulation, it is usually entirely closed while snow lies in the streets, the gutters being made the receptacles for the snow until it melts or is carted away.

Probably the best arrangement that can be employed for the average city building is the perforated hollow hitch-ing-post arrangement, or a perforated hollow stepping-block arrangement placed over the mouth of the fresh-air inlet pipe.

146. In order to have the drainage system efficiently ventilated, it is necessary that the fresh-air inlet be the full size of the main drain, and that its caliber be unobstructed. When a grating is used over the inlet, the perforations should be at least equal in area to the sectional area of the pipe itself. This will reduce the resistance to the inflowing air to the minimum.

Fig. 57.

To make a fresh-air inlet pipe self-cleaning, it should have a pitch down to the main house drain greater than the angle of repose for dirt, say a little over 45 degrees. The dirt falling in will then slide down into the house drain and be carried to the sewer by discharges from the house fixtures.

Another method is to so arrange the pipes that periodical flushes of clean water will pass down the fresh-air inlet pipe, and so carry away all dirt accumulations. This may be accomplished by discharging area storm water, or roof water, into the inlet pipe above the dirt accumulations.