WI T H unventi-lated traps evaporation of their seal goes on with extreme slowness, and with such traps containing a considerable body of water, no danger from this cause need be anticipated unless the building be left unoccupied and unwatched for years at a time. If the trap is adequately ventilated, however, its seal will be gradually lowered, and if the ventilating flue connects with the trap at or near the crown, the induced air current licks up the water with a speed proportional to the efficiency with which it performs its duty as a ventilator, and will destroy the seal of an ordinary small S trap in from four days to one or more weeks, according to the rapidity and dryness of the current. If the flue connects with the trap at any point below its seal it is powerless to prevent self-siphonage.

Fig. 279. Diagram illustrating the Air Movements

Fig. 279. Diagram illustrating the Air Movements above the Water Seal of a Back Vented S-Trap, and of a non-vented Antisyphon Trap. The former quickly losing and the latter retaining its seal.

Some years ago, after the enactment of the trap vent law, the Brooklyn newspapers published, in the interest of the public, the following warning, showing that the officers of the Public Health Department had already experienced some of the evils of the trap vent law: "The Commissioner of Health desires to direct the attention of householders to the danger of occupying houses which have been for any considerable time without occupants. It is the practice of many persons to leave their dwellings for several months in the summer, during which time the water in the traps of basins and other fixtures is liable to evaporate, and thus permit the unobstructed entrance of sewer gas into the dwellings. When the families return in the fall they are exposed to these sewer emanations, and it is believed that cases of serious, if not, indeed, fatal sickness have been due to this cause. Whenever houses are so left, provision should be made to have the traps filled at least every two weeks by a competent and trustworthy plumber, and if possible to have all the windows of the house opened for twenty-four hours before its re-occupation." This notice was given the public after the city had fastened upon them the trap vent law. They then felt constrained to advise every house owner, under peril of serious danger to health, to employ some honest and skilful expert to stand constantly on the alert to undo the danger done by this law. We will not calculate here the number of traps owned by the several million inhabitants of Greater New York, nor the exact size of the army of trustworthy plumbers which would be required to overhaul dwellings for the purpose of attending to apparatus intended to be made by the new law automatic. But were all the traps ventilated in accordance with this law, it is certain that their fortnightly refilling and the opening of all the house windows in consequence of their not having been refilled weekly would consume infinitely more time than would be required to clean out the same number of unventilated pot traps, if their use was the only alternative, as often as their condition required, and, as a collector of greasy sediment, the vent pipe is no better than an elongated cesspool trap itself. Had the Commissioner of Health published advice to the householders to employ some competent and trustworthy plumber to cork up or tear out once and for all from their houses all these incompetent and untrustworthy back vent pipes which were responsible for the trap seal evaporation, instead of raiding the premises every fortnight to try to repair in a measure the mischief they had done, he would have performed a really valuable service to the alarmed and unhappy absentees on their summer vacations, and have reassured the much-suffering public at large at the same time.

Sometimes an owner is advised to fill all the traps with oil as security against evaporation. This method is troublesome and expensive, and should the house owner or any of his friends or employes happen to require to visit the premises during the season of its inoccupancy, he would be obliged to obtain or carry with him on each occasion a gallon or so of oil and refill the traps of any fixtures used during such a visit. Inasmuch as such phenomenal foresight could hardly be expected, especially in cases of unexpected or unforeseen visits, the practical objections to this method are sufficiently obvious, though it often sounds well to propose it.

While it is evident that back venting must gradually destroy the seal it was commissioned to protect, it is also evident, as an examination of our Figure 279 will make clear, that a pot trap or an anti-siphon of good water capacity is capable of preserving its seal against loss by evaporation during an entire winter or summer season if unvented.

If vented, a current of air is constantly in motion over the seal when the vent pipe is actually performing the office for which it is installed, and it carries off vapor from the seal as fast as it rises.

One may easily test this by filling a large pot trap having arms long enough to reach to the floor and to a basin and placing it by the side of a cup of water as shown in Fig. 279. In my experiments for the City Board of Health the evaporation is shown to be very rapid from the sewer side of a vented trap and almost nothing from the house side of the same trap.

Experiments for the Board of Health on Evaporation

Produced by Back Venting.

These experiments were made on the basement floor. They were subdivided into (A) those in which the vent pipe was conducted into a cold flue, and (B) those in which it was conducted into a heated flue.

(A) Tests with a Cold Ventilating Flue. A 1-inch S trap having a seal 4 5/8 inches deep was attached to the end of the branch waste in the manner shown in Fig. 242, A 1 -inch rubber ventilatng pipe was taken from the 1-inch ventilating opening at the crown of the trap and conducted into a cold 4-inch galvanized iron ventilating flue, shown in the drawing. This flue passed through two occupied offices (basement and first floor) whose temperature was maintained at about 68 degrees F. during the term of the experiments, and through a chemical laboratory (second floor) whose temperature was maintained at about 60 degrees F. For the remainder of its height the flue passed through a Cellar and stairways whose temperature was maintained at about 45 degrees F. No artificial heat was applied to the flue. The velocity of the movement of the current of air in the flue was measured by the anemometer. The daily rate of loss of seal by evaporation and the velocity of the current in feet per minute is shown in Fig. 281.