The waste pipe or outlet of every plumbing fixture must have, as near as possible to the fixture, a trap, to prevent foul odors and sewer gas from issuing. The simplest form of trap, and in its improved forms one of the best, is the S-trap. (Fig. 44.) This consists of a piece of pipe bent into the form of the letter S, with a screw in or near the bottom to allow of cleaning out. The theory of this trap is that some of the water which passes through will remain in the trap, as shown in the cut. In actual use, however, if enough in volume runs through to completely fill the outlet, the falling water will create a vacuum at A, which will cause the outside air to force out the water in the trap until it falls below the bend B, thus destroying the seal. If, however, an inlet is provided at the highest point of the trap A, the vacuum in the outlet pipe will be filled from this source without disturbing the water in the trap. This inlet pipe must, of course, be carried to some main air pipe; and so it is customary to run beside the main soil pipe a line of "vent" pipes to which the different trap vents are attached, the main vent usually starting from the base of the soil pipe and entering the same again above the highest fixture. A greater danger exists where two or more fixtures are connected with the same soil pipe. In this case the seal of the lower fixture will be broken by reason of the column of water discharged from the upper fixture, creating a partial vacuum in the soil pipe and the outer air rushing in to fill the void by the easiest way, will force the water out of the trap below. (Fig. 45.) This is a real danger, and should be guarded against by venting the trap as described. Another way of guarding against syphonage is to make the trap so large that enough water will drain back from the outlet and sides of the trap to restore the seal. This is called a pot or cesspool trap. City plumbing laws in general require the venting of all traps, which should be done in all cases.


When the waste and air pipes are all in place and the connections for the various fixtures are put in, the whole system should be exposed to an effective test. The simplest and most efficient to employ at this stage is the water test called for by our specifications. This consists in closing all openings in the pipes and filling the whole system with water to the top. This may be done by attaching a hose at the bottom or filling from the top. The superintendent should be on hand to see that the water does not lower in the pipe after standing ten or fifteen minutes, when the system may be pronounced tight. If however, the water level drops it is evidence of a leak somewhere which must be sought out at once. In a tall line of piping the water pressure will be such that defective joints or sand holes in the piping will be discovered by a stream of water, or at least by the trickling of water down the pipe. Imperfect joints, in the cast iron pipes may be remedied by a more careful caulking, but the presence of sand holes can only be remedied by replacing the defective pipes which should be subjected again to the test until the system proves absolutely tight. After all fixtures are set and all connections made another test is usually made. This may be the "peppermint test" or the "smoke test". In the former test a vial of oil of peppermint, which is sold sealed for the purpose, is taken to the roof and the contents poured into the top of the soil pipe. A quantity of hot water is immediately poured in and the top of the pipe closed by stuffing in paper or rags. The vapor, charged with the odor of peppermint, is thus unable to escape and will penetrate the whole system.

Fig. 44. S Trap.

Fig. 44. S-Trap.

Waste Pipes And Traps 010057Fig. 45. Syphonage of Trap.

Fig. 45. Syphonage of Trap.

All drain, air or waste pipes and connections are immediately examined and any odor of peppermint detected will be evidence of a defect which must be remedied. Great care must be taken in applying this test which should be done by separate parties, one outside and one inside of the house, and no direct communication should be held until the test is pronounced satisfactory.

Another method of applying the peppermint test is to close all vent pipes and vaporize the oil of peppermint in the receiver of a small air pump, and then force the vapor into the system under pressure. The receiver is provided with a gauge so that any leak will cause a fall in the mercury and can then be located by the odor of the peppermint.

The smoke test is required in our case and is done by closing the system as for the peppermint test and forcing into the pipes smoke from oily waste or some similar substance by means of a bellows. When the pipes are filled and a slight pressure produced it is shown by a float which rises and remains in this position if the joints are tight. If there is a leak the float falls as soon as the bellows are stopped and the leak may be located by the issuing of smoke from the joint. Special machines are to be bought for making these tests, which should be done in the presence of the architect or superintendent.