This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
Drainage systems in all well-regulated cities are inspected and tested twice before being passed by the authorities as perfectly sanitary. These tests are: (1) A test of the roughing in, which consists of the iron or brass drains, soil, waste, and vent lines, and sometimes the fixture branches, also, before any pipes are concealed. (2) A test of the entire system when the fixtures are all set, the traps sealed with water, and the work otherwise complete.
The first or roughing test is accomplished by closing all branches and filling the system with water (if the weather permits), allowing the water to stand in the pipes for a certain time, depending on the inspector's judgment. Any leaks can be detected by water flowing from them. In applying this test, particular care must be taken to use plugs in the lower openings, which cannot be blown out by the heavy pressures that occur at these points.
Should the weather be too cold for the water test, the compressed-air test is applied. In this case air is pumped into the system until the pressure is 10 lb. by the gauge, when a valve between the pump and the system is closed. The presence of leaks is made manifest by the gauge indicating a decreasing pressure as the test continues. The location of leaks, however, is difficult unless some pungent volatile oil, as ether or oil of peppermint, is allowed to vaporize within the system and thus cause an odor in the vicinity of the leaks; or, the leaks may be located by the bubbles formed when a soap-and-water solution is applied to the joints with a brush.
The final test is the more important one. The chief objects are to positively ascertain (a) if the system when completed is gas-tight; (b) if the traps have perfect seals; (c) if every part of the system is trapped that should be trapped; (d) if any back-vent pipes are run into hollow partitions, attics, or chimneys. The test was formerly made by vaporizing oil of peppermint in the drainage system, without pressure, trusting to a diffusion of a pungent vapor to indicate any leaks. This form of peppermint test is now abandoned by modern sanitary engineers as untrustworthy, the smoke test being substituted. This is applied by blowing smoke through the system; when the smoke shows at the various vent outlets, they are closed tightly, and the drains are subjected to a smoke pressure of from 1 in. to 1 1/4 in. of water column. This pressure is sufficient to force smoke through the most minute leaks, but is not enough to blow through the seal of a good trap.
The best smoke machine for testing house drains consists of a double-action blower, combined with a smoke-generating chamber furnished with a balanced floating cover. Bach a blower will force air through the fire in a steady stream, and a uniform efflux of dense smoke is obtained. The advantage of the floating cover is that it will rise in its water seal when the desired pressure is obtained, and the pressure cannot be increased sufficiently to force the seals of traps, because the excess of smoke will escape to the atmosphere from under the cover. The smoke machine may be applied to the fresh-air inlet; or to one of the vent pipes above the roof - prefer-ably to the latter, as any smoke that escapes while lighting the fuel, which is oily cotton waste, cannot enter the building and thereby spoil the test.
House-drainage systems should be tested once a year, and a report of the sanitary condition should be furnished after each inspection and test. This action is made necessary by the fact that the plumbing is often abused to such an extent as to become dangerous; and settlement of buildings often causes leakage.