The air test is applied by closing all openings, and then forcing air into the drainage system by means of an air compressor, until there is a uniform pressure of about 10 pounds per square inch throughout the entire system. Any leakage will be indicated by the air gauge, which will show a falling pressure. This method of testing is used principally in cold climates, as it removes all possibility of damage from frost.

In warm climates the air test is seldom applied, chiefly on account of the difficulty in locating leaks. When applying an air test the first place to examine for leaks is around the testing apparatus. This being tight, the testing plugs should next be examined and made tight, after which the house drain, and then the soil waste and vent stacks should be examined. If the system leaks badly dash a bucketful of soap suds on the top of each stack in turn, and watch for bubbles as the soap suds follow down the stack. When the large leaks are located and made tight, the smaller ones can be found by daubing soap suds on the pipes and joints with a paint brush. The pressure must be maintained within the system during the search for leaks, otherwise bubbles will not form when suds are applied to a leak.

Final Tests are applied to drainage systems after all the fixtures have been permanently set and connected and the water turned on. Care should be taken to seal the traps with water, so that the smoke or chemical used in this test cannot blow through.

There are two kinds of final tests applied to drainage systems: The peppermint test and the smoke test. The peppermint test is applied, after closing the fresh air inlet, by emptying two ounces of oil of peppermint into each stack from the roof, then pouring a gallon of boiling water into each stack to vaporize the oil - after the water is poured in, the vent stacks are plugged, and leaks sought for inside of the building by the sense of smell. When buildings are over five stories in height two additional ounces of peppermint should be used for each additional five stories in height. Oil of peppermint should not be confused with essence of peppermint, which is a weak solution of oil and water, and possesses but a fraction of the strength of oil of peppermint.

The peppermint test is very unreliable, and cannot be depended upon to indicate small leaks, as it does not create a pressure within the system.

Smoke Test

A more reliable final test is the smoke test. It is applied by pumping the system full of a dense pungent smoke under a slight pressure, by means of a smoke machine. The pressure should be only sufficient to balance a column of water one inch high. This is sufficient pressure to develop any leaks, and is not enough to force the trap seals.