A smoke test forces non-toxic, artificially created smoke through waste and drain pipes under a slight pressure to find leaks. Plumes of smoke form where there are defects.
A smoke test can be performed when the plumbing is brand new, but more often it is used to find sewer gas leaks that may plague a building or an area. Any sign of smoke escaping can be considered a possible site for sewer gas to escape.
Sewer gas typically has a rotten egg smell and can contain methan gas which is explosive, or hydrogen sulfide gas which is deadly.
Plumbing smoke tests are also used to find places where pipes will spill fluid, and to check sanitary sewer systems for places where ground water and storm runoff can enter.
Smoke testing is particularly useful in places such as ventilated sanitary sewer systems, where completely sealing the system is not practical.
When smoke testing a sanitary sewer system, it is helpful to partially block off the section of sewer to be tested. This can be done by using a sand bag on the end of a rope. The sand bag is lowered into the manhole and swung into position to partially block lines. Completely blocking the line can cause water to back up and prevent smoke from escaping through defects.
Smoke testing may not be done after rain or when ground water is unusually high as this may also prevent detection of defects.
Plumes coming from plumbing vents or the interface between the fan shroud and manhole rim are normal, however smoke plumes outside of the manhole rim are not. Plumes are marked, usually with flags, and defects are noted using measurements from stationary landmarks like the corners of houses. The plumes or markers may also be photographed.