Large downdraft fans, usually powered by gasoline engines, are placed on top of open manholes at either end of the section to be tested. If possible all lines in the manholes except for the line between the manholes are partially blocked. Smoke is created using either a smoke bomb or liquid smoke. Smoke bombs are lit and placed on a grate or in a holder on top of each fan, while liquid smoke is injected into the fan via a heating chamber. The fans create a pressure differential that forces the smoke into the sewer at a pressure just above atmospheric. With properly installed plumbing, the traps will prevent the smoke from entering the house and redirect it out the plumbing vents. Defective plumbing systems or dry traps will allow smoke to enter the inside of the house.