A condensate drain trap is used to prevent sewer gas form escaping in the drainage of hvac systems. They are installed in the condensation relief line. These traps also prevent the accidental leakage form condensation overflow. They are available in either positive or negative flow styles. Drain traps come in abs and pvc tubing and are available at most hvac supply houses. More about condensation
. drain traps can be found on this site http://www.drainplus.com/.