What is a primary reason traps are not considered legal in plumbing?

Traps are a crucial component of plumbing systems, serving to prevent sewer gases from entering buildings by creating a water seal. However, if traps are not vented, it can lead to various problems that compromise their functionality.

Unvented traps can cause a vacuum effect, which prevents wastewater from flowing freely. This can result in siphoning, where the water seal is drawn out, allowing harmful sewer gases to escape into the living environment. Additionally, without proper venting, the airflow needed to equalize pressure in the drainage system is disrupted, potentially leading to slow drainage or backflow issues.

The importance of venting in maintaining the integrity of the plumbing system is why this is seen as a primary reason for traps being considered illegal in certain contexts. Proper venting is essential to ensure that traps work as intended and do not pose health risks or system failures.