When is the installation of a "P" trap mandatory in a drain line?

The installation of a "P" trap in a drain line is mandatory when the evaporator coil is located on the negative side of the blower. This is important because the negative pressure created by the airflow can cause the water in the drain line to be siphoned out, leading to potential air leaks and allowing contaminants to enter the duct system. The "P" trap serves to maintain a water seal, which prevents airflow from pulling out the water from the trap and helps to keep the drain line functioning properly by ensuring that it does not become obstructed.

In general, a "P" trap is a critical plumbing feature designed to retain a small amount of water, which acts as a barrier to gases that could escape from the sewer system into the occupied space. This protects indoor air quality and helps ensure the efficacy of the drainage system. Proper installation of the "P" trap also helps to minimize the risk of clogs and allows for better drainage of condensate that naturally results from the cooling process in HVAC systems.