What is the minimum size for a fresh air inlet relative to the building drain at the connection?

The correct size for a fresh air inlet in relation to the building drain is established to ensure proper ventilation and to allow for the adequate flow of air necessary to prevent siphoning of water from traps. When the fresh air inlet is 1/2 the diameter of the drain, it is generally large enough to prevent any negative pressure that could lead to a loss of water seal in traps. This ratio helps maintain the proper function of the plumbing system, as it allows fresh air to enter and alleviate pressure differentials that may occur, thus reducing the risk of sewer gas backflow.

In systems where the fresh air inlet does not meet this minimum size relative to the drain, there may be insufficient airflow, which could compromise the system's efficiency and effectiveness in venting sewer gases. Therefore, adhering to this guideline is crucial for maintaining proper plumbing practices and ensuring safety in the building.