If the building drain is 4 inches, what should be the size of the loop vent?

In plumbing systems, the size of a loop vent should generally be smaller than the size of the building drain it serves. The loop vent's primary function is to provide air to prevent siphoning of water from traps and to maintain proper plumbing system pressure.

For a building drain that measures 4 inches in diameter, standard plumbing codes typically recommend that the loop vent be sized at one-half or one-third the diameter of the building drain. Thus, a 2-inch vent is appropriate, as it allows for adequate air flow while complying with code requirements.

Choosing a 3-inch or 4-inch vent would be unnecessarily large for this application and could lead to functional issues. A 1-inch vent would be insufficient to maintain the necessary air pressure and could hinder the drainage system's ability to function properly.