What is the definition of a relief vent?

A relief vent is defined as a vent whose primary function is to provide the circulation of air between drainage and vent systems. This role is essential in plumbing systems, as it helps to maintain proper air circulation and pressure balance, preventing vacuum or siphoning conditions that could hinder drainage flow.

By ensuring that air can freely enter and exit the drainage system, a relief vent reduces the chances of water seals in trap fixtures being lost due to negative pressure. This helps in preventing the backflow of sewer gases into occupied spaces and enhances the overall efficiency of the plumbing system.

The other options are not accurate in capturing the full scope of what a relief vent does. While drainage flow and gases are important aspects of venting systems, a relief vent's main purpose is to facilitate air circulation, which in turn supports the functionality of the entire drainage system.