Which vent type is used to relieve pressure by allowing air movement between drainage and vent systems?

• A. Dry Vent
• B. Main Vent
• C. Relief Vent
• D. Fresh Air Inlet

Answer: (C)

Relief vents exist specifically to relieve pressure differences in a drainage system by letting air move between the drainage line and the vent system. When water flows or sewer gases push on the system, pressure can build or drop, risking trap siphon or back pressure. The relief vent provides an air path to equalize those pressures, protecting trap seals and keeping the system functioning smoothly.

Other options describe different venting roles: a dry vent is a vent that carries no drainage water and serves venting in specific fixture configurations, a main vent serves the broader system rather than providing a localized air relief between drain and vent, and a fresh air inlet introduces outside air into the drainage system but isn’t the dedicated air-relief path between drainage and vent lines.