Which device in venting system helps prevent back-pressure and siphonage, protecting traps?

Which device in venting system helps prevent back-pressure and siphonage, protecting traps?

• A. Thermostatic Mixing Valve
• B. Vent Cap
• C. Air Admittance Valve
• D. Hydraulic Jack

Answer: (C)

Air pressure balance in the drainage system is what keeps trap seals from being siphoned or overwhelmed by downstream pressure. An air admittance valve serves this purpose as a one-way valve installed on the drain. When flowing water creates negative pressure behind the moving column, the valve opens and lets air in, equalizing pressure so the trap water isn’t sucked out. It stays closed to prevent sewer gases from escaping when there isn’t a need for air. This mechanism protects traps without requiring vent pipes that vent to the outdoors. The other options don’t fit this role: a thermostatic mixing valve focuses on water temperature, a vent cap simply covers a vent without actively balancing pressure, and a hydraulic jack is unrelated to venting.

Air pressure balance in the drainage system is what keeps trap seals from being siphoned or overwhelmed by downstream pressure. An air admittance valve serves this purpose as a one-way valve installed on the drain. When flowing water creates negative pressure behind the moving column, the valve opens and lets air in, equalizing pressure so the trap water isn’t sucked out. It stays closed to prevent sewer gases from escaping when there isn’t a need for air. This mechanism protects traps without requiring vent pipes that vent to the outdoors. The other options don’t fit this role: a thermostatic mixing valve focuses on water temperature, a vent cap simply covers a vent without actively balancing pressure, and a hydraulic jack is unrelated to venting.