What is the name of the device that prevents backflow in a drain that may experience back pressure?

• A. Backwater valve
• B. Check valve
• C. Air gap
• D. Relief valve

Answer: (A)

When a drain can experience back pressure from the sewer line, you need a device that acts as a one-way barrier in the drain. A backwater valve is designed for this exact scenario: it normally stays open to let waste flow out, but if pressure from downstream tries to push water back toward the building, the valve closes and prevents the backflow. This protects the home from sewer backflow and flooding.

A check valve serves a similar purpose on supply lines, but it’s not intended for sewer drains and can fail or foul when dealing with sewage. An air gap provides a physical separation to prevent backflow in potable-water systems rather than in a drain, so it doesn’t stop sewer backflow. A relief valve releases excess pressure within a vessel or piping, not restore or block reverse flow in a drain.

So, the device that best stops backflow in a drain under back pressure is the backwater valve.