When in doubt, what should be installed at the bottom of a suction line riser?

• A. Check Valve
• B. P-Trap
• C. Service Port
• D. Balancing Valve

Answer: (B)

The key idea here is creating a barrier to prevent gases or odors from moving up the suction line. Placing a P-trap at the bottom of a suction line riser provides a water seal that blocks any sewer gases or air from traveling up the line into the space or system. A trap holds a small amount of water, and that water barrier stops gases from passing through, which is why it’s the sensible choice when you’re dealing with a vertical suction run that could otherwise vent to the area.

A check valve protects against backflow, but its purpose is not to seal against gases or odors in the suction line. A service port is for testing or charging, not for sealing the line. A balancing valve adjusts flow, not provide a protective seal against gases. If the trap could be siphoned by pressure changes, it would also need proper venting or a trap primer to maintain the seal.