When are high point vents closed during filling?

• A. When the system is water solid.
• B. When all air has been vented.
• C. When the system is half full.
• D. After pump starts.

Answer: (A)

High point vents are used to purge air that collects at the top of a system as it’s being filled with water. They stay open during filling so that any trapped air can escape. The vent is closed when the system becomes water solid—meaning it is completely filled with water and no air pockets remain at the high points. Closing at that moment prevents water from blowing out of the vent once the pump runs and avoids future air pockets or cavitation.

If you closed the vent earlier because you thought all air had been vented, there could still be tiny air voids, and those can cause issues once the system is pressurized. Waiting until the system is fully flooded with water ensures the entire loop is primed before sealing the vents. Opening the vent after the pump starts would defeat the purpose during filling and could lead to unnecessary water loss or air ingress.