A boiler pressure relief valve that is dispensing intermittently is most likely caused by which condition?

• A. Relief valve seat is faulty
• B. Expansion tank being water logged
• C. Water is too hot
• D. Valve is stuck open

Answer: (B)

The situation tests how a boiler handles thermal expansion and how the expansion tank affects pressure control. As boiler water heats, its volume expands. The expansion tank provides an air cushion to absorb this extra volume so pressure stays within safe limits. If the expansion tank is waterlogged, that cushion is lost—the tank is filled with water and cannot absorb expansion. As the boiler cycles on and off, the lack of expansion space lets pressure spike whenever heat is added, triggering the pressure relief valve to vent. Then as the system cools and pressure drops, the valve closes until the next heating cycle, causing the relief to vent intermittently.

If the valve seat were faulty, you’d expect either constant dripping or leakage at low pressure, not tied to the heating cycle. If the water were too hot, the valve would tend to relieve more consistently while the water remains hot. If the valve were stuck open, it would keep discharging continuously. The intermittent venting matches the pattern you’d see when the expansion tank cannot absorb thermal expansion.