A plumber is inspecting a residential hydronic heating system and notes that the system pressure repeatedly drops below the minimum operating pressure of 85 kPa (12 psi) when the system cools down. There is no visible leakage. What is the most likely cause of this pressure drop?

Question: A plumber is inspecting a residential hydronic heating system and notes that the system pressure repeatedly drops below the minimum operating pressure of 85 kPa (12 psi) when the system cools down. There is no visible leakage. What is the most likely cause of this pressure drop?

Answer options: ✅ An improperly charged or failed expansion tank.

• The automatic fill valve is stuck open.
• Air in the system that wasn't properly purged.
• A faulty pressure relief valve.

Correct answer: An improperly charged or failed expansion tank.

Explanation: An improperly charged or failed expansion tank means it cannot absorb the water's volume changes effectively, leading to pressure fluctuations. If it is undercharged or failed, it won't push water back into the system as it cools, causing pressure to drop. An open fill valve would cause pressure to rise, not drop. While air can cause circulation issues, it's less likely to be the primary cause of repeated pressure drops below minimum operating unless it's being continuously introduced and compressing. A faulty pressure relief valve would typically weep or open at inappropriate times, but not cause the system pressure to drop below minimum when cooling, unless it's constantly leaking, which is a form of leak.