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The most relevant reason for an expansion tank filling with water is due to air leaks from the gauge glass fitting. Expansion tanks are designed to accommodate the expansion of water as it heats up, allowing for the absorption of excess pressure in a hydronic heating system. When the system is functioning properly, the tank should contain a mix of water and air, with the air acting as a cushion to absorb the pressure changes.

When there are air leaks, particularly from fittings like the gauge glass, air can escape from the system. This leakage reduces the air cushion effect, leading to higher water levels in the expansion tank as the system compensates for the loss of air. Consequently, without adequate air in the tank, it can become filled predominantly with water, leading to possible performance issues within the boiler system.

In contrast, options such as insufficient pressure, a faulty expansion valve, and a blocked drainage system would not directly cause the expansion tank to fill with water in the same manner. Insufficient pressure may affect the entire system's operation but does not inherently lead to excessive water in the expansion tank. A faulty expansion valve might not open or close appropriately, but this typically leads to other issues within the system rather than filling the expansion tank with water. Lastly, a blocked