What could cause an expansion tank to fill with water?

An expansion tank can fill with water primarily due to a lack of sufficient air cushion inside the tank, which is often caused by air leaks. If there are leaks from fittings such as the gauge glass fitting, it can allow air to escape while water enters. This reduces the volume of air that helps to absorb pressure changes in the system. Consequently, as the system heats up and water expands, there isn't enough air space to accommodate this expansion, leading to the tank filling with water.

Understanding the role of an expansion tank is crucial. Its primary function is to absorb the excess pressure created by heated water, which helps maintain system efficiency and safety. If air is allowed to escape, this function is compromised, and water will fill the tank rather than air, resulting in potential operational issues for the boiler system.