What occurs if the expansion tank's sight glass is full and system pressure is high?

When the expansion tank's sight glass is full and the system pressure is high, the relief valve will open to prevent excessive pressure buildup in the system. Expansion tanks are designed to accommodate the expansion of water as it heats and to maintain balanced system pressure. When the pressure exceeds the set threshold due to thermal expansion or other factors, the relief valve acts as a safety measure, automatically opening to release excess pressure. This action prevents damage to the system, including pipes, tanks, and other components.

The other scenarios described do not adequately address the functions of an expansion tank. For instance, if water were to leak from the tank, that would indicate a potential failure or defect, rather than a normal operational response to high pressure. The possibility of the tank freezing is irrelevant in this situation, as freezing is generally a concern with low temperatures rather than high pressure. Lastly, air entering the system typically results from issues like a leak or improper sealing, rather than being a direct consequence of high system pressure indicated by a full sight glass. Therefore, the opening of the relief valve is the appropriate response to protect the system from high pressure.