The purpose of an expansion tank is to?

The purpose of an expansion tank is to provide for the expansion and contraction of the water volume as the temperature varies. As water is heated, it expands. In a closed heating system, this expansion increases the pressure within the system. An expansion tank acts as a buffer, accommodating this expansion to maintain safe pressure levels and preventing potential damage to the system. It allows space for the increased volume of water caused by heating, ensuring the system operates efficiently and safely.

When the water cools and contracts, the expansion tank can also help maintain pressure equilibrium. This functionality is critical in ensuring the longevity of the heating system and its components. Other options mentioned do not pertain directly to the primary function of an expansion tank in a hydronic or steam heating system.