When water is heated, the space required for each molecule increases. Any attempt to prevent this expansion will be met by tremendous forces. If a strong metal container is completely filled with liquid water and sealed from the atmosphere, it will experience a rapid increase in pressure as the water is heated. If this pressure is allowed to build, that container will eventually burst. 

To prevent such a result, closed-loop hydronic systems are equipped with an expansion tank. The tank provides a “cushion” of air — a highly compressible fluid — against which the expanding water can push without creating large pressure increases in the system. Think of the air in the tank as a spring. As the system’s water expands, this “spring” gets compressed. When the water cools and contracts, the “spring” returns to its original condition.