What is the function of an expansion tank in a closed-loop system?

The function of an expansion tank in a closed-loop system is to absorb excess pressure caused by thermal expansion of the water. In a closed-loop HVAC or hydronic heating system, as water heats up, it expands. This increase in volume can lead to a rise in pressure which, if not managed, can damage the system components or lead to leaks.

The expansion tank accommodates this expansion by providing an additional space for the excess water to flow into, thereby preventing a dangerous increase in pressure. It typically contains a diaphragm or bladder that separates the air cushion from the water side, allowing for the absorption of the expanded water while maintaining system pressure at safe levels. This helps maintain the integrity and efficiency of the system, ensuring it operates smoothly and safely.