What is the purpose of an expansion tank in a hot-water heating system?

The purpose of an expansion tank in a hot-water heating system is to allow for water expansion. As water is heated, it expands in volume. Without a system to accommodate this expansion, pressure can build up in the system, potentially leading to damage or even failure of components such as pipes, valves, or the boiler itself.

The expansion tank serves as a buffer, providing a space where this expanded water can go, thereby helping to maintain proper pressure levels within the heating system. This is crucial for the safety and efficiency of the system because it prevents excessive pressure that could cause leaks or ruptures. Therefore, the correct answer highlights the primary function of the expansion tank in managing the physical properties of water as it heats, ensuring the longevity and reliability of the heating system.