How is the expansion tank affected if the isolating valve for it is closed?

When the isolating valve for the expansion tank is closed, the water in the heating system can no longer flow into or out of the tank. This prevents the tank from receiving excess water, especially during periods when the water heats up and expands, which may lead to an increase in pressure within the system.

In most systems, the expansion tank is designed to accommodate this expansion of water. However, if the isolating valve is closed, any additional water that cannot exit due to thermal expansion will cause the expansion tank to fill up with water.

This situation can lead to a risk of creating excess pressure in the system, as the tank cannot properly perform its function of relieving pressure. Therefore, the correct understanding is that when the isolating valve is closed, the expansion tank will fill with water, as it cannot discharge the expanding water to maintain system balance.