All closed-loop hydronic systems require an air space to absorb the increased volume of water as it warms during system operation. In most hydronic systems, this task is handled by an expansion tank. In most modern hydronic systems, it’s handled by a diaphragm-type expansion tank.
The intended benefit of a diaphragm-type expansion tank is that it can be pre-pressurized with air before it is installed. The air is held captive in the space between the steel tank shell and flexible rubber diaphragm (Shown in Figure 1).