An expansion vessel is a safety and comfort component that compensates for volume variations due to the expansion and contraction of water in heating and cooling systems, thus maintaining system pressure within a controlled range. In domestic hot water systems, the specific volume increases when the water temperature rises from 10°C to 80°C. If this increase is not absorbed by an expansion vessel, it causes the safety valves to open, putting excessive strain on the equipment and generating noise and pressure fluctuations. The expansion vessel safely absorbs this volume, reducing pressure variations and ensuring the proper functioning of pumps, boilers, and heat exchangers.

Open expansion vessels operate in open air. In conventional fixed-height heating systems, they naturally reduce pressure to atmospheric level, necessitating the installation of pipes on the roof, thus increasing heat loss and the risk of evaporation. Closed expansion vessels, on the other hand, maintain pressure within a controlled range using a pressure gauge and a safety valve. By preventing contact between the water and air, they limit corrosion and evaporative losses. Their compact installation near the boiler room reduces piping and insulation costs. The closed model is preferred in most current applications; the open model is used in specific cases dictated by architectural and operational constraints.

In closed tanks, the internal volume is divided by a flexible membrane separating the water compartment from the gas compartment. When the system heats up, the water stretches the membrane, compressing the gas compartment and causing a controlled increase in pressure. Conversely, during cooling, the water is extracted, and the gas compartment refills the tank. The proper operation of this mechanism depends on adjusting the membrane's pre-charge pressure to match the static pressure. The pre-charge value is generally chosen to be slightly lower than the static pressure, taking into account the head at the highest point of the system. Too low a value would lead to premature wear of the membrane, while too high a value would reduce the tank's capacity. Before commissioning, the pre-charge must be checked using a pressure gauge when the system pressure is zero.

Three essential pieces of information are needed for sizing: the total volume of water circulating in the system, the minimum and maximum operating temperatures, and the operating pressure range defined by the safety valve setting. Water expands by approximately 3 to 4% between 10 and 80 °C; the volume required to compensate for this expansion is determined based on the tank capacity. A practical method for initial sizing is to multiply the total water volume by the estimated percentage of expansion, then divide the result by the tank's acceptance rate. The final selection should be made taking into account the manufacturer's curves, the preload, and the maximum operating pressure. The pressure class is selected based on the system's safety valve setting and the maximum possible peak pressures.

The closed expansion vessel must be installed in a vibration-free and easily accessible location near the boiler or heat generator. The connections must be equipped with shut-off valves, a pressure gauge, and a safety valve. Proper positioning of automatic filling units, air separators, and sediment filters contributes to pressure stability; thermal expansion protection devices must also be considered for domestic hot water production applications. The load-bearing capacity of the vessel's feet, insulation and condensation control in freezing environments, the necessary clearance for maintenance, and the doorway dimensions must be defined during the project design phase.