expansietank

Expansietank, or expansion tank, is a pressure vessel used in closed heating, cooling or potable-water systems to accommodate the thermal expansion of water as it heats. It helps maintain stable system pressure and protects pumps, valves and other components from overpressure and damage.

There are two main designs. Bladder or membrane expansion tanks contain a flexible membrane that separates the water from a pressurized air chamber. Open, or vented, expansion tanks are used in older, open-vented systems and are connected to the atmosphere rather than sealed to the water side.

Operation: As the water in the system warms, it expands and slightly increases pressure. The extra volume is absorbed by the air cushion in the tank, preventing large pressure rises. The membrane keeps water from entering the air side; the tank must be pre-charged to a specified pressure when cold.

Installation and sizing: The tank is fitted to the cold-water inlet of a closed-loop system or to the boiler return line. Sizing depends on the system’s total water content and the maximum expected expansion, following manufacturer or code guidelines. A separate tank is typical for each heating circuit.

Maintenance and safety: Tanks should be inspected regularly for leaks or corrosion, and the pre-charge should be checked with a pressure gauge and adjusted to the recommended value when the system is depressurized. A waterlogged or failed tank should be replaced to restore proper operation.