Expansietanks

Expansietanks are pressure vessels used in closed fluid systems to accommodate the thermal expansion of the contained liquid. When a heating or cooling system warms water, its volume increases; without space for this expansion, system pressure can rise enough to trigger relief valves, damage components, or cause leaks. The expansion tank provides a dedicated volume for the extra water, using a compressible gas or a flexible membrane to absorb the pressure increase.

There are two main configurations. Closed expansion tanks contain a diaphragm or bladder that separates the system water from an air or gas chamber, allowing the expanding water to press on the membrane while keeping the fluid isolated. Open expansion tanks, common in older or gravity-fed installations, are vented to atmosphere and do not require a pre-charged gas chamber. In some systems, a simple metal tank without a membrane can also serve as an expansion reservoir, but it is less common in modern installations.

Sizing and placement are guided by system volume, the maximum operating pressure, and the expected temperature rise. The pre-charge pressure in the tank’s air chamber should be set below the system’s cold operating pressure. Tanks are typically installed near the boiler, heat exchanger, or pump discharge to minimize piping losses and simplify maintenance.

Maintenance involves regular inspection for corrosion, leaks, or membrane failure, and periodic verification and adjustment of the pre-charge pressure. If a tank loses its membrane or becomes waterlogged, it should be replaced to restore proper protection against pressure spikes.