Why an expansion vessel is needed
Water expands as it is heated. In a sealed circuit, an expansion vessel provides a compressible volume to accommodate that change and keep pressure within the intended operating range.
What happens when it is not working correctly?
If a vessel loses its pre-charge, is incorrectly sized or has a failed diaphragm, pressure may rise excessively as the system heats. This can lead to safety-valve discharge followed by low pressure when the system cools.
Do heat pumps have more than one vessel?
They can. Ground source systems may have separate expansion arrangements for the heating circuit and collector/brine circuit. Hot-water systems can also incorporate separate expansion equipment.
Diagnosis matters
Repeated pressure loss is not proof of an expansion-vessel fault; leaks and other issues can cause similar symptoms. The system should be tested rather than components replaced by assumption.
Our ground source experience
Envirowarm's NIBE servicing work has included checking expansion vessels, brine circuits and heating pressures. Read our ground source servicing guide.