BoilerFaultCodesUnical 21 Fault Code
The Unical 21 fault code means: your boiler is struggling to move water around the system correctly, which is preventing it from heating up properly.. It appears on 2 Unical models in our database.
Technical description: POOR wATER CIRCULATION Poor circulation in primary circuit
DIY fix possible
Check that your boiler pressure is between 1.0 and 1.5 barEnsure all radiator valves and internal stop taps are fully openBleed your radiators to remove any trapped air pockets
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Unical models that show the 21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Unical 21 fault code information on this page is sourced and cross-checked against the official Unical service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026