21 High

Unical 21 Fault Code

The Unical 21 fault code means: your boiler is temporarily stopping because water is not flowing properly through the system, which can cause it to overheat.. It appears on 2 Unical models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: POOR WATER CIRCULATION Poor circulation in primary circuit. Check pump operation (12) and speed, if there are any obstructions or system closure.
Typical repair cost £150-£350 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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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 31 August 2026