What does the Unical 21 fault code mean?

Your boiler is temporarily stopping because water is not flowing properly through the system, which can cause it to overheat.

How much does it cost to fix Unical error code 21?

Typical repair cost for the Unical 21 fault is £150-£350 including labour.

Manufacturer manual

POOR WATER CIRCULATION Poor circulation in primary circuit. Check pump operation (12) and speed, if there are any obstructions or system closure.

From this model's manual

Verified against the Unical/Unical Alkon C 28 He installation & service manual.

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Boiler cover options

A cover plan would handle faults like 21: no call-out fees, no surprise bills.

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Seeing a different code? Every code this model can display, in one place.

On a combi it is usually the diverter valve. On a system boiler, the motorised valve.

Quarter turn on the valve, wait for water, then check the boiler pressure.

Each noise points to a different fault.