H:01 .21 High

Baxi H:01 .21 Fault Code

Also written as H0121 — same fault.

The Baxi H:01 .21 fault code means: your boiler is struggling to control the temperature of your hot water, causing it to heat up much faster than it safely should.. It appears on 8 Baxi 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: DHW flow temperature increasing too fast
Typical repair cost £150-250 (including labour)

Engineer required

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

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Baxi models that show the H:01 .21 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Baxi H:01 .21 fault code information on this page is sourced and cross-checked against the official Baxi service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026