21:1 to 21:100 Low

Viessmann 21:1 to 21:100 Fault Code

Also written as 211TO21100 — same fault.

The Viessmann 21:1 to 21:100 fault code means: your boiler is reminding you that it is due for its routine annual safety service based on the number of hours it has been running.. It appears on 1 Viessmann model in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: The number of hours run before the burner should be serviced is adjustable from 100 to 10000 hours (each step represents 100 hours)
Typical repair cost £100-£180 (including labour)

Engineer required

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

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Viessmann models that show the 21:1 to 21:100 fault code

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

Source

Viessmann 21:1 to 21:100 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026