BoilerFaultCodesViessmann 6C:100 Fault Code
Also written as 6C100 — same fault.
The Viessmann 6C:100 fault code means: your boiler's internal pump for hot water is stuck running at full speed, which usually means there is a communication error or a part is beginning to fail.. It appears on 5 Viessmann models in our database.
Technical description: Set speed internal DHW loading pump 100%
What causes the Viessmann 6C:100 fault?
This fault occurs when the boiler's control board loses its ability to regulate the speed of the internal hot water pump, causing it to run at maximum power constantly. The most common reason is a communication failure between the pump and the main computer, often caused by a loose wiring connection or a failing electronic controller within the pump itself.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Viessmann models that show the 6C:100 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 6C: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