BoilerFaultCodesViessmann 5b:1 Fault Code
Also written as 5B1 — same fault.
The Viessmann 5b:1 fault code means: your boiler has detected that the hot water cylinder is incorrectly positioned in the system's plumbing layout, which may cause inefficient heating of your water.. It appears on 5 Viessmann models in our database.
Technical description: DHW cylinder connected downstream of the low loss header
What causes the Viessmann 5b:1 fault?
This fault occurs when the boiler's control system detects that the hot water cylinder is plumbed into the wrong part of the heating circuit, specifically after a device called a low loss header. It typically happens because the system's software settings don't match the physical pipework, or a temperature sensor is sending incorrect data back to the main board. Essentially, the boiler is trying to heat your water but cannot confirm the correct flow path to do so safely.
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 5b:1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 5b:1 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