Medium Warning
5B:1 on Viessmann Vitodens 333-F

Your boiler is confused about where your hot water tank is located within the system pipework, often due to a setup error or a sensor problem.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

DHW cylinder connected downstream of the low loss header

What causes this 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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