Low Warning
5b:1 on Viessmann Vitodens 300-W

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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

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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