Medium Warning
6C:100 on Viessmann Vitodens 300-W

Your boiler's hot water pump is working as hard as it can to move heat, but something is preventing the water from cooling down or circulating correctly.

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

Set speed internal DHW loading pump 100%

What causes this 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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