9b:70 Low

Viessmann 9b:70 Fault Code

Also written as 9B70 — same fault.

The Viessmann 9b:70 fault code means: your boiler is notifying you that it has received a request for heat from an external source, such as a thermostat or control system, that is lower than the minimum temperature the boiler is currently programmed to provide.. It appears on 5 Viessmann models in our database.

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: Minimum set boiler water temperature in case of external demand

What causes the Viessmann 9b:70 fault?

This fault occurs when there is a mismatch between the boiler's internal computer and the external smart controls or thermostat. It usually happens because the minimum heat level requested by the external system is set higher than what the boiler is programmed to allow, causing a configuration conflict.

Typical repair cost £0-80 (including labour)

DIY fix possible

Check your external thermostat or heating controller settings to ensure the target temperature isn't set unusually low.Restart the boiler to see if the advisory code clears.No immediate action is required if your heating and hot water are currently working as expected, as this is often an advisory status code.

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Viessmann models that show the 9b:70 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann 9b:70 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