High
E291 on Navien NCB

Your boiler has detected that the hot water flow and the cooler return water are being identified in the wrong order, likely due to a sensor misreading or a plumbing configuration issue.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Supply/Return inversion limit

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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