How much does it cost to fix Navien error code E291?

Typical repair cost for the Navien E291 fault is £120-250 including labour.

E291 High

Navien E291 Fault Code

Q: What does the Navien E291 fault code mean?

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.

The Navien E291 fault code means: 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.. It appears on 6 Navien models in our database.

Technical description: Supply/Return inversion limit

Typical repair cost £120-250 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Navien models that show the E291 fault code

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

NCB Repair: £120-250
NCB 28 HE Repair: £150-300
NCB 35 HE Repair: £150-300
NCB 40 HE Repair: £150-300
NCB H Repair: £150-300
NHB 29 Repair: £150-300

Source

Navien E291 fault code information on this page is sourced and cross-checked against the official Navien service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026