High
E445 on Navien NCB 28 HE

Your boiler is having trouble managing the flow of water between your heating system and your hot water taps, which may cause your heating or hot water to stop working properly.

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

Abnormal operation: Bypass mixing valve

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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