High Lockout
F29 on Saunier Duval Isotwin F30E

Your boiler has shut down because a safety component has detected that the internal temperature reached a level high enough to potentially damage the system.

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

Thermal fuse. Replace the heat exchanger

What causes this fault?

This fault occurs when a safety sensor detects that the heat exchanger has reached an dangerously high temperature, often due to a lack of water flow or a buildup of debris. Because this component is designed to permanently break to protect the boiler from melting or catching fire, it means the system has suffered a significant overheat event. The most common reasons are a failed pump, a severe blockage in the pipework, or a lack of water in the system.

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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Related fault codes

F5View →F20View →

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