High Lockout
F.22 on Vaillant Thermocompact

Your boiler has detected that there is not enough water pressure or flow to operate safely and has shut down to prevent overheating.

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

Dry fire protection | No water in appliance, pump cable defective, pump blocked or defective, pump output too low

What causes this fault?

This fault occurs when the boiler detects a critical lack of water or a failure in the circulation system, preventing it from moving heat away from the burner. The most common reason is a significant drop in system pressure, but it can also be triggered if the pump has seized or failed to start. Without sufficient water flow, the internal sensors shut the system down immediately to prevent the heat exchanger from melting or cracking.

This fault is frequently seen in autumn when heating systems are turned back on after being idle all summer, which can cause the internal pump to seize.

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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