High
AL20 on Vokera Unica HE

Your boiler has sensed that it is starting to overheat and has shut itself down as a safety precaution.

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.

What the manual says

Manufacturer manual

Limit thermostat fault

From this model's manual
Verified against the Vokera Unica He installation & service manual, page 28.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This fault occurs when the temperature inside the boiler exceeds a safe limit, causing a safety sensor to trip and shut the system down. The most common reasons are a pump failure that prevents hot water from moving away from the burner, or a build-up of sludge and limescale that restricts flow through the heat exchanger.

This fault is frequently seen when the heating is first turned back on in autumn, as pump components can seize up after being idle during the summer months.

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

AL21View →AL28View →AL34View →

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