BoilerFaultCodes.com
S.21 Low Information

Your boiler is currently preparing itself to provide hot water quickly and is not actually reporting a fault.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Pump and fan running (warm start)

What causes this fault?

This code appears when the boiler activates its pre-heat function to ensure hot water is delivered to your taps as quickly as possible. It is a standard operational sequence where the pump and fan run briefly to warm up the internal heat exchanger. Your boiler hasn't stopped working; it is simply preparing itself for use.

Typical repair cost £0 (including labour)

DIY fix possible

Check if the comfort mode or pre-heat setting is turned onWait a few minutes to see if the status changes to a normal heating codeMonitor for any actual fault codes starting with F if the boiler fails to fire

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