S.21 High
Your boiler is struggling to start the fan when you turn on your hot water taps, meaning the system cannot safely clear exhaust gases to begin heating.
Technical description: DHW mode – fan start up error
Typical repair cost: £180-450 (including labour)
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Protect yourself from future repair bills
A boiler cover plan would cover faults like S.21 — no call-out fees, no surprises.
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Parts commonly replaced
- Fan Assembly
- Printed Circuit Board (PCB)
- Wiring Harness
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