BoilerFaultCodesViessmann A.18 Fault Code
Also written as A18 — same fault.
The Viessmann A.18 fault code means: your boiler is struggling to drain away the water created during normal operation, which could cause internal damage or blockages if not addressed.. It appears on 4 Viessmann models in our database.
Technical description: Possible condensate backup in the heat cell. Measure: Check the combustion chamber and condensation drain.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Viessmann models that show the A.18 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann A.18 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
- Viessmann Vitodens 100-W WB1A manual (PDF)
- Viessmann Vitodens 200-W B2HF System manual (PDF)
- Viessmann Vitodens 200-W B2KF Combi manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026