F.23 / F23 — Vaillant ecoTEC exclusive 832/838
What it means: The boiler suspects a water shortage because the flow and return temperatures are too far apart.
Typical causes
- Pump seized or faulty
- Pump delivering too little flow
- Flow and return sensors fitted the wrong way round
When to treat it as unsafe
Treat as unsafe if there is any sign of overheating, or if the boiler fired with poor water circulation.
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NwadFix is not affiliated with or endorsed by Vaillant. Manufacturer and model names are used only to identify the appliances an entry refers to.
Last reviewed: 13 October 2026 by a Gas Safe Engineer
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