Defro
Condensation in Flue / Low Return Temperature Corrosion Risk
Condensation or water dripping from flue connection and back of furnace
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Return water temperature too low, Oversized chimney causing cooling, Poor insulation of flue, Incorrect mixing valve setting
How to Fix / Troubleshooting
Safety: Ensure condensate is not leaking onto electrical components.
- Check that the three‑way mixing valve is adjusted to maintain the minimum return temperature recommended by Defro (often around 55–60°C for solid fuel).
- Insulate exposed sections of the flue to reduce cooling.
- Verify that the boiler is not operating continuously at very low output; adjust heating curves or room thermostat settings.
- Clean any condensate‑soaked soot from the flue to prevent corrosion.
Persistent condensation may require system redesign (e.g., loading valve or buffer tank) by a heating specialist.
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Repair Difficulty
Medium
3/5
Required Part
Three‑Way Mixing Valve
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