A09
Fondital
DHW Temperature Sensor / Modulation Fault
Boiler displays A09, domestic hot water temperature unstable or excessively hot
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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
Faulty DHW NTC sensor on Fondital plate heat exchanger, Scaled or partially blocked DHW plate heat exchanger, Sticking gas valve not modulating correctly, Incorrect sensor wiring
How to Fix / Troubleshooting
Safety: Risk of scalding. Test hot water carefully and avoid maximum tap opening until resolved.
User checks:
- Flow rate: Open hot taps moderately instead of fully to see if temperature stabilises.
- Reset: Reset the boiler once. If A09 returns, professional service is needed.
Technician actions:
- Measure resistance of the DHW NTC sensor and compare with Fondital temperature/resistance tables.
- Descale or replace the DHW plate heat exchanger if restricted by limescale.
- Check gas valve modulation via flue gas analysis and adjust or replace the gas valve if necessary.
- Inspect wiring between the DHW NTC and main PCB.
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Repair Difficulty
Medium
3/5
Required Part
DHW NTC Sensor
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