A09
Herz
DHW Temperature Sensor Fault (NTC DHW)
Boiler locks out with A09, DHW temperature unstable or not reaching setpoint
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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
Defective DHW NTC sensor in Herz plate heat exchanger, scale buildup around sensor pocket, loose sensor connection, damaged sensor cable
How to Fix / Troubleshooting
Safety: Isolate power and close cold water inlet to the boiler before working on DHW components.
- Check DHW sensor location: On combi models, locate the DHW NTC on or near the plate heat exchanger. Inspect for leaks or corrosion.
- Inspect wiring: Ensure the sensor cable is intact and firmly connected to the PCB.
- Descale DHW side: If DHW temperatures fluctuate, the plate heat exchanger may be scaled. Flush with an approved descaling solution using a pump kit (technician recommended).
- Replace NTC: If readings are clearly wrong or A09 persists, replace the Herz DHW NTC sensor and re-test.
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Repair Difficulty
Medium
3/5
Required Part
Herz DHW NTC sensor
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