Saunier Duval
Unstable Domestic Hot Water Temperature
Hot water temperature fluctuates or runs hot then cold
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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
Partially blocked plate heat exchanger, Faulty DHW NTC sensor, Sticking diverter valve, Inadequate flow rate through boiler
How to Fix / Troubleshooting
Safety first: Risk of scalding if water suddenly becomes very hot. Test temperature carefully.
Steps to check:
- Ensure the cold water inlet isolation valve to the boiler is fully open.
- Open a single hot tap fully and observe if the boiler maintains a stable flame and temperature.
- Clean any inlet filters or strainers on the cold water inlet (isolate water first).
- If radiators warm up when hot water is drawn, the diverter valve may be leaking or sticking and needs professional attention.
- Descaling or replacing the plate heat exchanger may be required if scale is heavy.
Warning: Do not adjust internal temperature limiters yourself; use only the user controls.
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Repair Difficulty
Hard
4/5
Required Part
Plate Heat Exchanger / Diverter Valve
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