Saunier Duval Short cycling / unstable operation

Water heater frequently cycles on and off while a tap is open

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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

Flow rate near minimum threshold, Oversized heater for demand, Faulty flow sensor giving intermittent signal, Incorrect modulation settings

How to Fix / Troubleshooting

Safety first: Turn off power before accessing internal components.

Steps:

  • Increase flow: Open the hot tap more fully to keep flow above the minimum required by the Saunier Duval heater.
  • Clean flow sensor: Remove and clean the DHW flow sensor/turbine to ensure a stable signal.
  • Check NTCs: Verify that temperature sensors are correctly mounted and not loose, which can cause rapid temperature swings.
  • Professional adjustment: A technician may need to check modulation parameters and gas settings.

Warning: Frequent cycling reduces component life. Address the root cause rather than ignoring the symptom.

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Repair Difficulty

Medium 3/5

Required Part

DHW Flow Sensor
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