Defro Short Cycling / Oversizing or Control Issue

Boiler starts and stops frequently in short cycles, even when heating demand is constant.

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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

Boiler output too high for small system, Incorrect minimum power setting on Defro PCB, Faulty room thermostat placement, Flow sensor or NTC misreading temperature

How to Fix / Troubleshooting

Safety: Parameter changes should be made by someone familiar with boiler controls.

  • Thermostat location: Ensure the room thermostat is not located near heat sources or in direct sunlight, which can cause rapid cycling.
  • Check bypass: Confirm that a bypass radiator or automatic bypass valve is installed and open, allowing minimum flow through the Defro boiler.
  • Adjust parameters: In the installer menu, a technician can reduce maximum heating output or adjust anti-cycling time according to the Defro manual.
  • Sensor check: Verify that flow and return NTC sensors are properly attached and reading realistic temperatures.
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Repair Difficulty

Medium 3/5

Required Part

Defro automatic bypass valve
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