Thermona Short Cycling / Oversized Boiler or Control Issue

Boiler cycles on and off rapidly (short cycling) without maintaining stable temperature.

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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 system size, Incorrect Thermona parameter settings, Faulty room thermostat placement, Blocked bypass or no bypass circuit

How to Fix / Troubleshooting

Safety: Parameter changes should be made by competent persons following the Thermona manual.

  • Check thermostat location: Ensure the thermostat is not near a heat source or in a draught, which can cause rapid on/off commands.
  • Open more radiators: Ensure enough radiators are open to absorb the boiler's minimum output.
  • Adjust boiler output (engineer level): Reduce the maximum CH output in the Thermona installer menu if the boiler is oversized for the system.
  • Install/adjust bypass (engineer level): Fit or adjust an automatic bypass valve to maintain minimum flow through the boiler when TRVs close.
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Repair Difficulty

Medium 3/5
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