Thermona Heating Circuit Isolation / Diverter Valve Stuck on DHW

Boiler runs but radiators do not get hot, while domestic hot water works normally (combi models).

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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

Stuck Thermona diverter valve in DHW position, Closed heating flow or return valves, Faulty room thermostat or wiring, Faulty diverter motor

How to Fix / Troubleshooting

Safety: Isolate power before removing boiler covers.

  • Check heating controls: Ensure the room thermostat is calling for heat and the programmer is set to heating ON.
  • Verify valves: Confirm that the heating flow and return isolation valves under the boiler are fully open.
  • Observe diverter valve (engineer level): With the front cover removed, switch between DHW and CH demands and watch the Thermona diverter motor. If it does not move, the motor or valve may be stuck.
  • Replace diverter head (engineer level): If the motor is faulty, replace the diverter valve actuator. If the valve body is seized, replace the complete diverter valve assembly.
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Repair Difficulty

Medium 3/5

Required Part

3-Way Diverter Valve Actuator
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