Junkers Overheating Rooms / Control Overshoot

Furnace display shows normal operation but radiators overheat, room becomes too hot

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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 heating relay on Junkers PCB, Faulty room thermostat sensor, Incorrect control parameters, Three-way valve stuck in heating position

How to Fix / Troubleshooting

Safety first: Turn down thermostat and, if necessary, switch off the furnace.

  • 1. Thermostat check: Verify that the room thermostat is not located near heat sources or in direct sunlight. Replace if it does not respond to setpoint changes.
  • 2. Control settings: Check Junkers controller settings for heating curve and room influence. Adjust to more moderate values.
  • 3. Three-way valve: On combi or system boilers with a diverter valve, check if the valve is stuck in heating position. Replace the valve motor or cartridge if necessary.
  • 4. PCB relay: If the boiler continues to fire for heating even with no demand, the heating relay on the PCB may be stuck and the board should be replaced by a technician.
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Repair Difficulty

Medium 3/5

Required Part

Three-Way / Diverter Valve
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