Mysa Stuck Output Triac / Relay Welded Closed

Heater runs continuously and overheats the room even when the thermostat is set low or turned off.

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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

Shorted triac on thermostat PCB, Relay contacts welded due to overload, Miswired line and load conductors bypassing control

How to Fix / Troubleshooting

Safety first: Turn OFF the breaker immediately to stop heating. Do not leave a heater running uncontrolled.

Steps to diagnose:

  • Verify wiring: With power OFF, confirm that LINE and LOAD wires are correctly placed on the thermostat terminals and that no wires are tied together in a way that bypasses the thermostat.
  • Test heater with thermostat removed: Disconnect the thermostat and cap the LINE and LOAD wires separately. Restore power briefly. If the heater still runs, the issue is in the wiring or heater, not the thermostat.
  • Inspect thermostat: Check for burn marks or melting near the output components on the PCB.

If the heater only runs uncontrollably when the thermostat is connected, the output triac/relay is stuck closed. Replace the thermostat and verify the heater load is within the rated amperage to prevent recurrence.

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Repair Difficulty

Hard 4/5

Required Part

Output Triac / Solid-State Relay Assembly
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