Thermia No Call for Heat Recognized

Thermostat calls for heat but furnace does not respond; no inducer or blower activity, yet control board has power.

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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

Broken W (heat) wire, Faulty thermostat, Miswired thermostat to Thermia control board, Failed low-voltage input circuit on control board

How to Fix / Troubleshooting

Safety first: Turn off power before changing wiring.

  • 1. Verify thermostat operation: Confirm the display is on and set to HEAT with a setpoint above room temperature.
  • 2. Check W circuit: At the Thermia control board, measure 24 VAC between R and C. Then check between W and C when there is a call for heat. If no voltage, the thermostat or wiring is faulty.
  • 3. Jumper test: With power off, install a jumper between R and W at the furnace. Restore power. If the furnace starts, the thermostat or cable is the issue.
  • 4. If jumper does not start furnace: The control board's heat call input may be defective and require replacement.
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Repair Difficulty

Medium 3/5

Required Part

Wall Thermostat
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