TP-Link Kasa
No Heat Call Output or Open W Circuit
Thermostat calls for heat but furnace does not start
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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
Loose or broken W wire at thermostat, Blown low-voltage fuse on furnace board, Misconfigured system type in thermostat, Failed W output relay on thermostat
How to Fix / Troubleshooting
Safety: Turn off power to furnace before opening panels or touching wiring.
- Confirm call for heat: Set thermostat to Heat and raise setpoint at least 3–5°F above room temperature. Verify the thermostat indicates a heating call.
- Check W wiring: At the thermostat, ensure the W (or W1) wire is secure. At the furnace board, confirm the thermostat W conductor is firmly attached.
- Fuse inspection: Check the furnace control board fuse and replace if blown, then look for shorts in thermostat wiring.
- System type: In the Kasa app, verify the thermostat is configured for the correct system (conventional vs heat pump) and that W is enabled as a heat call.
- Technician test: A technician can jumper R to W at the furnace board to verify furnace operation. If furnace runs with jumper but not via thermostat, replace the thermostat.
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Repair Difficulty
Hard
4/5
Required Part
Heating Call (W) Circuit / Thermostat Relay
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