Geo Humidifier Control Output Not Activating

Thermostat fails to power the connected whole‑home humidifier when humidity is low and a call is expected.

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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

Humidifier wired to wrong Geo terminals, AUX/HUM output not configured for humidifier, Humidifier current draw exceeds thermostat rating, Failed AUX/HUM driver on Geo PCB

How to Fix / Troubleshooting

Safety first: Turn off power to the HVAC system and humidifier at the breaker before working on wiring.

  • Step 1 – Verify wiring: Confirm the humidifier control wires are connected to the correct AUX or HUM terminals on the Geo wall plate and to the humidifier solenoid or relay.
  • Step 2 – Configure output: In the thermostat equipment settings, set the AUX/HUM output function to Humidifier and configure any required parameters (e.g., heat‑only operation).
  • Step 3 – Test humidifier manually: Temporarily jumper the humidifier control leads at the furnace or humidifier to verify the humidifier itself operates.
  • Step 4 – Use isolation relay if needed: If the humidifier coil draws more current than the thermostat AUX output rating, install an isolation relay controlled by the Geo AUX output.
  • Step 5 – Replace thermostat if driver failed: If wiring and configuration are correct but the AUX/HUM output never energizes, the driver on the Geo PCB may be defective. Replace the thermostat head unit.
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Repair Difficulty

Hard 4/5

Required Part

AUX/HUM Output Circuit
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