Google Nest Accessory Control Wiring or Configuration Error

Nest thermostat fails to control a whole-home humidifier or dehumidifier

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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/dehumidifier wired to wrong Nest terminal, Accessory requires separate transformer, Nest accessory output not enabled in settings, Incorrect relay type (powered vs. dry contact)

How to Fix / Troubleshooting

Safety warning: Turn off power to the furnace/air handler and accessory before working on wiring.

  • Identify accessory type: Determine whether the humidifier/dehumidifier uses 24V control or dry contacts, and whether it has its own transformer.
  • Check Nest wiring: Verify the accessory is connected to the Nest's * (star) or dedicated accessory terminal as per Google's wiring diagrams.
  • Enable accessory in settings: On the Nest, go to Settings > Equipment and configure the accessory type and operation (humidify vs. dehumidify).
  • Test operation: Temporarily set humidity targets to force the accessory on and confirm operation.

If the accessory still does not respond, a relay or transformer may be required; consult the accessory and Nest installation manuals or an HVAC professional.

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Repair Difficulty

Hard 4/5

Required Part

Nest Thermostat Base (* / Accessory terminal), Whole-Home Humidifier/Dehumidifier Relay
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