Possible Configurations - nest Learning thermostat Installation And Configuration Manual

Possible configurations:

Stand Alone Humidifier, 1 Wire
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This system has a humidifier that is separate from the HVAC system.
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Install the hum wire directly into the * terminal at Nest.
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Correctly ground the other wire from the Humidifier.
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No relay is recommended.
Stand Alone Humidifier, 2 Wire
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This system has a humidifier that is separate from the HVAC system.
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Install the 2 hum wires directly into the relay.
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Run the single wire from the relay to the * terminal at Nest.
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Nest recommends the White Rogers 90-290Q or similar 24VAC HVAC relay.
Nest Learning Thermostat installation & configuration guide Page 16