nest Learning thermostat Installation And Configuration Manual page 19

Built-in Humidifier System, 2 Wire
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This system has a humidifier built in to its HVAC system and 2 separate hum wires.
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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.
Built-in Humidifier System, 1 Wire
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This system has a humidifier built in to its HVAC system and only 1 hum wire.
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Connect the hum wire from the built-in system directly to the * terminal at the Nest.
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No relay is recommended.
Nest Learning Thermostat installation & configuration guide Page 17