Thank you for your email regarding your model 600 Aprilaire humidifier. Your model 600 can be set up as an automatic or manual control. In automatic there is an outdoor sensor connected to the control which will allow the control to automatically adjust the humidity level as the outdoor temperature changes. In manual mode you will need to adjust the humidifier control as the outdoor temperature changes. If not, you may over humidify your home. If you choose to run in manual mode simply remove the outdoor sensor from the terminals labeled odt and connect the manul model sensor instead.
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