How does the Aprilaire model 400 measure humidity?

Monday, December 12, 2011 by Aprilaire Team
Consumer question:

How does the 400M measure the Humidity in the return air vent.

Right now It will only work when the dial is set all the way 45o


Aprilaires' response:

Thank you for contacting us regarding your model 400M humidifier. The manual control that comes with the 400M contains a membrane that expands and contracts depending on the amount of moisture in the air. It takes this measurement from the air that is surrounding the control, whether it is air from the duct or from the room the control is mounted in. If the control is turning the humidifier on at 45%, that means that the control is sensing the current relative humidity is between 40 and 45%. As the air in your home becomes drier, the humidifier will begin to run on a lower number.

If you have any further questions regarding this or any of our products, please feel free to contact our customer service department at your earliest convenience.

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