If your air supply vent is in the floor, instead of clipping the Comfort Sensor to the outside of the vent, if you can remove the vent cover, you may place it inside. Assuming the duct is metal, you can secure it to the wall of the metal boot directly in the airflow using the built in magnets.
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​Make sure the sensor is placed horizontally and NOT vertically. Ensure too that the Comfort Sensor is positioned in the center of the metal boot wall.