About the Aprilaire 4655 manual digital control humidistat Aprilaire has been providing top of the line humidifiers and humidistats for over 60 years now. The Aprilaire 4655 manual digital control humidistat is designed to be used with low voltage service to offer control of your humidification equipment, and is made to be used with the Aprilaire humidifier 4655 500. The Aprilaire manual humidistat is designed to be installed on the cold air return duct or on an interior wall in the living area of the home. To determine the relative humidity in the home, slowly turn the knob on the control clockwise and counterclockwise and listen for 2 clicks. The actual relative humidity in the home is between the 2 clicks. It is important to anticipate a drop in outdoor temperature and reduce the humidity setting in order to avoid excess condensation. The excess condensation occurs because the indoor relative humidity is higher than what the home can support for the current outdoor temperature. To adjust the humidity setting, simply rotate the control knob to the desired setting. Provided the HVAC system fan is operating and the humidification level is lower than the new set point, the manual humidistat will activate the humidifier.