Aprilaire 4655 Manual Humidistat The indoor humidity should be set to match the outdoor temperature to prevent water from running down the sides of your windows When the relative humidity reverses the Aprilaire 4655 manual digital control humidistat will close the control switch. The Aprilaire 4655 manual digital control humidistat can either be wall mounted or mounted on the return Plenum This even water distribution ensures maximum evaporation from your humidifier