The Montgomery Ward SXZ-58517 Humidifier is a whole house humidifier designed to be installed on a forced air heating system for a home or office. Maximum humidifier efficiency depends upon the tightness of your home, the hardness of your water and a variety of external factors. The humidifier filter on your Montgomery Ward SXZ-58517 Humidifier must be replaced at least once per year for optimum performance and to increase the life of your humidifier and heating equipment.