The Skuttle 81 humidifier is a whole house humidifier designed to be installed on a forced air heating system for a home or office. This model is mounted UNDER the supply duct and has a rotating pad in it. Some parts are still available for it but others are not. The unit itself was discontinued years ago and can be replaced by the model 55UD should you need to replace it. The humidifier filter on your Skuttle 81 humidifier must be replaced at least once per year for optimum performance and to increase the life of your humidifier and heating equipment.