Thank you for your email regarding your model 560 Aprilaire humidifier. The model 560 was discontinued in 2002 and replaced by the model 600. Both models will effectively humidity a tight home up to 4000 square feet with a .50 air change rate. As homes age they can become less tight and humidity can escape. We design all our Aprilaire humidifiers to maintain an indoor humidity level of 35% at an indoor room temperature of 70F anytime the outdoor temperature is 20F or above. The operating time of the HVAC system will directly affect the operating time of your model 560.