It has two separate compressors: a smaller one for milder weather and a second larger one for extremely hot days. It’s like having two units in one. When matched with a variable speed indoor unit from Trane, the XL20i has another special feature. It's a method of enhancing airflow called Comfort-R, and it gives you greater humidity control in cooling. This is accomplished by running the fan at a slower speed during start-up. Comfort-R also provides warmer air during heating start-up. These managed cycles reduce temperature variances for the optimum in home comfort.