An air conditioner controls the temperature, humidity and general quality of the air in your home. As part of a central heating and cooling system, an air conditioner draws heat out of the house and transfers cooler air inside. When comparing Trane air conditioning systems, check the SEER (Seasonal Energy Efficiency Ratio). Systems usually range from 13 to 23. A higher SEER rating means greater energy efficiency, similar to how a car with higher miles per gallon gets better fuel efficiency.