Direct expansion evaporator coils are incorporated into forced-air systems that can supply a mix of outdoor air and “return” air. These systems typically use furnaces to provide heating. Efficiency is measured in Energy Efficiency Ratio (EER) or Seasonal Energy Efficiency Ratio (SEER). Baseline systems often have a SEER of 10. High-efficiency units, available with SEERs of 16 and above, typically include higher efficiency compressors (i.e. scroll compressors) and condensing and evaporating fans, and increase the surface area of the condensing unit. Variable speed compressors and fans, as well as NEMA Premium motors, also increase efficiencies.