Heat Pumps are typically incorporated into forced-air systems that supply a mix of outdoor air and "return" air. Baseline systems have a Seasonal Energy Efficiency Ratio (SEER) of 10 and a coefficient of performance of 2.5. Efficiency measures include high-efficiency units with SEERs of up to 18 (on water-based systems) with a COP of 4. Air-based systems can attain SEERs of up to 12.5 and COPs of 4. Water-based systems can employ variable speed circulation of condenser water, earth-coupled heat exchangers, and high efficiency boilers. Additional efficiencies can be obtained through the use of variable speed control of ventilation fans and NEMA Premium^TM fan motors.