Standard Air Conditioners

• Central air conditioners are either split-system or packaged models. Split-system models have an outdoor cabinet that contains the compressor and condenser. An indoor cabinet houses the evaporator coil and fan. Heat pumps are sometimes included with these units.
  
  Packaged models have all components in one unit, usually located outside the house on a concrete slab or sometimes on the roof. Gas furnaces or electric heating coils can be included with packaged models.
  
  

Geothermal Systems

• Geothermal cooling and heating systems run water filled pipes underground to tap the earth's constant 50-degree temperature, according to an October 2009 article in Popular Mechanics. The water circulates into the geothermal unit, which uses the water and a refrigerant to heat and cool air. Geothermal systems are less expensive to operate because the system's condenser uses less energy to cool or heat the refrigerant and water--it's already at a constant 50 degrees.
  
  

Ductless Systems

• Ductless central air conditioning systems are common in Asia and Europe where space for ductwork is limited, but are used less frequently in the United States. A ductless system has one compressor located outdoors, similar to a split-system air conditioner, but it has several small evaporator coils located throughout the building, each serving its own blower unit mounted on exterior walls.
  
  

Size and Efficiency

• Ask your contractor to calculate the correct air conditioner tonnage for your building's needs. According to Tim Carter at Ask the Builder, to calculate tonnage the contractor needs to consider not only the building's square footage, but also how many windows it has and which directions they face, the type and amount of insulation used, how many rooms the building has and the number and placement of vents throughout.
  
  Units that are too small will consume too much energy while attempting to cool your building. Those that are too large will cycle on and off too quickly, preventing enough air from reaching the evaporator coil and not removing humidity from the air, leaving the interior air feeling cold and clammy.
  
  Regardless of which air conditioning option you choose, be sure to select the highest efficiency rating you can afford. Air conditioners are rated according to their Seasonal Energy Efficiency Ratio (SEER). According to the U.S. Department of Energy, SEER indicates the amount of energy needed to provide a specific cooling output. The higher the SEER, the greater the air conditioner's efficiency. The minimum SEER permitted as of 2009 is 13.