Split systems are the most common type of central air conditioner found in the U.S. Inside the house, tucked in a cabinet, is the evaporator coil, which removes heat and moisture from the air. Outside the house, a metal case contains the condenser coil, which releases the heat, and the compressor, which pumps refrigerant between the two coils.
The indoor component of the air conditioner is connected to a network of ducts, and a blower circulates the cold air through them to reach all parts of the house. This type of system is the most economical to install in a house with a central furnace, because it can share the ductwork used by the heating system.
Heat pumps are a variant on the traditional split system. This is essentially an air conditioner that can work in both directions. During hot summer months, it pumps heat out of the house and releases it outside. In the wintertime, it extracts heat from the outdoor air and uses it to warm the house. Heat pumps can be used effectively for both heating and cooling in mild climates.
However, heat pumps do not generally work well when temperatures stay below freezing for a long time, so they are not the best choice for cold climates. A specialized type of heat pump, called a ground-source or geothermal heat pump, may be more suitable for colder environments. It works by drawing heat out of the ground rather than the air. However, you must first locate a contractor who specializes in this type of equipment.
Packaged central air conditioners combine the evaporator, condenser and compressor in a single unit. The air conditioner is usually placed on a roof or a concrete slab near the foundation. Ducts running through the exterior wall or roof draw air from inside the house and return cooled air indoors. This type of air conditioner can also be used in small commercial buildings. When combined with a set of heating coils or a natural gas furnace, it eliminates the need for a separate furnace inside the building.
For houses that do not have ductwork, a ductless mini-split system can be a good choice. Like a basic split system, the ductless mini-split combines an outdoor compressor and condenser with one or more indoor air-handling units. These units are mounted high on the wall and have blowers attached. Tubing connects the indoor and outdoor units and circulates refrigerant between them. Each indoor unit is installed in a separate room and cools that room only, much like a window air conditioner.
The main advantage of these systems is that they can be installed without tearing up walls to install ductwork. They also allow the flow of cold air to be controlled independently in each room (or shut off altogether in empty rooms). Mini-split systems are more expensive than ducted central air conditioning systems, costing roughly 30 percent more for the same amount of cooling power. However, they are also more efficient, since they avoid the energy loss associated with ductwork. Another drawback is that it can be hard to find qualified contractors to install and service a mini-split system.