Central air conditioning is, of necessity, a split system, with some components installed outdoors and others indoors. The heaviest, noisiest, heat-shedding components–the compressor and condenser coil–are installed outdoors, while the evaporator coil is installed indoors, usually in the form of an A-frame in the plenum of a forced-air furnace. In this case, the furnace's blower moves warm air over the coils and distributes the chilled air. The indoor and outdoor segments of the system are typically joined by two refrigeration lines and a low-voltage relay cable. If a home is heated by some means other than forced air–baseboard or radiant floor heat, for example–the evaporator coil is typically mounted in a dedicated blower unit, which pushes the cooled air through conventional ductwork. Most blower units are installed in attics and crawlspaces and are connected to flexible, insulated ductwork, which is the easiest and least costly to install, especially in retrofit situations. In houses built without ductwork, and where conventional ductwork would be too costly or too inefficient to install, a ductless central-air system is now possible. Ductless systems have long been popular in Europe and Asia, where building methods discourage ductwork, but are relatively new in the United States. The Carrier Corp. is one of several companies now making ductless systems for the U.S. market. In these systems, a single outdoor compressor serves several, smaller evaporator coils located indoors, each in its own box and each with its own blower fan. These components are installed on exterior walls, usually on the upper half, where much of the heat accumulates. They're finished unobtrusively, but they can be quite large, often measuring 6 x 18 x 24 in. Condensation lines are routed outside, along with the refrigeration and electrical lines. Ductless systems can also provide heat, either through resistance coils in the wall units or in heat-pump fashion. The advantages of ductless air over window air are that ductless systems move the noisiest components outdoors, they can provide heat, they distribute the air more evenly, and they don't block the better half of a window. Installation costs vary widely, but ductless systems can be more expensive than ducted systems. It's a sliding scale, with each home's variables–primary building material, number of rooms, house size and layout–coming into play. They're reliable and efficient, but they're not likely to replace ducted systems in the United States. They're considered problem solvers, with the problem being fairly rare.
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