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Installing central air conditioning in a home that does not have forced air ducts can be difficult. Ductless mini-split heat pumps provide a unique solution to bringing central air conditioning into homes. By piping refrigerant to individual coils within air handlers mounted throughout a home (rather than a single refrigerant coil/air handler and central ductwork), mini-split heat pumps do not require ductwork for central air conditioning. Because the refrigerant lines take up much less room than do typical ducts, much less effort is required in installation in a retrofit.
Ductless mini-split systems combine the flexibility of room air conditioners with the whole house cooling of central systems. Although some systems provide heating and cooling, ductless mini-split heat pumps are usually installed primarily for cooling.
In a conventional heat pump, a single indoor unit (refrigerant coil and air handler) and single outdoor unit (condenser and compressor) serve the entire house. Air is cooled at the evaporator coil and distributed around the house via ductwork. In ductless systems, there is (usually only) one outdoor unit serving multiple indoor units (each containing a refrigerant coil and blower). Refrigerant is piped from the outdoor unit through small-diameter insulated refrigerant lines directly to individual rooms or zones. Cooled air is blown into the room by a fan in the individual evaporator units. The term "mini" is used to describe the small indoor units located in each room or zone.
While distribution energy losses in conventional systems have been estimated as high as 30 percent, distribution losses for ductless systems are about one to five percent.
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Because they do not rely on ductwork (which is often leaky and can account for a third of the energy usage for heating and cooling), mini-split heat pumps can boost energy efficiency. The ability to control each zone separately can also contribute to energy efficiency.
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Ductless heat pumps are installed using conventional methods for heat pump/air conditioner installation. However, extra care must be taken to prevent refrigeration leaks and to ensure proper operating pressures.
The cost of ductless heat pumps has declined as the technology has become established in the marketplace. A 2004 polling of ductless heat pump suppliers showed costs for ductless heat pumps to run between $500 and $900 per ton, depending upon the type of system and the number of zones desired per unit.
Ductless systems can reduce energy costs for heating and cooling over conventional heat pumps. Cost will vary with equipment efficiency rating and the leakiness (and location) of ducts to which it is compared.
Few, if any, code or regulatory barriers limit ductless technology. The National Appliance Energy Conservation Act of 1992 requires that split systems have a minimum SEER of 10.0 and HSPF of 6.8. In 2006, these minimum requirements will increase to 13 SEER and 7.7 HSPF. The Air-Conditioning and Refrigeration Institute provides a list of air conditioners and heat pumps and their rated efficiency.
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Ductless systems are relatively easy to install. It takes two installers about a day to install a system having up to three zones. Wiring for power and controls is easier than with a conventional unit since wires can be run along with the refrigerant lines. Refrigerant lines from outdoor units can span up to one hundred feet to indoor units.
Indoor units are about six to eight inches deep and are mounted flush on a wall or ceiling, or recessed in a drop ceiling. A three-inch hole behind the unit is used for attaching wiring, refrigerant lines, control cables, and a condensate drain.
Warranty varies with manufacture. A typical warranty is one year on indoor units and up to six years on compressors.
Ductless system benefits include ease of installation-air ducts are bulky and require special structural attention, while split system piping can often be routed through walls and joists. Further, split systems allow zone control for increased comfort and efficiency. System energy losses are reduced because distribution takes place through insulated refrigeration lines rather than ductwork. Aesthetics are improved over window units and no windows need to be blocked. Each zone has its own thermostat, so occupants can modify temperatures in each zone.
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