Residential air conditioners are sized from the smallest portable window unit up to huge central air systems that can weigh 5 tons or more. Self contained units have the advantage of low initial cost, mobility and easy installation, while central air units cool larger volumes at lower overall operating costs. Bridging the gap between these extremes is the split ductless air conditioner.
Ducted systems use the same ductwork that forced air heating uses by simply adding a cooling unit to the furnace supply, allowing coverage for an entire living space. Equivalent cooling with window units would require an air conditioner in almost every room. Unfortunately, the initial outlay for a central air system, along with maintenance and repair costs makes this solution prohibitively expensive when air conditioning isn't a high priority. For instance in motel rooms that may or may not be rented any particular night or in areas of the country with shorter or cooler summers.
The split ductless system uses the same principle of cooling that central air does, but is much smaller and much less expensive to install. It consists of an inner unit that is wall mounted with a fan and cooling coils that is fed by an outer unit (hence the "split"). No air is exchanged between the two parts, only refrigerant. This means that no ducts are required; each split unit serves one room or area. On the inside are a cooling coil and a fan. On the outside is a compressor and heat exchange. No air is exchanged between the two. A second line for condensation drainage can also be installed in parallel with the refrigerant line2.
There are two main advantages of a split ductless system over a window unit: the system doesn't require a window and is mounted on any outside wall and because the compressor is outside, it is much quieter than self-contained window air conditioners. The major disadvantage is lack of portability - a split ductless system is permanent (or nearly so).
Zoning
Because energy efficiency is getting great attention, both for cost savings and environmental concerns, split ductless air conditioners are becoming more popular. They offer the ability to "zone" a house into separate areas and customize cooling needs for each area.
Zoning is accomplished by using one exterior compressor unit that feeds several interior cooling units, each in its own zone. Each zone can then be regulated with a separate thermostat and remote control. A bedroom which isn't utilized in the daytime can be set to a higher temperature and the door closed to limit air circulation to the rest of the house. This saves on overall use and subsequent cost. Central air systems cannot be controlled in this manner - they are either "all on" or "all off."
Zones can also be added when an addition is put on a house, provided the outside unit can take additional capacity. Zones can also be switched on or off (as long as the total number of lines in use remains within specifications) for when a guest is visiting (guest room) or other purposes.
One further consideration is that zones and capabilities can be added piecewise, so that the cost of installation can be spread out over one or more years.
Types and styles
Because they are meant to only serve a small room, split ductless systems are not made in large sizes. Customers prefer the unobtrusive nature of a toaster sized coil and blower fan. The available output in BTUH (British Thermal Units per Hour) is up to about 12,000. This is enough for a single room or "zone." The lower capacity for individual interior units can be dealt with by running multiple interior units to a single exterior compressor.
For example, one system by Sanyo3 (Quad Zone 7/7/7/7 kBTU) runs four interior units (up to 7,000 BTUH each) to one outside compressor with a total capacity of 24,000 BTUH. A single refrigerant line runs to each of the four units.
One style that is popular with motels that have an outside wall is a combination heater/air conditioner set up in a baseboard style. In the summer months it runs as a split ductless air conditioner and in the winter months the cycle is reversed and the same unit then acts as a heater.
When used in this dual configuration the products are more properly called heat pumps - they "pump heat" either into or out of the house. One consideration is the distance between indoor and outdoor units. Since pumping long distances uses more energy, and because refrigerant lines lose heat, the limit is usually fifty feet. This is a consideration when considering locations for both indoor and outdoor units.
Prices run into the multiple thousands for a multiline system, but savings can be had because of less cost to install compared to central air systems and lower operating costs - while central air systems max out at a SEER (Seasonal Energy Efficiency Ratio) of 20, split air systems can go as high as 23. For comparison, a window unit will not exceed a SEER of 10 - 13. Single unit systems start at about $1,0004.
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