One of the big disadvantages of a window air conditioner is the fixed location in a window. This means the unit is often stuck in a location that isn’t the best, it blocks the view and looks ugly, it can be loud and it has to be removed in winter months. All of these disadvantages are overcome with a split system.
Another name for a window air conditioner is a unitary system. The compressor, heating and cooling coils, and the fan is all built into a single box. A split system, as the name implies, has the condenser and heat exchange outside with the cooling coils and fan located inside the building.1 Inside units are usually referred to as cassettes and because they just have a cooling coil and a fan, they can be quite small and unobtrusive.
Separating the outside and inside units means there is less contact between the inside and the outside air. This helps efficiency by using available building insulation to keep the cool side cool. The cassettes are very quiet compared to a window unit, and they can be placed on interior walls or even in the ceiling. With proper planning, this also adds to cooling efficiency.
Split systems do not require duct work like central air and this is one of two major advantages over traditional systems. Interior cassettes are fed by small tubes that carry refrigerant from the outside condenser/heat exchanger. This provides the necessary cooling capacity. Warm refrigerant then flows out again through another tube. These two tubes, along with an electrical supply for the fan, are then the only connections needed and this is why they can be placed easily in convenient locations.
The second major advantage over central air conditioning is the ability to zone split systems. When more than one cassette is running off the same outside condenser, each can be programmed separately with a thermostat dedicated to that particular zone. This allows control over separate rooms and even a shutdown of one or more zones when rooms are not in use. This latter ability makes them ideal for offices or hotels where rooms may not be used for some time.2
Bridging the gap between a window air conditioner and a full blown central air system (with ducts) a split system is within the scope of a home handyman and many homeowners will install systems themselves. This is a considerable cost savings. Installation consists of an outdoor unit with power supply and one or more 3” holes for tubing to run into the house. If supplies to cassettes will be hidden, electricity and tubing will have to be run through interior walls.
A final advantage is the ability of a split system to serve as both an air conditioner in the summer months and a heater in the winter. This is accomplished by switching functions at the outside compressor which then functions as a heat pump. Generally, cassettes are provide less heat than is necessary, but they are a useful addition to other systems.
The major disadvantage of a split system is the initial cost which generally runs as much as 30% more than a central air system when ducts are already in place for heating. In some applications, this price differential is made up for with efficiencies because of the zoning feature and split systems can run a very high SEER rating.
The length of tube that can be run will be limited depending on the unit and it may not be possible to place cassettes everywhere in a building where they are wanted. Refrigerant has to be pumped without too much change in temperature and runs are limited to about 50 feet in length, although extensions are available up to 90 feet with qualified installation.3
Homeowners who install systems themselves face a few difficulties. Unit placement and tube runs can be mishandled leading to costly re-dos. Permits for outdoor units or roof installations may be required and manufacturer warranties are sometimes voided or limited when a non-licensed person installs the equipment.
Water buildup from condensation on indoor cassettes can be an issue and drainage must be allowed for. This might need a separate run of tubing or direct drainage to the outside.
Indoor cassettes cool the air directly. There is no air exchange with the outside. Although this adds efficiency, some users find indoor air gets “stale” over time. Some units do allow for this when they are mounted on an outside wall, although these are more common in Asia than in the US.
Technically, a central air conditioning system is a type of split system, only the coolant comes into one central location (the furnace) and all cooling is done there. Zoning is only possible by blocking outflow from ducts. There is an advantage in that existing heating ductwork can be used to move cool air throughout the house and the blower used for the furnace can serve the same function for cold air.
Efficiencies are less than for smaller split systems and professional installation is required. Current government Energy Star recommendations are to replace or upgrade an existing furnace when installing a central air conditioning system.4