The outdoor condenser/compressor unit of a mini-split system can be mounted high on a garage wall to provide clearance beneath it.
Window units and mini-split heat pumps both have good qualities
Central air conditioners don't always cool all rooms adequately in a house. This is especially true for added rooms and for second-floor rooms. Cool air is denser than warm air, so it tends to drop to the first floor through cracks, gaps and the stairs. Also, second-floor ceilings are exposed to the hot underside of the roof and can stay warm well into the evening.
Pros and cons
Whether you install a mini-split heat pump or a window air conditioner depends on what you want and need. Most people install a window air conditioner to provide extra cooling at a low initial cost. Energy efficiency is not the primary concern. Mini-split heat pumps offer more efficiency and bonus features (heating and cooling, quiet operation, flexible installation, and control), but they cost more upfront.
A window unit generally sells for less than $300; mini-splits can run to more than $1,000, plus the cost of installation. Also, unlike a window unit, mini-splits can't be moved once installed.
I have a two-story house with a central heat pump. I recently installed an LG Art Cool mini-split system for the master bedroom. I selected the smaller output 9,000 Btu per hour model, with a seasonal energy efficiency ratio (SEER) of 28 and inverter compressor technology — twice as efficient as the central heat pump. I chose the heat pump version so it can also heat efficiently during winter.
A window air conditioner has all its components — compressor, air circulation fan, condenser fan, etc. — in the cabinet mounted in the window. Though it is insulated against heat flow and sound, it still is not ideal for energy efficiency. The newest ones are fairly quiet but still may be annoying in a bedroom at night.
A mini-split system is similar to a central air conditioner or heat pump, with the condenser fan, coils and compressor in an outdoor unit, which is flat and small. Some models allow the outdoor unit to be placed up to 100 feet from the room or group of rooms to be cooled or heated. This virtually eliminates noise coming indoors from these components.
Instead of having the indoor cooling coil in an air-duct system as with your existing central air conditioner, the coil is mounted in a fan unit on the wall or ceiling of the room. It's connected to the outdoor unit by refrigerant and electric lines.
Mini-split systems can also be used to cool an entire house by installing indoor wall units in several rooms. This is commonly done in houses that use baseboard electric or hydronic heat, which lack a duct system. Installing a duct system for central air-conditioning in an existing two-story house can be an expensive project.
In addition to the high SEER rating, installing a mini-split unit allows for zone cooling, which can also lower your electric bill.
Inverter compressor technology is the newest and most efficient trend in air conditioning. The compressor runs at variable speeds to provide for variable cooling output. Once the room cools down to the thermostat setting, the inverter compressor speed slows to keep the room at that temperature. Its remote control includes a dehumidification setting for the summer, ideal for allergy sufferers who are sensitive to high humidity but don't want a cold room.
There also is a "jet" setting that switches either the heating or cooling mode output and fan speed to high for quickly changing the temperature. This feature conserves energy because you can turn it on only when you use the room. For more information, visit energy.gov.