Ask yourself what you want from a supplemental air-conditioning unit before buying one—window units and mini-split heat pumps both have good qualities.

Q: We added a room, but our central air conditioner doesn’t cool it well. Our second-floor master bedroom also does not stay cool. Does it make more sense to install a window air conditioner or a mini-split system?

A: This is a common problem, especially for second-floor rooms. Cool air is denser than warm air, so it can drop to the first floor through cracks, gaps and stairs. Second-floor ceilings are also exposed to the hot underside of the roof, and tend to stay warm well into the evening.

Whether you install a mini-split heat pump or a window air conditioner depends on what you need and want. Most people install a window air conditioner to provide extra cooling in a room at a low initial cost, and energy efficiency is not their primary concern. Mini-split heat pumps offer many bonus features (heating and cooling, quiet operation, flexible installation, control) and increased efficiency, but at a higher initial cost.

The main drawback for mini-split heat pumps is cost. A window unit generally sells for under $300; mini-splits can run to $1,000 or more, plus the installation cost. Also, unlike a window unit, mini-splits can’t be moved once they are installed.

I have a two-story house with a central heat pump. I recently installed an LG Art Cool® mini-split system in the master bedroom. I selected the smaller output 9,000 Btu-per-hour model, which has 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. While it is insulated against heat flow and sound, it still is not ideal for energy efficiency. The newest ones are fairly quiet, but may still be annoying in a bedroom. When it’s not running, more outdoor road noise can also be heard.

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. Mine is mounted high on the garage wall so I can walk under it on an existing walkway.

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 indoor noise pollution from these components.

Instead of having the indoor cooling coil in an air-duct system, as with most central air conditioners, 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. Only a 3-inch-diameter hole needs to be cut through the wall, and the condensate drain from the evaporator coils can go out through the same hole.

Mini-split systems can also be used to cool an entire house by installing indoor wall units in several rooms. The conditioned air will naturally circulate throughout the house. This is commonly used in homes with baseboard electric or hydronic heat, which lacks 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 of your house, which can also lower your electric bill. In my case, there’s no need to keep the downstairs cool all night when I’m in the upstairs bedroom. The mini-split system allows me to set the central heat pump thermostat higher at night so it runs very little. This provides substantial electricity savings.

Inverter compressor technology is the newest, 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.

The remote control has several modes of operation, including a dehumidification setting for summer, which slows the fan speed so the indoor air is dehumidified more than it is cooled. This is ideal for allergy sufferers who are sensitive to high humidity but don’t want a cold room.

There’s also a “jet” setting that switches either the heating or cooling mode output and fan speed to high for changing the temperature quickly. This conserves energy because you can turn it on only when you use a particular room instead of keeping the room hot or cold continuously.

To learn more, visit:

  • > Products > Find ENERGY STAR Products > Ductless Heating and Cooling

Companies offering mini-split units:

  • Carrier, 800-227-7437,
  • Fujitsu General, 888-888-3424,
  • LG, 888-865-3026,
  • Mitsubishi Electric, 800-433-4822,
  • Samsung, 888-699-4351,

Send inquiries to James Dulley, Michigan Country Lines, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit ames Dulley is a nationally recognized mechanical engineer writing about home energy issues for the National Rural Electric Cooperaive Association.