A central air conditioner consists of either a split-system unit or a packaged unit.

In a split-system central air conditioner, an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator. In many split-system air conditioners, this indoor cabinet also contains a heat pump. The air conditioner's evaporator coil is installed in the cabinet or main supply duct of the heat pump.

In a packaged central air conditioner, the evaporator, condenser, and compressor are all located in one cabinet, which usually is placed on a roof or on a concrete slab next to the house's foundation. This type of air conditioner also is used in small commercial buildings. Air supply and return ducts come from indoors through the home's exterior wall or roof to connect with the packaged air conditioner, which is usually located outdoors.

Buying a New Air Conditioner
Today's best air conditioners use 30% to 50% less energy to produce the same amount of cooling as air conditioners made in the mid-1970s. Even if your air conditioner is only 10 years old, you may save 20% to 40% of your cooling energy costs by replacing it with a newer, more efficient model.

When buying a new air conditioner, avoid window units and look for a split or central system with a variable speed inverter compressor. Ensure you select a model with a Seasonal Energy Efficiency Ratio (SEER) of 18 or more. SEER indicates the relative amount of energy needed to provide a specific cooling output. Many older systems have SEER ratings of 6 or less. If you must purchase a less efficient window unit, ask for an Energy Efficiency Ratio (EER) of 12 or more.

Proper sizing and installation are key elements in determining air conditioner efficiency. Too large a unit will not adequately remove humidity. Too small a unit will not be able to attain a comfortable temperature on the hottest days. Improper unit location and lack of insulation can greatly diminish efficiency.

Other features to look for when buying an air conditioner include:

• A thermal expansion valve and a high-temperature rating (EER) greater than 12, for high-efficiency operation when the weather is at its hottest
• A fan-only switch, so you can use the unit for nighttime ventilation to substantially reduce air-conditioning costs
• A filter check light to remind you to check the filter after a predetermined number of operating hours
• An automatic-delay fan switch to turn off the fan a few minutes after the compressor turns off.

Installation and Location of Air Conditioners
An air conditioner is installed correctly will perform efficiently for years with only minor routine maintenance. Modern energy-efficient air conditioners can perform almost as poorly as older inefficient models if they are not installed correctly.

When installing a new central air conditioning system, be sure that your contractor:

• Allows adequate indoor space for the installation, maintenance, and repair of the new system
• Locates the condensing unit where its noise will not keep you or your neighbors awake at night, if possible
• Locates the condensing unit where no nearby objects will block airflow to it
• Verifies that the newly installed air conditioner has the exact refrigerant charge and airflow rate specified by the manufacturer
• Locates the thermostat away from heat sources, such as windows or supply registers.

Using Your Air Conditioner
Take the following steps to reduce the energy consumption of your air conditioner:

• Cool as few rooms as possible
• Only use air conditioning when necessary
• Reduce the need to use air conditioning by shading windows that catch a lot of sunlight and using natural ventilation or fans where possible
• Try to use the warmest temperature setting that is comfortable
• Prevent cool air from escaping by keeping doors and windows closed
• Clean the filters regularly

Information on this webpage has been referenced from the US Department of Energy website;