An air conditioner cools your home by pumping cold refrigerant through a series of coiled tubes, one inside your house called the evaporator, the other outside your house called condenser. The evaporator and the condenser are simply coils of copper tubing surrounded by aluminum cooling fins, much like an automotive radiator. A pump, known as the compressor circulates a heat transfer fluid, called refrigerant, through the tubes. When the refrigerant evaporates inside the evaporator it pulls heat from the air and cools the air. Ouside the house, the refrigerant releases its heat into the air via the condenser, reverts back to a liquid and is forced back into the house for more cooling.

Residential air conditioners come packaged a couple of different ways. The quick and dirty way to cool your home is with window-mounted room air conditioners. For more evenly distributed whole-house cooling, you're going to want to opt for a split or packaged central air system.

Room air conditioners are easy for almost anyone to install and operate plus they are inexpensive. Unfortunately, as the name implies, they are only good for cooling the room where they're located. They're available in 115 and 220-volt varieties.

Central air conditioners can circulate cool air throughout an entire house because they are plugged into a network of ducts that draw warm air and feed cool air. In a house equipped with a forced hot air heating system the installation of central air conditioning is relatively simple. In a house equipped with baseboard hot water or baseboard electric, the homeowner will have to go to the trouble and expense of installing the ducts.

Central air systems come in two varieties. Split system central air conditioning systems

utilize an outdoor metal cabinet which contains the condenser and compressor, and an indoor cabinet that contains the evaporator. In many split-system air conditioners, this indoor cabinet also contains a furnace or the indoor part of a heat pump. The air conditioner's evaporator coil is installed in the cabinet or main supply duct of this furnace or heat pump. If your home already has a furnace but no air conditioner, a split-system is the most economical central air conditioner to install.

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. Packaged air conditioners often include electric heating coils or a natural gas furnace. This combination of air conditioner and central heater eliminates the need for a separate furnace indoors.

If you are installing a new central air conditioning system, you may want to consider the size of the unit as well its efficiency -- both of which will have a direct impact on its price.

Air conditioners are rated by the number of BTUs they can remove in an hour. They might also be rated by the ton, which amounts to 12,000 BTUs per hour.

It's almost impossible to offer a general rule of thumb for determining air conditioner size because so many variables can come into play. For instance, the size of the unit will be based on how large your home is and how many windows it has, how much of your home is in the shade, the amount of insulation in your home's ceiling and walls and how much heat your major appliances generate.

There are formulas available to help homeowners determine this, but its probably best to let a reputable air conditioning contractor do the work for you. It is an important decision however, because too small an air conditioner will be overwhelmed and function poorly while too large an air conditioner will cost more and use more power to operate.

Equally important is the air conditioner's efficiency rating which should be posted on the air conditioners Energy Guide Label. More efficient air conditioners will cost more but you will reap benefits in reduced energy consumption over the life of the unit. It's not a bad idea to buy the most efficient unit you can afford.

Government standards require room air conditioners built after January 1, 1990, to have an energy efficiency rating of 8.0 or greater. Select a room air conditioner with an EER of at least 9.0 if you live in a mild climate. If you live in a hot climate, select one with an EER over 10.

Owners of older room air conditioner can realize energy savings by replacing their existing units with more modern ones. According to the Association of Home Appliance Manufacturers the average EER of room air conditioners rose 47 percent from 1972 to 1991. If you own a 1970s-vintage room air conditioner with an EER of 5 and you replace it with a new one with an EER of 10, you will cut your air conditioning energy costs in half.

National minimum standards for central air conditioners require an energy efficiency rating of 9.7 and 10.0, for single-package and split-systems, respectively. There are many units available with ratings as high as 17 however. According to the Association of Home Appliance Manufacturers, in 1979, the SEERs of central air conditioners ranged from 4.5 to 8.0. Replacing a 1970s-era central air conditioner with a SEER of 6 with a new unit having a SEER of 12 will cut your air conditioning costs in half.

Perhaps the single most important part of buying a new central air conditioning system is choosing the right contractor.

Don't be afraid to ask for references and be sure to get a written warranty. If you are replacing an existing system with a new one be sure to tell your contractor any problems you may have had with the old system so that he can address them during the new installation.

If you suspect your air conditioner needs a tune up, the Home Show is a great place to find a trained professional who can handle the job.

When interviewing prospective contractors be sure that they're capable of, checking the refrigerant, testing for leaks, capturing any refrigerant that may need to be evacuated from the system, measuring the system's flow, servicing the air conditioner's motor and checking its belts for tightness

An older air conditioning unit may provide years of trouble-free use, however, their operation can be hampered by a variety of conditions including:

Refrigerant leaks -- If your air conditioner is low on refrigerant, either it was undercharged at installation, or it leaks. If it leaks, simply adding refrigerant is not a solution. A trained technician should fix any leak, test the repair, and then charge the system with the correct amount of refrigerant. Remember that the performance and efficiency of your air conditioner is greatest when the refrigerant charge exactly matches the manufacturer's specification, and is neither undercharged nor overcharged.

Inadequate maintenance -- If you allow filters and air conditioning coils to become dirty, the air conditioner will not work properly, and the compressor or fans are likely to fail prematurely.

Electric control failure -- The compressor and fan controls can wear out, especially when the air conditioner turns on and off frequently, as is common when a system is oversized. Because corrosion of wire and terminals is also a problem in many systems, electrical connections and contacts should be checked during a professional service call.

There isn't all that much that a homeowner can do to keep an air conditioning system healthy and happy, but there are a few basic maintenance checks that can be performed.

Perhaps the most important and simple maintenance measure that will ensure the efficiency of your air conditioner is to routinely replace or clean its filters. Clogged, dirty filters block normal air flow and reduce a system's efficiency significantly. With normal air flow obstructed, air that bypasses the filter may carry dirt directly into the evaporator coil and impair the coil's heat-absorbing capacity. Filters are located somewhere along the return duct's length. Common filter locations are in walls, ceilings, furnaces, or in the air conditioner itself.

Some types of filters are reusable; others must be replaced. They are available in a variety of types and efficiencies. Clean or replace your air conditioning system's filter or filters every month or two during the cooling season. Filters may need more frequent attention if the air conditioner is in constant use, is subjected to dusty conditions, or you have fur-bearing pets in the house.

The air conditioner's evaporator coil and condenser coil collect dirt over their months and years of service. A clean filter prevents the evaporator coil from soiling quickly. In time, however, the evaporator coil will still collect dirt. This dirt reduces air flow and insulates the coil which reduces its ability to absorb heat. Therefore, your evaporator coil should be checked every year and cleaned as necessary.

Outdoor condenser coils can also become very dirty if the outdoor environment is dusty or if there is foliage nearby. You can easily see the condenser coil and notice if dirt is collecting on its fins.

You should minimize dirt and debris near the condenser unit. Your dryer vents, falling leaves, and lawn mower are all potential sources of dirt and debris. Cleaning the area around the coil, removing any debris, and trimming foliage back at least 2 feet (0.6 meters) allow for adequate air flow around the condenser.

The aluminum fins on evaporator and condenser coils are easily bent and can block air flow through the coil. Air conditioning wholesalers sell a tool called a "fin comb" that will comb these fins back into nearly original condition.

An enormous waste of energy occurs when cooled air escapes from supply ducts or when hot attic air leaks into return ducts. Recent studies indicate that 10 percent to 30 percent of the conditioned air in an average central air conditioning system escapes from the ducts.

For central air conditioning to be efficient, ducts must be airtight. Hiring a competent professional service technician to detect and correct duct leaks is a good investment, since leaky ducts may be difficult to find without experience and test equipment. Ducts must be sealed with duct "mastic." The old standby of duct tape is ineffective for sealing ducts.

Obstructions can impair the efficiency of a duct system almost as much as leaks. You should be careful not to obstruct the flow of air from supply or return registers with furniture, drapes, or tightly fitted interior doors. Dirty filters and clogged evaporator coils can also be major obstructions to air flow.

The large temperature difference between attics and ducts makes heat conduction through ducts almost as big a problem as air leakage and obstructions. Ducts in attics should be insulated heavily in addition to being made airtight.

Finally, the overall efficiency of any air conditioner depends largely on its operator. Installing a programmable thermostat will keep you from forgetting to turn the AC down when you aren't home, saving energy as well as wear and tear on the machine.

Simply drawing the blinds to prevent sunlight from warming your house will help make your air conditioner last longer and draw less power too.

-- Information for this article was provided Federal Department of Energy