Central air conditioning uses a considerable amount of electricity, particularly in warmer climates, and is one of the most expensive home appliances to operate. The difference between central air conditioning and a room (or window) air conditioner is the distribution system. A central air conditioner uses a blower and ductwork to distribute chilled, dehumidified air throughout the entire house. In contrast, a room air conditioner, typically installed in a window or, in some cases, through a wall, will cool a small area, generally a single room or perhaps two to three adjacent rooms. Central air conditioning is becoming much more common in this country, and is now present in over 47% of existing homes and over 77% of new homes!

Standard central air conditioners consist of three main parts: a condenser, an evaporator, and a compressor. The compressor and condenser are often integrated into a single unit that is typically installed outside the home but occasionally in an attic or on the roof. The evaporator is the cooling coil, normally installed in the air handling system. A refrigerant is circulated between these components. The air handler blower draws air from various parts of the home through return air ducts. The air then passes over the evaporator coil which has been chilled by the refrigerant. Heat is absorbed from the air, and moisture is drained away.

The conditioned air is then routed back to the rooms through the supply air ductwork. The refrigerant, which is converted to a vapor by the heat, is pressurized by the compressor and moved to the outside coil, or condenser, where the heat is dissipated. As the heat is removed by the condenser, refrigerant turns back into a liquid and then is sent (by way of the copper pipes) to a metering device (that is usually inside the cooling coil cover). And the process starts over again.

Energy Efficiency of Central Air Conditioning Systems
The efficiency of central air conditioners is measured in a Seasonal Energy Efficiency Rating (SEER). Government standards currently require a minimum SEER of 13. Unfortunately, manufacturers can label such units "high efficiency", even though they are the minimum allowed by law. High-efficiency air conditioners are now available that are 50% to 75% more efficient than units that were available five years ago. In fact, central air conditioners are now made with SEER ratings of up to 23. For air conditioners that are at or near the end of their useful lives, it is often cost-effective to replace them with high-efficiency equipment. When buying from a commercial source, choose models that qualify for the EPA/ENERGY STAR® label. Some manufacturers and retailers display the label on complying models. Alternatively, look at the yellow EnergyGuide label to identify models with efficient EER ratings.

Efficiency Losses
Unfortunately, the rating on the unit does not tell the whole story. Several factors can have a dramatic impact on the efficiency of (and hence the cost to operate) an air conditioning system. Duct leakage, particularly in the attic, garage, or crawlspace, can lead to losses of 30% or more! Unless the duct system was specially sealed, it is very likely that it leaks. Even in brand new systems, the majority leak a significant amount of air where the pieces are assembled. Other problems include improper refrigerant charge, low airflow, dirty indoor and outdoor coils, and recirculation. Even something as simple as a system that is larger than needed can cost more to run: see the section on replacement systems for more on proper A/C sizing.

Zoning
The addition of zones in central air conditioners is not usually recommended. This is because airflow across the indoor coil is critical to efficient operation, and zoned systems reduce airflow when only some zones are calling for cooling. Since the efficiency is reduced, there really is not much energy savings. If multiple zones are desired in the home, it is better to install two or more small central air systems to provide independent control of different areas.

When to Replace an Air Conditioning System
If you are the owner of an older air conditioning system, you may be wondering if it will last another year. You are concerned that it may stop working in the middle of the summer leaving you and your home hot and soggy. If this is the case, it's probably time to call on an expert to consider replacement or repair.

Home air conditioning systems can be expected to last for 15 years or more. Good maintenance practices will, of course, provide an even longer service life. Deciding when to replace an old cooling system is not easy. Unless your present system is old and in very poor working condition, it may be hard to justify a new high efficiency system on energy savings alone. If your system doesn't seem to be working as well as it used to, you should have a service person look at it to determine the likely cause of the problem. If there is a significant problem, simply comparing the price to repair the system with the cost of replacement will give you a good idea of what you should do. Keep in mind, however, that, due to improvements in efficiency, a new system will have the added benefit of reducing your operating cost. If your system is over 10 years old, you may want to consider a new unit. Especially if you have a large home with a high cooling load, the annual dollar savings from installing a new system may pay for itself in a short time.

Factors to Consider When Replacing an Air Conditioning System
Before you purchase a new system you should consider improving the efficiency of your home by adding insulation and sealing air leaks. This may have an impact on the size (and cost) of the new air conditioning system.

When planning for the replacement of your central air conditioning system, you will have an opportunity to make some efficiency improvements. You must first consider the tradeoff between the efficiency and cost of the new system. Furthermore, if your outdoor (condenser) unit is mounted on the roof or in an attic, you may want to move it to a cooler location when you install a new unit. That alone can make a large improvement in efficiency. If you are thinking of just changing the outdoor unit; keep in mind that the indoor unit may be just as old. Changing the outdoor unit to a newer high efficiency style may not give you the results you are paying for due to the mismatching of SEER ratings and age of equipment between units.

Sizing of Air Conditioning Systems
Central air conditioning system sizes are measured in tons. One ton of cooling is 12,000 Btu per hour, which is equivalent to the rate of cooling produced by one ton of ice melting over the course of a day. The larger the home, the larger the air conditioning unit that will be needed. Correct sizing is very important; an air conditioner that is too large will cycle on and off and be much less efficient. On the other hand, if it is too small it may not produce sufficient cooling.

Cooling Load Calculations
If you are installing a new air conditioning system or replacing an existing one, any contractor who is bidding on the job should perform a detailed cooling load calculation in order to select the proper system. They should not base their recommendation on simple "rules of thumb" which tend to oversize the equipment, and they should not simply replace the system with a new one of the same size. This is because all houses are different, the existing air conditioning system may have been oversized to begin with, and the house may have had insulation added or new windows installed since the original system was put in.

The contractor should measure your home to determine size of windows, the direction they face, the type of glass, the size of each room, the areas of walls and ceilings, and the amount of insulation. Preferably, they should do a room-by-room calculation to determine whether the existing ducts are adequate for the new system. In most cases, the time it takes them to perform these calculations should be offset by the savings in a smaller installed system.

Oversizing
Bigger is NOT necessarily better! Oversizing is common, because it is a way of compensating for potential distribution problems such as uninsulated or leaky ductwork. A new system will be much more efficient to operate if those problems are fixed first. An air conditioner that is too big will cycle on and off much more often, resulting in "blasts" of cold air, reducing efficiency, and increasing stress on components. In humid climates, oversized air conditioning will do a poor job of lowering the humidity, which is just as critical to comfort as the temperature setting in the room! In fact, a slightly undersized air conditioner will be just as comfortable—if not more—than an oversized air conditioner.

System Maintenance
Proper maintenance of an air conditioner will improve its efficiency and extend its life. This should be done at least every other year and it will require the services of a professional. The service call will involve a general tune-up, inspection, and cleaning of the system. Every tune-up should include an air filter change, inspection of the inside and outside coils (and cleaning if necessary), a check for proper air flow, and a check of all electrical connections.

In addition, the first time your air conditioner is serviced, the technician should check the refrigerant levels by testing something called "superheat" or "subcooling" (depending on the type of unit). Once they verify the proper refrigerant charge, no service person should attach refrigerant gauges again, unless you notice a decrease in performance or a sudden increase in your electric bills.

Also have the technician show you how to change the air filter. Dirty and clogged air filters result in improper supply of conditioned air and are the largest source of air conditioning service calls. In most cases, filters are easy to replace and result in more effective air conditioning. We recommend checking the air filters monthly, and replacing at least once a season as needed. Always follow the manufacturers instructions when replacing filters.When choosing an air filter, you have to be concerned about both filtering effectiveness and resistance to air flow. "Washable" electrostatic air filters create a lot of resistance which can reduce air flow and operating efficiency. On some systems, depending on the size of the ductwork, even a pleated filter can cause air flow problems. This can be checked by observing airflow with one and then the other filter in place. Air filters can be either permanent or disposable. Permanent electrostatic air filters, which require frequent cleaning, rely on static electricity to attract charged particles in the air stream. Permanent filters cost approximately $60 to $300, depending on size. High efficiency filters do not need to be replaced as often, but still should be checked at the beginning of the cooling season and replaced if dirty. Disposable panel air filters can be purchased at your local hardware store and range in price from as little as $1 to well over $40. The less expensive disposable fiberglass filters are not as efficient, and may cost you more in the long run. Upgrading the efficiency of the filter to remove smaller particles can help maintain the equipment as well as the health of employees and consumers.

Always keep the air vents open in a central air conditioning system, and don't obstruct them with furniture or draperies. In addition, do not attempt to cover or shade the outdoor coil; even a deck built above the unit, with open sides, can cause recirculation and reduce efficiency dramatically. Keeping leaves and dirt from accumulating on the sides of the outdoor unit helps to maximize heat transfer and minimize cooling bills.