Central Air Conditioners

 
Central Air Conditioners

Introduction
Cooling residential and commercial buildings in California typically draws 30% of the state's peak summer energy demand. Heating and cooling can account for more than 40% of a homeowner or business' annual energy bill. By installing a new central air conditioner in your home or business, you can dramatically reduce your annual cooling costs, save electricity and reduce smog causing emissions.

Technology Options
Air conditioners employ the same operating principles and basic components as a home refrigerator. An air conditioner cools a home with a cold indoor coil called the evaporator. The condenser, a hot outdoor coil, releases the collected heat outside. The evaporator and condenser coils are serpentine tubing surrounded by aluminum fins. The tubing is usually made of copper. A pump, called the compressor, moves a heat transfer fluid (or refrigerant) between the evaporator and the condenser by forcing the liquid refrigerant through the circuit of tubing and fins in the coils. The refrigerant evaporates in the indoor evaporator coil, pulling heat out of indoor air and thereby cooling the home. The hot refrigerant gas is pumped outdoors into the condenser where it reverts back to a liquid and gives up its heat to the air flowing over the condenser's metal tubing and fins.

As an air conditioner cools the air, it also dehumidifies the air, because warm air passing over the indoor evaporator coil cannot hold as much moisture as it carried at a higher temperature, before it was cooled. The extra moisture condenses on the outside of the coils and is carried away through a drain.

There are two general types of central air conditioners: split systems and packaged systems.
Split Systems An outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator. In many split-system air conditioners, the 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 the furnace or heat pump. If a home already has a furnace but no air conditioner, a split system is the most economical central air conditioner to install.
Packaged Systems 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 foundation. This type of air conditioner also is used in small commercial buildings. Air supply and return ducts come from indoors through an 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.
High-Efficiency Improvements
High-efficiency air conditioners usually contain one or more of the following technical improvements: High-efficiency compressors use less energy than traditional models to move the refrigerant; variable-speed fans can cycle between high speed (when cooling is most needed) and a lower speed for maintaining that cooling (this reduces on/off cycling, a major electrical draw); high-efficiency motors operate the fan using less electricity; and improved heat exchangers extract more heat out of the air that is drawn into the unit.

Efficiency Ratings
Air conditioners are rated by the number of British Thermal Units (Btu) of heat they can remove per hour. Another common rating term for air conditioning size is the "ton," which is 12,000 Btu per hour. Each air conditioner has an energy efficiency rating that lists how many Btu per hour are removed for each watt of power it draws. For central air conditioners, this is called the Energy Efficiency Ratio (EER) or the Seasonal Energy Efficiency Ratio (SEER), which is adjusted for a representative residential cooling load during the cooling season. The higher the EER/SEER, the more efficient the central air conditioner is.These ratings are posted on an EnergyGuide Label, which should be conspicuously attached to all new air conditioners.

National minimum standards for central air conditioners require a SEER of 9.7 for a packaged system and 10.0 for a split system. But there is a wide selection of units with SEERs reaching nearly 17.

Before 1979, the SEERs of central air conditioners ranged from 4.5 to 8.0. Replacing a 1970s-era central air conditioner having a SEER of 6 with a new unit having a SEER of 12 or better may cut your air conditioning costs in half.

publishes a guide to the most energy-efficient appliances. Visit its for the top-rated models in various sizes.

Efficiency Benefits
Central air conditioners that are in the top 25% of efficient models may carry the label. To qualify, they must have a minimum SEER of 12 for packaged systems (also 10.5 EER) and 13 for split systems (also 11 EER). Air conditioners that bear the ENERGY STAR label may be twice as efficient as some existing systems. 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. A new central air conditioner may also perform more quietly and provide better humidity control than your old model.

The Federal Energy Management Program (FEMP) of the U.S. Department of Energy (DOE) has prepared the following cost effectiveness example comparing air conditioning units that meet the federal minimum standard, the ENERGY STAR standard and the best available efficiency product. The values are based on a central air conditioner with a 36,000 Btu/hour capacity and national averages for cooling loads and electricity prices. Actual savings and energy use will vary with climate and usage.

Cost Comparison of Standard and ENERGY STAR Qualified Central Air Conditioners

Performance
Federal Minimum
ENERGY STAR Minimum
Best Available

EER
9.2
11.0
14.5

SEER
10.0
13.0
16.5

Annual Energy Use
3,600 kWh
2,770 kWh
2,000 kWh

Annual Energy Cost
$216
$166
$131

Lifetime Energy Cost
$2,350
$1,800
$1,420

Lifetime Energy Cost Savings
-
$550
$930Source: DOE, FEMP

Use ENERGY STAR's central air conditioner calculator to estimate how much money and energy you can save with a highly efficient unit.

Purchasing Tips

Select the right size air conditioner by considering these factors:

In which climate is the building located?
What is the size of the building? How many windows are in the building?
How much shade is on the building's windows, walls and roof?
How much insulation is in the building's ceiling and walls?
How much air leaks into the building from the outside?
How much heat do the occupants and appliances in the building generate?
Be aware that a large air conditioner will not provide the best cooling. An oversized air conditioner costs more to buy and cycles on and off more frequently, reducing its efficiency. Frequent cycling makes indoor temperatures fluctuate more and results in a less comfortable environment. Frequent cycling also inhibits moisture removal and wears out the compressor and electrical parts more rapidly. A larger air conditioner also uses more electricity and creates added demands on electrical generation and delivery systems.
Choose your contractor carefully. Choosing a contractor may be the most important and difficult task in buying a new central air conditioning system. Make sure to get bids from at least three contractors and ask prospective contractors for recent references. If you are replacing your central air conditioner, tell your contractor what you liked and did not like about the old system. If the system failed, ask the contractor to find out why. The best time to fix past problems is when a new system is being installed. Visit the Air Conditioning Contractors of America for help in selecting a good contractor.
Consider upgrading windows, insulation, roof or duct work to realize full potential savings from a new central air conditioner. And, consider coupling your central air conditioner with a new ENERGY STAR programmable thermostat, which can provide significant savings (up to $100).