  Many central air conditioning systems locate the evaporator, condenser, and compressor in a single cabinet. Credit: tommymac.us |
|
Air conditioners employ the same operating principles and basic components as your home refrigerator. An air conditioner cools your 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. This tubing is usually made of copper. A pump, called the compressor, moves a heat transfer fluid (or refrigerant) between the evaporator and the condenser. The pump forces the refrigerant through the circuit of tubing and fins in the coils. The liquid 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 giving up its heat to the air flowing over the condenser’s metal tubing and fins.
Room Air Conditioners
The basic types of air conditioners are room air conditioners, split-system central air conditioners, and packaged central air conditioners. Room air conditioners cool rooms rather than the entire home. If they provide cooling only where they’re needed, room air conditioners are less expensive to operate than central units, even though their efficiency is generally lower than that of central air conditioners.
Smaller room air conditioners (those drawing less than 7.5 amps of electricity) can be plugged into any 15- or 20-amp, 115-volt household circuit that is not shared with any other major appliances. Larger room air conditioners (those drawing more than 7.5 amps) need their own dedicated 115-volt circuit. The largest models require a dedicated 230-volt circuit.
Central Air Conditioners
Central air conditioners circulate cool air through a system of supply and return ducts. Supply ducts and registers (openings in the walls, floors, or ceilings covered by grills) carry cooled air from the air conditioner to the home. This cooled air becomes warmer as it circulates through the home; then it flows back to the central air conditioner through return ducts and registers. A central air conditioner is 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 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.
Today’s best air conditioners use 30 percent to 50 percent 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 percent to 40 percent of your cooling energy costs by replacing it with a newer, more efficient model.
Sizing Air Conditioners
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.
How big should your air conditioner be? The size of an air conditioner depends on:
• how large your home is and how many windows it has;
• how much shade is on your home’s windows, walls, and roof;
• how much insulation is in your home’s ceiling and walls;
• how much air leaks into your home from the outside; and
• how much heat the occupants and appliances in your home generate.
An air conditioner’s efficiency, performance, durability, and initial cost depend on matching its size to the above factors.
Make sure you buy the correct size of air conditioner. Two groups—the Air Conditioning Contractors of America (ACCA) and the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE)—publish calculation procedures for sizing central air conditioners. Reputable air conditioning contractors will use one of these procedures, often performed with the aid of a computer, to size your new central air conditioner.
Be aware that a large air conditioner will not provide the best cooling. Buying an oversized air conditioner penalizes you in the following ways. It costs more to buy a larger air conditioner than you need. The larger-than-necessary air conditioner 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. In humid climates, removing moisture is essential for acceptable comfort. In addition, this cycling wears out the compressor and electrical parts more rapidly. Additionally, a larger air conditioner uses more electricity and creates added demands on electrical generation and delivery systems.
Air Conditioner Efficiency
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 room air conditioners, this efficiency rating is the Energy Efficiency Ratio, or EER. For central air conditioners, it is the Seasonal Energy Efficiency Ratio, or SEER. These ratings are posted on an Energy Guide Label, which must be conspicuously attached to all new air conditioners. Many air conditioner manufacturers are participants in the voluntary EnergyStar® labeling program. EnergyStar-labeled appliances mean that they have high EER and SEER ratings.
In general, new air conditioners with higher EERs or SEERs sport higher price tags. However, the higher initial cost of an energy-efficient model will be repaid to you several times during its life span. Your utility company may encourage the purchase of a more efficient air conditioner by rebating some or all of the price difference. Buy the most efficient air conditioner you can afford, especially if you use (or think you will use) an air conditioner frequently and/or if your electricity rates are high.
Room Air Conditioners—EER
Room air conditioners generally range from 5,500 Btu per hour to 14,000 Btu per hour. National appliance standards require room air conditioners built after January 1, 1990, to have an EER 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.
The Association of Home Appliance Manufacturers reports that 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.
Central Air Conditioners—SEER
National minimum standards for central air conditioners require a SEER of 9.7 and 10.0, for single-package and split-systems, respectively. But you do not need to settle for the minimum standard—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 with a SEER of 6 with a new unit having a SEER of 12 will cut your air conditioning costs in half.
Hiring Professional Service
When your air conditioner needs more than the regular maintenance described previously, hire a professional service technician. A well-trained technician will find and fix problems in your air conditioning system. However, not all service technicians are competent. Incompetent service technicians forsake proper diagnosis and perform only minimal stop-gap measures. Insist that the technician:
• check for correct amount of refrigerant;
• test for refrigerant leaks using a leak detector;
• capture any refrigerant that must be evacuated from the system, instead of illegally releasing it to the atmosphere;
• check for and seal duct leakage in central systems;
• measure air flow through the evaporator coil;
• verify the correct electric control sequence and make sure that the heating system and cooling system cannot operate simultaneously;
• inspect electric terminals, clean and tighten connections, and apply a non-conductive coating if necessary;
• oil motors and check belts for tightness and wear; and
• check the accuracy of the thermostat.
Choosing a Contractor
Choosing a contractor may be the most important and difficult task in buying a new central air conditioning system. 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 existing problems is when a new system is being installed.
When designing your new air conditioning system, the contractor you choose should use a computer program or written calculation procedure to size the air conditioner. Expect to receive a written contract listing the main points of your installation that includes the results of the cooling load calculation. Reputable installers will also give you a written warranty on equipment and workmanship, and allow you to hold the final payment until you are satisfied with the new system.
Avoid making your decision solely on the basis of price. The quality of the installation should be your highest priority, because quality will determine energy cost, comfort, and durability.
Installation and Location of Air Conditioners
If your air conditioner is installed correctly, or if major installation problems are found and fixed, it will perform efficiently for years with only minor routine maintenance. However, many air conditioners are not installed correctly. As an unfortunate result, modern energy-efficient air conditioners can perform almost as poorly as older inefficient models.
Be sure that your contractor performs the following procedures when installing a new central air conditioning system:
• allows adequate indoor space for the installation, maintenance, and repair of the new system, and installs an access door in the furnace or duct to provide a way to clean the evaporator coil.
• uses a duct-sizing methodology such as the Air Conditioning Contractors of America (ACCA).
• ensures there are enough supply registers to deliver cool air and enough return air registers to carry warm house air back to the air conditioner.
• installs duct work within the conditioned space, not in the attic, wherever possible.
• seals all ducts with duct mastic and heavily insulates attic ducts.
• locates the condensing unit where its noise will not keep you or your neighbors awake at night, if possible.
• places the condensing unit in a shady spot, if possible, which can reduce your air conditioning costs by 1 percent to 2 percent.
• verifies that the newly installed air conditioner has the exact refrigerant charge and air flow rate specified by the manufacturer.
• locates the thermostat away from heat sources, such as windows, or supply registers.
If you are replacing an older or failed split system, be sure that the evaporator coil is replaced with a new one that exactly matches the condenser coil in the new condensing unit. (The air conditioner’s efficiency will likely not improve if the existing evaporator coil is left in place; in fact, the old coil could cause the new compressor to fail prematurely.)
If you install a new room air conditioner, try to locate the air conditioner in a window or wall area near the center of the room and on the shadiest side of the house. Also, minimize air leakage by fitting the room air conditioner snugly into its opening and sealing gaps with a foam weather stripping material.
Paying attention to your air conditioning system saves you money and reduces environmental pollution. Notice whether your existing system is running properly, and maintain it regularly. Or, if you need to purchase a new air conditioner, be sure it is sized and installed correctly and has a good EER or SEER rating.
The EnergyWise House in an ongoing project that examines energy efficient building techniques, materials, and methods.
Credit: Energy Efficiency and Renewable Energy Clearinghouse
