Central air conditioners circulate cool air through a system of supply and return ducts. Supply ducts and registers (i.e., 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.
Types of Central Air Conditioners
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.
Choosing or Upgrading Your Central Air Conditioner
Central air conditioners are more efficient than room air conditioners. In addition, they are out of the way, quiet, and convenient to operate. To save energy and money, you should try to buy an energy-efficient air conditioner and reduce your central air conditioner's energy use.
If you are considering adding central air conditioning to your home, the deciding factor may be the need for ductwork.
If you have an older central air conditioner, you might choose to replace the outdoor compressor with a modern, high-efficiency unit. If you do so, consult a Reid Brothers’ heating and cooling consultant to assure that the new compressor is properly matched to the indoor unit. However, considering recent changes in refrigerants and air conditioning designs, it might be wiser to replace the entire system.
Today's best air conditioners use 30%–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%–40% of your cooling energy costs by replacing it with a newer, more efficient model.
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, lack of insulation, and improper duct installation can greatly diminish efficiency.
When buying an air conditioner, look for a model with a high efficiency. Central air conditioners are rated according to their seasonal energy efficiency ratio (SEER). SEER indicates the relative amount of energy needed to provide a specific cooling output. Many older systems have SEER ratings of 6 or less. The minimum SEER allowed today is 13. Look for the ENERGY STAR® label for central air conditioners with SEER ratings of 13 or greater, but consider using air conditioning equipment with higher SEER ratings for greater savings.
Other Features to Look For When Buying an Air Conditioner:
· A thermal expansion valve and a high-temperature rating (EER) greater than 11.6, for high-efficiency operation when the weather is at its hottest
· A variable speed air handler for new ventilation systems
· A unit that operates quietly
· 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
If your air conditioner is installed correctly, or if major installation problems are found and fixed, it will perform efficiently for years with annual routine maintenance.
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
· 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
· Locates the condensing unit where no nearby objects will block the flow of air to it
· 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.)