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Air Conditioning 101

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Air conditioning is fairly simple when you get down to the basics. All air conditioners, whether they're small window units or central systems that cool an entire house, work on a similar cooling principle, much like your household refrigerator. And just like refrigerators, they come in different sizes, functionalities, and energy efficiencies. In order to ensure a long and healthy life for your air conditioning system, there are a few basic maintenance checks that should be performed—some by you and some best left for the pros.

Types of Air Conditioners

There are 2 basic types of air conditioners—room air conditioners and central air conditioners. There are several different kinds within each type.

Room air conditioners are specialized units designed and sized to cool one room or a large open area. They are installed either in windows or through walls. They do not use ducts to distribute air, but rely on a fan to blow air across and around the area. Small capacity units can be plugged into standard power outlets, while larger units may need to be wired in.

Central air conditioners today are generally "split systems," in which the condensing unit of the air conditioner is located outside the home and the evaporator coil part is located inside in the air handler. The handler generally is located somewhere in the home such as the garage, attic, or a hall closet. The duct system generally runs through the attic, crawl space, or living area of the home to distribute conditioned air to the various rooms.

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. Packaged air conditioners often include electric heating coils or a natural gas furnace, which eliminates the need for a separate heater indoors.

How It Works

Air conditioners and refrigerators are very similar in mechanics; the latter just happens to come in a more attractive casing. Instead of cooling just the small, insulated space inside of a refrigerator, an air conditioner cools a room, or an entire house.

All air conditioners, whether they're small window units or central air systems, work on a similar principle. 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 the condenser are coils of copper tubing surrounded by aluminum cooling fins.

A pump, called the compressor, moves refrigerant gas 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. Outside the house, the refrigerant releases its heat into the air through the condenser, reverts back to a liquid, and is forced back into the house for more cooling.

An air conditioner's capacity is measured in British thermal units, or Btu’s. A Btu is the amount of heat required to raise by 1° the temperature of a pound of water. So if you have an air conditioner rated at 10,000 Btu, it has the ability to cool 10,000 lbs.—about 1,200 gal.—of water 1° in an hour. A 60,000-Btu system is generally adequate to cool a 2,000 ft2 house.

Care and Maintenance

Dirty coils and improper refrigerant levels can cause your system to cool less efficiently than it should. An inefficient system means higher energy bills. Your compressor works harder than it should, and the life span of your unit is shortened.

With good maintenance, your air conditioner should last about 15 years. You can keep your system running efficiently with these easy tips:

  • Check all hose connections for leaks, and make sure the condensate tube is draining freely.
  • Change or clean the air filter once a month. This is especially important during the summer when dust and allergens are prevalent.
  • Keep plants and shrubbery at least 2 ft. away for adequate airflow around the entire unit.
  • Vacuum the air vents and registers, or grills, regularly, and consider a professional duct cleaning every few years.
  • Keep furniture and drapes away from registers.

There are other maintenance procedures that are best handled by a professional service provider. Consider having a professional inspection done once a year by an experienced technician to ensure that your system is running efficiently. Some technicians perform free inspections, while others charge a fee for service calls. Any repairs are likely to result in additional costs.

WARNING: If you work on your air conditioning system yourself, shut off the power at the service panel before you start.

Air Conditioning Problems

According to the Department of Energy, if your air conditioner fails, it is usually due to lack of routine maintenance or for one of the common reasons listed below:

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 going to work. A trained technician should fix the leak, test the repair, and then charge the system with the correct amount of refrigerant. The performance and efficiency of your air conditioner is best when the refrigerant charge matches the manufacturer's specification exactly, and is neither undercharged nor overcharged.

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. Corrosion of wire and terminals is also a problem in many systems, so electrical connections and contacts should be checked during a professional service call.

Repair or Replace?

As a general rule, if your unit is less than 5 years old, it’s probably better to repair it. If it’s over 8 years old and the repairs are major, consider replacing it. If it’s somewhere in between, it depends on your warranty and the price of the failing parts.

If you are replacing your air conditioner, look for the 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.

If you have an older unit with a SEER rating of less than 8, the increased efficiency of a new one can reduce your monthly power bill. According to the Department of Energy, each increase of 1 point on the SEER rating results in an approximate 10% increase in efficiency. Look for the ENERGY STAR® label for central air conditioners with SEER ratings of 12 or greater.