How does the system work?

Air conditioners employ the same operating principles and basic components as your home refrigerator. Refrigerators use energy (usually electricity) to transfer heat from the cool interior of the refrigerator to the relatively warm surroundings of your home; likewise, an air conditioner uses energy to transfer heat from the interior of your home to the relatively warm outside environment.

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 your home. The hot refrigerant gas is pumped outdoors into the condenser where it reverts back to a liquid, giving up its heat to the outside air flowing over the condenser's metal tubing and fins.

Throughout the second half of the 20th century, nearly all air conditioners used chlorofluorocarbons (CFCs) as their refrigerant, but because these chemicals are damaging to Earth's ozone layer, CFC production stopped in the United States in 1995. Nearly all air conditioning systems now employ halogenated chlorofluorocarbons (HCFCs) as a refrigerant, but these are also being gradually phased out, with most production and importing stopped by 2020 and all production and importing stopped by 2030.

Production and importing of today's main refrigerant for home air conditioners, HCFC-22 (also called R-22), will begin to be phased out in 2010 and will stop entirely by 2020. However, HCFC-22 is expected to be available for many years as it is recovered from old systems that are taken out of service. As these refrigerants are phased out, ozone-safe hydrofluorocarbons (HFCs) are expected to dominate the market, as well as alternative refrigerants such as ammonia.

Maintaining Your Air Conditioner

An air conditioner's filters, coils, and fins require regular maintenance for the unit to function effectively and efficiently throughout its years of service. Neglecting necessary maintenance ensures a steady decline in air conditioning performance while energy use steadily increases.

Air Conditioner Filters

The most important maintenance task that will ensure the efficiency of your air conditioner is to routinely replace or clean its filters. Clogged, dirty filters block normal air flow and reduce a system's efficiency significantly. With normal air flow obstructed, air that bypasses the filter may carry dirt directly into the evaporator coil and impair the coil's heat-absorbing capacity. Keeping the filter clean can lower your air conditioner's energy consumption by 5%–15%.

For central air conditioners, filters are generally located somewhere along the return duct's length. Common filter locations are in walls, ceilings, furnaces, or in the air conditioner itself. Room air conditioners have a filter mounted in the grill that faces into the room.

Some types of filters are reusable; others must be replaced. They are available in a variety of types and efficiencies. Clean or replace your air conditioning system's filter or filters every month or two during the cooling season. Filters may need more frequent attention if the air conditioner is in constant use, is subjected to dusty conditions, or you have fur-bearing pets in the house.

Air Conditioner Coils

The air conditioner's evaporator coil and condenser coil collect dirt over their months and years of service. A clean filter prevents the evaporator coil from soiling quickly. In time, however, the evaporator coil will still collect dirt. This dirt reduces air flow and insulates the coil, reducing its ability to absorb heat. To avoid this problem, check your evaporator coil every year and clean it as necessary.

Outdoor condenser coils can also become very dirty if the outdoor environment is dusty or if there is foliage nearby. You can easily see the condenser coil and notice if dirt is collecting on its fins.

You should minimize dirt and debris near the condenser unit. Your dryer vents, falling leaves, and lawn mower are all potential sources of dirt and debris. Cleaning the area around the coil, removing any debris, and trimming foliage back at least 2 feet (0.6 meters) allow for adequate air flow around the condenser.

Coil Fins

The aluminum fins on evaporator and condenser coils are easily bent and can block air flow through the coil. Air conditioning wholesalers sell a tool called a "fin comb" that will comb these fins back into nearly original condition.

Condensate Drains

Occasionally pass a stiff wire through the unit's drain channels. Clogged drain channels prevent a unit from reducing humidity, and the resulting excess moisture may discolor walls or carpet.

Window Seals for Room Air Conditioners

At the start of each cooling season, inspect the seal between the air conditioner and the window frame to ensure it makes contact with the unit's metal case. Moisture can damage this seal, allowing cool air to escape from your house.

Common Problems with Air Conditioners

One of the most common air conditioning problems is improper operation. If your air conditioner is on, be sure to close your home's windows and outside doors. For room air conditioners, isolate the room or a group of connected rooms as much as possible from the rest of your home.

Other common problems with existing air conditioners result from faulty installation, poor service procedures, and inadequate maintenance. Improper installation of a central air conditioner can result in leaky ducts and low airflow. Many times, the refrigerant charge (the amount of refrigerant in the system) does not match the manufacturer's specifications. If proper refrigerant charging is not performed during installation, the performance and efficiency of the unit is impaired. Unqualified service technicians often fail to find refrigerant charging problems or even worsen existing problems by adding refrigerant to a system that is already full.

Air conditioner manufacturers generally make rugged, high quality products. If your air conditioner fails, begin by checking any fuses or circuit breakers. Let the unit cool down for about five minutes before resetting any breakers. If a central air conditioner's compressor stops on a hot day, the high-pressure limit switch may have tripped; reset it by pushing the button, located in the compressor's access panel. Other common reasons for air conditioner failures are 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 a solution. A trained technician should fix any leak, test the repair, and then charge the system with the correct amount of refrigerant. Remember that the performance and efficiency of your air conditioner is greatest when the refrigerant charge exactly matches the manufacturer's specification, and is neither undercharged nor overcharged. Refrigerant leaks can also be harmful to the environment.

Inadequate Maintenance

If you allow filters and air conditioning coils to become dirty, the air conditioner will not work properly, and the compressor or fans are likely to fail prematurely.

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

Sensor problems

Room air conditioners feature a thermostat sensor, located behind the control panel, which measures the temperature of air coming into the evaporative coil. If the sensor is knocked out of position, the air conditioner could cycle constantly or behave erratically. The sensor should be near the coil but not touching it; adjust its position by carefully bending the wire that holds it in place.

Drainage problems

When it's humid outside, check the condensate drain to make sure it isn't clogged and is draining properly. Room air conditioners may not drain properly if not mounted level.

Hiring Professional Services for Air Conditioner Maintenance

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 do the following:

• 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
• Check the accuracy of the thermostat.

Central Air Conditioners

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.

Air conditioners help to dehumidify the incoming air, but in extremely humid climates or in cases where the air conditioner is oversized, it may not achieve a low humidity. Running a dehumidifier in your air conditioned home will increase your energy use, both for the dehumidifier itself and because the air conditioner will require more energy to cool your house. A preferable alternative is a dehumidifying heat pipe, which can be added as a retrofit to most existing systems.

Types of Central Air Conditioners

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. In an average air-conditioned home, air conditioning consumes more than 2000 kilowatt-hours of electricity per year, causing power plants to emit about 3500 pounds of carbon dioxide and 31 pounds of sulfur dioxide.

If you are considering adding central air conditioning to your home, the deciding factor may be the need for ductwork. See the section on limitations when replacing existing systems for more information.

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 local heating and cooling contractor 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.

New residential central air conditioner standards went into effect on January 23, 2006. Air conditioners manufactured after January 26, 2006 must achieve a Seasonal Energy Efficiency Ratio (SEER) of 13 or higher. SEER 13 is 30% more efficient than the previous minimum SEER of 10. The standard applies only to appliances manufactured after January 23, 2006. Equipment with a rating less than SEER 13 manufactured before this date may still be sold and installed.

The average homeowner will remain unaffected by this standard change for some time to come. The standards do not require you to change your existing central air conditioning units, and replacement parts and services should still be available for your home's systems. The "lifespan" of a central air conditioner is about 15 to 20 years. Manufacturers typically continue to support existing equipment by making replacement parts available and honoring maintenance contracts after the new standard goes into effect.

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 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) Manual D
• 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.
• Places the condensing unit in a shady spot, if possible, which can reduce your air conditioning costs by 1%–2%.
• 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.)

To improve energy efficiency for an existing home, thermal images from an energy house inspection or home energy audit will reveal many of the leaks in the thermal envelop. After the review you will have a roadmap to proper weatherization. Whether you are the do it yourself, "DYI" person or want to hand off to a contractor or handyman, the roadmap will help you make the best decision first. Fixing whats found will greatly improve your comfort, savings and reduce your carbon footprint.

www.eere.energy.gov