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Air Conditioning Units - Heating, Ventilation and Cooling Units- HVAC

Summary: This fact sheet (from provides information on how to make your air conditioning more energy efficient.

Energy-Efficient AC Units

Window air conditioning unitAre you considering buying a new AC unit? Or, are you dissatisfied with the operation of your current AC unit? Are you unsure whether to fix or replace it? Are you concerned about high summer utility bills? If you answered yes to any of these questions, this article can help. With it, you can learn about various types of AC systems and how to maintain your AC unit, hire professional air conditioning services, select a new air conditioner, and ensure that your new air conditioner is properly installed.

Understanding AC Units

Many people buy or use AC units without understanding their designs, components, and operating principles. Proper sizing, selection, installation, maintenance, and correct use are keys to cost-effective operation and lower overall costs.

This publication discusses both central and room AC units. Heat pumps, which provide both home cooling and heating, are not covered in this article.

How AC Units Work

AC Units 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.

Types of AC Units

The basic types of air conditioners are room AC units, split-system central AC units, and packaged central AC units.

Room AC Units
Room AC units cool rooms rather than the entire home. If they provide cooling only where they're needed, room AC units are less expensive to operate than central units, even though their efficiency is generally lower than that of central AC units.

Smaller room AC units (i.e., 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 AC units (i.e., those drawing more than 7.5 amps) need their own dedicated 115-volt circuit. The largest models require a dedicated 230-volt circuit.

heat pump ac unitCentral AC units
Central AC units 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 AC unit to the home. This cooled air becomes warmer as it circulates through the home; then it flows back to the central AC unit through return ducts and registers. A central AC unit is either a split-system unit or a packaged unit.

In a split-system central AC unit, 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 AC unit'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 AC unit, a split-system is the most economical central air conditioner to install.

In a packaged central AC unit, 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 AC unit 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 AC unit, which is usually located outdoors. Packaged AC units often include electric heating coils or a natural gas furnace. This combination of AC unit and central heater eliminates the need for a separate furnace indoors.

Portable AC Units - There is growing popularity for smaller and easily portable air conditioning units. Such units allow one to select a portable air conditioner appropriately sized to the area to be cooled; and even relocate it as cooling needs change.

Maintaining Existing AC Units

Older AC units may still be able to offer years of relatively efficient use. However, making your older AC unit last requires you to perform proper operation and maintenance.

Air Conditioning Problems
One of the most common air conditioning problems is improper operation. If your AC unit is on, be sure to close your home's windows and outside doors.

Other common problems with existing AC units result from faulty installation, poor service procedures, and inadequate maintenance. Improper installation of your AC unit can result in leaky ducts and low air flow. 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. Service technicians often fail to find refrigerant charging problems or even worsen existing problems by adding refrigerant to a system that is already full. AC unit manufacturers generally make rugged, high quality products. If your AC unit fails, it is usually for one of the common reasons listed below:

  • refrigerant leaks. If your AC unit 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 AC unit is greatest when the refrigerant charge exactly matches the manufacturer's specification, and is neither undercharged nor overcharged.
  • inadequate maintenance. If you allow filters and air conditioning coils to become dirty, the AC unit 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 AC unit 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.

Regular Maintenance

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.

AC Unit Filters
The most important maintenance task that will ensure the efficiency of your AC unit 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. Filters are located somewhere along the return duct's length. Common filter locations are in walls, ceilings, furnaces, or in the air conditioner itself.

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 AC unit is in constant use, is subjected to dusty conditions, or you have fur-bearing pets in the house.

AC Unit Coils
The AC unit'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 which reduces its ability to absorb heat. Therefore, your evaporator coil should be checked every year and cleaned 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.

Sealing and Insulating Air Ducts

An enormous waste of energy occurs when cooled air escapes from supply ducts or when hot attic air leaks into return ducts. Recent studies indicate that 10% to 30% of the conditioned air in an average central air conditioning system escapes from the ducts.

For central air conditioning to be efficient, ducts must be airtight. Hiring a competent professional service technician to detect and correct duct leaks is a good investment, since leaky ducts may be difficult to find without experience and test equipment. Ducts must be sealed with duct "mastic." The old standby of duct tape is ineffective for sealing ducts.

Obstructions can impair the efficiency of a duct system almost as much as leaks. You should be careful not to obstruct the flow of air from supply or return registers with furniture, drapes, or tightly fitted interior doors. Dirty filters and clogged evaporator coils can also be major obstructions to air flow.

The large temperature difference between attics and ducts makes heat conduction through ducts almost as big a problem as air leakage and obstructions. Ducts in attics should be insulated heavily in addition to being made airtight.

Buying New AC Units

Today's best AC units use 30% to 50% less energy to produce the same amount of cooling as AC units made in the mid 1970s. Even if your AC unit 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.

Sizing AC Units
AC units 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 AC unit be? The size of an AC unit 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 AC unit's efficiency, performance, durability, and initial cost depend on matching its size to the above factors.

Make sure you buy the correct size of AC unit. 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 AC units. Reputable air conditioning contractors will use one of these procedures, often performed with the aid of a computer, to size your new central AC unit.

Be aware that a large AC unit will not provide the best cooling. Buying an oversized AC unit penalizes you in the following ways.

  • It costs more to buy a larger AC unit 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.
  • A larger air conditioner uses more electricity and creates added demands on electrical generation and delivery systems.

AC Unit Efficiency

Each AC unit has an energy-efficiency rating that lists how many Btu per hour are removed for each watt of power it draws. For room AC units, this efficiency rating is the Energy Efficiency Ratio, or EER. For central AC units, 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 AC units. Many AC unit manufacturers are participants in the voluntary EnergyStar® labeling program. EnergyStar-labeled appliances mean that they have high EER and SEER ratings.

In general, new AC units 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 AC unit by rebating some or all of the price difference. Buy the most efficient AC unit you can afford, especially if you use (or think you will use) an AC unit 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 AC units 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% from 1972 to 1991. If you own a 1970s-vintage room AC unit 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 AC Units—SEER
National minimum standards for central AC units 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 AC units ranged from 4.5 to 8.0. Replacing a 1970s-era central AC unit 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 AC unit, 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 AC unit;
  • provide a written contract listing the main points of your installation that includes the results of the cooling load calculation;
  • 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 AC Units

If your AC unit 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 AC units are not installed correctly. As an unfortunate result, modern energy-efficient AC units 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 AC unit.
  • 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% to 2%.
  • verifies that the newly installed AC unit 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 AC unit'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 AC unit, try to:

  • locate the AC unit in a window or wall area near the center of the room and on the shadiest side of the house.
  • minimize air leakage by fitting the room AC unit snugly into its opening and sealing gaps with a foam weatherstripping 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 AC unit, be sure it is sized and installed correctly and has a good EER or SEER rating.


Evaporative Coolers

An evaporative cooler (also called a "swamp cooler") is a completely different type of AC unit that works well in hot, dry climates.

These units cool outdoor air by evaporation and blow it inside the building, causing a cooling effect much like the process when evaporating perspiration cools your body on a hot (but not overly humid) day. When operating an evaporative cooler, windows are opened part way to allow warm indoor air to escape as it is replaced by cooled air.

Evaporative coolers cost about one-half as much to install as central AC units and use about one-quarter as much energy. However, they require more frequent maintenance than refrigerated air conditioners and they're suitable only for areas with low humidity.

Using Your AC Unit

An air conditioner will cool the air in your home fairly quickly. For economical operation, turn it on only when your home is occupied. You may consider installing a programmable thermostat. These allow you to set the time when the AC unit will turn on, such as 30 minutes before you arrive home from work on a hot day. During the day, keep the drapes or blinds closed on windows that face east, south, and west. This will help reduce solar heat gain into your home.


News & Notes has compiled the data herein from the U.S. Department of Energy Website only for the convenience of our readers. For currently accurate, up-to-date and/or additional information visit


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