U.S. Department of Energy - Energy Efficiency and Renewable
Energy Energy Savers
02/07/12
Energy-Efficient Air Conditioning
Summary: This fact sheet will provide you with information on
how to make your air conditioning system more energy efficient.
To view illustrations, you can download the PDF version (PDF 233
KB) of this fact sheet (Download Acrobat Reader). See Related
Links below for more publications on related topics, which
aren't included in the PDF version.
Are you considering buying a new air conditioner? Or, are you
dissatisfied with the operation of your current air conditioner?
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 publication can help. With it, you can
learn about various types of air conditioning systems and how to
maintain your air conditioner, hire professional air
conditioning services, select a new air conditioner, and ensure
that your new air conditioner is properly installed.
Understanding Air Conditioners
Many people buy or use air conditioners 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 air
conditioners. Heat pumps, which provide both home cooling and
heating, are not covered in this publication. For more
information, see Air-Source Heat Pumps and Geothermal Heat Pumps
under Related Links below.
How Air Conditioners Work
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.
Types of 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 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 (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 air conditioners (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.
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. 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.
Maintaining Existing Air Conditioners
Older air conditioners may still be able to offer years of
relatively efficient use. However, making your older air
conditioner 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 air
conditioner is on, be sure to close your home's windows and
outside doors.
Other common problems with existing air conditioners result from
faulty installation, poor service procedures, and inadequate
maintenance. Improper installation of your air conditioner 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. Air conditioner manufacturers generally
make rugged, high quality products. If your air conditioner
fails, it is usually 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 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. 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. 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.
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. 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 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 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 Air Conditioners
Today's best air conditioners use 30% to 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% to 40% 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. 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 (see Source List in this
publication). 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% 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; 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 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. 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 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.
minimize air leakage by fitting the room air conditioner 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 air conditioner, be
sure it is sized and installed correctly and has a good EER or
SEER rating.
Sidebars:
Evaporative Coolers
An evaporative cooler (also called a "swamp cooler") is a
completely different type of air conditioner 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 air conditioners 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 Air Conditioner
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 air
conditioner will turn on, such as 30 minutes before you arrive
home from work on a hot day. For more information programmable
thermostats, see Related Links below. 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.
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Source List
There are many groups offering information on air conditioning.
The following groups are just a few that can assist you in
increasing your air conditioning efficiency. Some of these
organizations provide only materials appropriate for
professionals in the air conditioning industry.
DOE Office of Energy Efficiency and Renewable Energy (EERE) Web Site
Your comprehensive online resource for DOE's energy efficiency
and renewable energy information.
Air Conditioning and Refrigeration Institute (ARI)
ARI represents manufacturers of air conditioning, refrigeration,
and heating equipment and has consumer brochures on a variety of
topics.
American Society of Heating, Refrigeration, and Air Conditioning
Engineers (ASHRAE)
ASHRAE is organized solely for the purpose of advancing the arts
and science of heating, ventilation, air conditioning, and
refrigeration for the public's benefit through research,
standards writing, continuing education, and publications.
Air Conditioning Contractors of America (ACCA)
The ACCA is the most active and widely recognized organization
representing contractors in the heating, ventilation, air
conditioning, and refrigeration (HVACR) industry.
EnergyStar?
ENERGY STAR?-labeled products, including heating and cooling
equipment, use less energy than other products and save you
money on utility bills. These products are made by all major
manufacturers and are available at stores everywhere.
Florida Solar Energy Center (FSEC)
FSEC provides information on building in hot, humid climates.
For more information about cooling and air conditioning, as well
as on a wealth of other energy-efficiency topics, contact:
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Reading List
The following publications provide more information on air
conditioning. The list does not cover all the available books,
reports, and articles on air conditioning, nor is the mention of
any publication a recommendation or endorsement.
ASHRAE Journal
Consumer Guide to Home Energy Savings
American Council for an Energy-Efficient Economy
Home Energy Magazine
Consumer Reports
"Chill Out: A Guide to Air Conditioning," (63:6) pp. 36-42, June
1998. "Cool It (Room/Window Air Conditioners)" (64:6) pp. 35-37,
June 1999.
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Related Links
Here's more information on cooling your home energy efficiently:
Air-Source Heat Pumps
Automatic and Programmable Thermostats
Cooling Your Home Naturally
Cooling Your Home with Fans and Ventilation
Geothermal Heat Pumps
Insulation
Sizing Residential Heating and Air Conditioning Systems
Weatherize Your Home—Caulk and Weather Strip
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This fact sheet was produced for the U.S. Department of Energy
(DOE) by the National Renewable Energy Laboratory, a DOE
national laboratory.
DOE/GO-10099-379 FS 206 June 1999
NOTICE This report was prepared as an account of work sponsored
by an agency of the United States government. Neither the United
States government nor any agency thereof, nor any of their
employees, makes any warranty, express or implied, or assumes
any legal liability or responsibility for the accuracy,
completeness, or usefulness of any information, apparatus,
product, or process disclosed, or represents that its use would
not infringe privately owned rights. Reference herein to any
specific commercial product, process, or service by trade name,
trademark, manufacturer, or otherwise does not necessarily
constitute or imply its endorsement, recommendation, or favoring
by the United States government or any agency thereof. The views
and opinions of authors expressed herein do not necessarily
state or reflect those of the United States government or any
agency thereof.
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