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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:
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How large your home is and how many windows it has;
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How much shade is on your home's windows, walls, and
roof;
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How much insulation is in your home's ceiling and
walls;
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How much air leaks into your home from the outside; and
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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 EER 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:
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Check for correct amount of refrigerant;
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Test for refrigerant leaks using a leak detector;
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Capture any refrigerant that must be evacuated from the
system, instead of illegally releasing it to the atmosphere;
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Check for and seal duct leakage in central systems;
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Measure air flow through the evaporator coil;
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Verify the correct electric control sequence and make
sure that the heating system and cooling system cannot operate
simultaneously;
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Inspect electric terminals, clean and tighten
connections, and apply a non-conductive coating if necessary;
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Oil motors and check belts for tightness and wear; and
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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:
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Use a computer program or written calculation procedure
to size the air conditioner;
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Provide a written contract listing the main points of
your installation that includes the results of the cooling load calculation;
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Give you a written warranty on equipment and
workmanship; and
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Allow you to hold the final payment until you are
satisfied with the new system.
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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:
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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.
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Uses a duct-sizing methodology such as the Air
Conditioning Contractors of America (ACCA) Manual D.
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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.
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Installs duct work within the conditioned space, not in
the attic, wherever possible.
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Seals all ducts with duct mastic and heavily insulates
attic ducts.
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Locates the condensing unit where its noise will not
keep you or your neighbors awake at night, if possible.
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Places the condensing unit in a shady spot, if
possible, which can reduce your air conditioning costs by 1% to 2%.
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Verifies that the newly installed air conditioner has
the exact refrigerant charge and air flow rate specified by the
manufacturer.
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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:
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Locate the air conditioner in a window or wall area
near the center of the room and on the shadiest side of the house.
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Minimize air leakage by fitting the room air
conditioner snugly into its opening and sealing gaps with a foam
weather stripping material.
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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.
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.
For Additional Information on Portable Air Conditioners:
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