Hello kwts and Thanks for your question I just had central air
installed in my home yesterday and with the temperatures this
weekend, it wasnt a moment too soon!
The calculation that you request is available, although it is a very
complex procedure, well, not so complex as involved (as you will see).
But before getting to the calculations, if you'd like, a good place to
start is at the Department of Energy Site to get the basics down,
assuming that you are just beginning your search. They have an
informative bit on Sizing Residential Heating and Air Conditioning
Systems which can be found at:
Make sure you buy the correct size of air conditioner. Two groupsthe
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.
HERE IS THE CALCULATION: The ACCA Manual J is the most cited source
for sizing air conditioning installations a short form calculator
is available at the following site:
This is a pretty complex procedure, but it appears to give a
scientific approach to the question. I would recommend the instruction
tab at the top right a little pop-up will guide you through the
HERE'S ANOTHER CALCULATION: HVAC Computer Systems has software with a
free download available, though they are looking for name, address,
etc. They can be found at http://www.heat-loss.com/hvac/tofc2.asp
The following information was furnished by the U.S. Department of
Energy, which I will quote here verbatim it seems like excellent
Correctly Sizing Heating and Air Conditioning Systems
Building owners should insist that contractors use a correct sizing
calculation before signing a contract. This service is often offered
at little or no cost to homeowners by gas and electric utilities,
major heating equipment manufacturers, and conscientious heating and
air conditioning contractors. Manual J, published by the Air
Conditioning Contractors of America (ACCA), is the most common method
in use in the United States. There are also many user-friendly
computer software packages or worksheets that can simplify the
calculation procedure. You should make sure that the procedure used by
the contractor follows Manual J or one of the approved standards in
the bibliography below.
Many factors effect a homes heating or cooling requirement or "load."
A good estimator will measure walls, ceilings, floor space, and
windows to determine the room volumes, and will assess the R-value of
the homes insulation, windows, and building materials. A close
estimate of the buildings air leakage is also necessary. A blower
door test is the best way to measure air leakage.
A good estimate will also include an inspection of the size, condition
of seals on joints and insulation, and location of the distribution
ducts in forced air systems. The placement of supply and return
registers, should be appropriate for the system type and size.
The orientation of the house also effects heat gain and heat loss
through windows. Overhangs can reduce solar gain through windows. Make
sure the contractor uses the correct design outdoor temperature and
humidity for your area. Using a higher summer design temperature
results in oversizing air conditioners.
Any bid should include an agreement to provide written calculations
(listing the procedures and standards that will be followed),
equipment and installation warranties, a payment schedule, and a firm
completion date. When the contractor is finished, get a copy of their
calculations, assumptions, and the computer printout or finished
worksheet. This is your only proof that they did the job right.
Sizing Heaters and Air Conditioners: Quick but Inaccurate Methods
The following are some of the "quickie" methods some contractors may
use to size a system. They are also somewhat useful for very rough
sizing. NEVER use any of these to determine the final size.
The contractor walks in the house, looks at the existing unit, and
recommends that the replacement unit be the same size, or larger. This
obviously does not take into account any improvements made to the
house or mistakes made in sizing the original unit.
The contractor asks you how many square feet of living space there are
in your house, then tells you what size unit you need. This is called
"sizing by square footage" and is the most commonly used inaccurate
method of sizing. A typical value used for air conditioners is one ton
(12,000 Btu/hour) per 500 square feet (46 m2). This does not take into
account differences among house orientation, insulation levels,
design, construction, and energy efficiency or intended use of the
system. You may get different answers from different contractors who
use this technique. In that case, they may have a different "rule of
thumb," or one of them may be using the "lowest cost" method. This
involves adjusting the square footage rule so that whatever the
contractor has in their warehouse becomes the right size for you.
Since the "in-stock" unit costs the contractor (but not necessarily
you) less to install, this becomes the "lowest cost" method.
Another rough method for sizing heating systems involves a prepared
chart such as the one below. You use the chart in the following way.
First, determine the floor area of all the heated rooms, and the
levels of insulation in the floors, walls, and ceilings. Next, find
the category that best describes the house. Then, multiply both the
higher and lower numbers for heat loss in Btu per hour per square foot
(from the table) by the floor area of the home to give you a rough
range for the heating load.
Home Type or Characteristics ..... Heat loss (Btu/hr/ft2)
1) No insulation in walls, ceilings, or floors; no storm windows;
windows and doors fit loosely .... 90 to 110
2) R-11 insulation in walls and ceilings; no insulation in floors over
crawl spaces; no storm windows; doors and windows fit fairly tight.
..... 50 to 70
3) R-19 insulation in walls, R-30 in ceilings, and R-11 in floors;
tight-fitting storm windows or double pane windows. ..... 29 to 35
4) "Superinsulated" house with R-24 wall insulation, R-40 in ceilings,
and R-19 in floor; tight-fitting storm windows or double pane windows;
vapor barrier sealed carefully during construction. ..... 21 to 25
5) Earth-sheltered house with little exposure; well insulated. .....
10 to 13
For example, if a homes energy-saving features are best described by
#2, and the home has a heated space of 1,500 square feet (139.35 m2),
then 1,500 ´ 50 and 1,500 ´ 70 is the heating load range. Roughly
75,000 to 105,000 Btu/hour (18,900 to 26,460 kilocalories/hour.)
Although a chart like this looks official, not all houses fit the
profile given. There is also no accounting for the other factors
To save some time the above methods are often used for a first "guess"
or rough estimate. If so, then it should be plainly stated to you that
this is the case. The final bid should be based on the results of the
use of a procedure such as Manual J or those listed below.
Here is a source list of various vendors of information:
Ask an Energy Expert
DOE's Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
Phone: 1-800-DOE-EREC (363-3732)
Fax: (703) 893-0400
Consumer Energy Information Web site
Energy experts at EREC provide free general and technical information
to the public on many topics and technologies pertaining to energy
efficiency and renewable energy.
DOE's Energy Efficiency and Renewable Energy Network (EREN .
Air Conditioning and Refrigeration Institute (ARI)
4301 N. Fairfax Drive, Suite 425
Arlington, VA 22203
Fax: (703) 528-3816
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
1791 Tullie Circle, N.E.
Atlanta, GA 30329
Fax: (404) 321-5478
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)
1513 16th Street NW
Washington, DC 20036
Fax: (202) 234-4721
The ACCA is the most active and widely recognized organization
representing contractors in the heating, ventilation, air
conditioning, and refrigeration (HVACR) industry.
ENERGY STARŽ-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)
1679 Clear Lake Road
Cocoa, FL 32922
Fax: (407) 638-1010
FSEC provides information on building in hot, humid climates.
I hope this helps you in sizing the unit to fit your needs. I you
require any other information, PLEASE ask for clarification and I will
get it right back to you.