So it started out as a way of making people comfortable. Right? Well, not exactly. In 1882, thanks to Thomas Edison, the first electric power plant opened in New York making it possible for the first time to have an inexpensive source of energy for residential and commercial buildings. And by 1889, central station refrigeration was used in large cities to preserve foods and documents. It was well known that a cool surrounding could preserve foods and other perishables for a long time. But what was not well known was how humidity and heat were related. Then in 1902, Willis Carrier built the first air conditioner to combat humidity inside a printing company. Controlling the humidity in printing companies and textile mills was the start of environment management.

U.S. Department of Energy - Energy Efficiency and Renewable
Energy Energy Savers

02/07/12

Sizing Residential Heating and Air Conditioning Systems

Older space conditioning systems (more than 10 years old) are
often unreliable and much less efficient than a modern system.
When it's time for a new replacement, choosing one of the
correct size (heating and/or cooling output) is critical to
getting the best efficiency, comfort, and lowest maintenance and
operating costs over the life of the new system. Some national
surveys have determined that well over half of all HVAC
contractors do not size heating and cooling systems correctly.

The most common sizing mistake is in oversizing. This not only
makes the new system cost more to install, but also forces it to
operate inefficiently, break down more often, and cost more to
operate. Oversized heating equipment also often creates
uncomfortable and large temperature swings in the house.
Oversized air conditioners (and heat pumps) do not run long
enough to dehumidify the air, which results in the "clammy"
feeling and unhealthy mold growth in many air-conditioned houses.

It is the installer/contractor's job to perform the correct
sizing calculation for the building. However, many installers
only check the "nameplate" (the label on the unit that has the
Btu per hour output among other things) of the existing system
and sell you one just like it, or even worse, one that's larger.
This is a not a correct sizing method and not in your best
interests!

Before the era of tightly constructed homes, it was not uncommon
to install furnaces and air conditioners that had two to four
times the necessary capacity. Also, since many people have added
new windows, caulking, weatherstripping, and insulation to their
homes, going by the nameplate is likely to result in an
oversized system. Making improvements such as these to reduce
heat loss in the winter and heat gain in the summer should allow
you to install a smaller systems while still being comfortable,
as well as saving large amounts of energy.

Correct system sizing requires consideration of a many more
factors than simply reading the nameplate of the existing unit.
Key factors for correctly sizing a heating and cooling system
include:

The local climate Size, shape, and orientation of the house
Insulation levels Window area, location, and type Air
infiltration rates The number and ages of occupants Occupant
comfort preferences The types and efficiencies of lights and
major home appliances (which give off heat). 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 affect a home's 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 home's insulation, windows, and
building materials. A close estimate of the building's 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 affects 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 home's 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 mentioned above.

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.

Bibliography The following publications provide additional
information about load calculations and sizing heating or air
conditioning systems. The publications are based upon standards
approved by professional organizations.

Comfort, Air Quality, and Efficiency by Design, Manual RS, Air
Conditioning Contractors of America (ACCA), 1999. 80 pp.
(approx.) Available from ACCA (see Source List below).

Cooling and Heating Load Calculation Principles, American
Society of Heating, Refrigerating, and Air-Conditioning
Engineers, Inc. (ASHRAE), 1998. 248 pp. Available from ASHRAE,
Email: ashrae@ashrae.org.

Heat Loss Calculation Guide (for hydronic heating systems),
Hydronics Institute, 1998.

Residential Duct Systems, Manual D, (New ed.), Air Conditioning
Contractors of America (ACCA), 2000. 298 pp. Available from ACCA
(see Source List below).

Residential Equipment Selection, Manual S, (2nd ed.), Air
Conditioning Contractors of America (ACCA). 115 pp. Available
from ACCA (see Source List below).

Residential Load Calculation, Manual J, (8th ed.), Air
Conditioning Contractors of America (ACCA), 1988. 126 pp.
Available from ACCA (see Source List below).

Source List Air Conditioning Contractors of America (ACCA)
Email: info@acca.org

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This fact sheet was reviewed for accuracy in November 2003.

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
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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.