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Sizing
Residential Heating and Air Conditioning Systems
When a heating or air conditioning system
becomes old and unreliable, or rising utility bills are emptying your wallet,
replacing your system with a more efficient one becomes an option. Although
newer models are more efficient, you must install the correct size in order to
realize all of the savings from improved energy efficiency. Correctly sizing a
heating system is also important because too small a system will not adequately
heat the home during the coldest days of winter, or supply enough air
conditioning in mid-August. Over-sizing the heating system is wasteful, not only
because a larger system normally costs more to install, but also because it does
not operate efficiently and, consequently, costs more to operate. Over-sized air
conditioners (and heat pumps), in addition to costing more to buy and operate,
do not run long enough to dehumidify the air. This results in the
"clammy" feeling of some air conditioned houses.
The starting point for determining a
properly sized unit is to check the "nameplate rating" of the existing
system. The "nameplate" is a metal tag attached to the unit, usually
near the control valves or wiring box on a furnace. On air conditioners or
air-to-air heat pumps there will be nameplates on both the outdoor (compressor)
units and the indoor (air handler) units. The nameplate will tell you the
maximum Btu per hour output as well as other useful information about the unit.
One ton of air conditioning equals 12,000 Btu/hour of cooling capacity. DO NOT
ASSUME the existing unit is the right size. In the past, few if any new homes
had adequate levels of insulation or air infiltration sealing. It was not
uncommon to install furnaces and air conditioners with twice the needed capacity
to make up for the large losses found in most production-built houses. As fuel
costs rose, most people did at least some caulking, weatherstripping, and
insulating. A full load (heat loss and heat gain) calculation should always be
done on your home, especially if you believe that the insulation or windows of
your home have been upgraded since the original heating or cooling systems were
installed.
Sizing Heaters and Air Conditioners:
Quick but Inaccurate Methods
Some contractors regularly use quick and
incorrect methods when sizing a heater or air conditioner. Surveys by the
Florida Solar Energy Center showed that one third of the contractors in that
state used incorrect methods. The survey included only those contractors who
were members of a professional contractors' association and who responded to
the survey. The percentage of all contractors who use incorrect methods is
probably much higher. Some of these methods, together with a brief description
of their weaknesses, are listed below:
- The contractor walks in the house,
looks at the existing unit, and recommends that the replacement unit be
the same size. 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. He (or she) 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 houses in design, construction, or energy efficiency.
- You may get different answers from
different contractors who use the previous 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 an in-stock unit is the right size for your
house. Since the in-stock system costs the contractor (but not necessarily
the homeowner) the least, this becomes the "lowest cost" method.
- A more complex (but still inaccurate)
method involves a prepared chart such as the one below, which is for
heating systems. 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 (under
description) that best describes the home. Then, multiply both the upper
and lower values for heat loss in Btu per hour per square foot (from the
table) by the floor area of the home to roughly estimate the required
heating range.
| 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 litle
exposure; well insulated |
10 to 13 |
For example, if a home's energy-saving
features are best described by category 2, and the home has a heated space of
1,500 square feet (139.35 m2), then the design heating load is
roughly 75,000 to 105,000 Btu/hour (18,900 to 26,460 kilocalories/hour) (1,500
x 50 and 1,500 x 70). Although a chart like this looks official, not all
houses fit the profile given. There is also no accounting for the thermostat
temperature setting, the location of the house, the shape of the house, or
many other factors.
The above methods can be used for a
first "guess" or rough estimate over the phone. You cannot expect
contractors to go to the home of everyone that calls them for an estimate. DO
NOT USE THESE ESTIMATES for the final sizing.
Correctly Sizing Heating and Air
Conditioning Units
Before making a final decision on any
heating or cooling system, the design heating load and/or the design cooling
should be calculated accurately. Homeowners should insist upon a correct
system sizing 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. Many user-friendly computer
software packages or worksheets 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 home's design
heating or cooling load. A good heating technician will measure walls,
ceilings, floor space, and windows for the accurate determination of room
volumes. Also, a good estimate takes into account the R-value of the home's
insulation, windows, and building materials. An estimate of the building's air
leakage is necessary. A blower door test is the best measurement. The duct
sizes and insulation, as well as the location of the registers and returns,
should be appropriate for the system type and size. ACCA, American Society of
Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE), or other
approved standards (see Source List below) should be followed. If you have an
air-to-air heat exchanger (heat recovery ventilator), you need to account for
heating and cooling the ventilation air. The orientation of the house also
effects heat gain and heat loss through windows. This should be noted and
entered into the calculation. 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
over-sizing air conditioners. Underestimating the latent (humidity) load
(energy used by the air conditioner to remove moisture from the air) results
in undersized air conditioners.
Any bid should include an agreement to
provide written calculations (with 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 output or finished worksheet. This
is your only proof that they did the job right.
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. This bibliography was reviewed in October 1997.
ANSI/AHAM RAC-1-1992, Room Air
Conditioners, Association of Home Appliance Manufacturers (AHAM), 1992.
Available from AHAM, 20 North Wacker Drive, Chicago, IL 60606, (312) 984-5800
x315. 25 pp., $7.50.
ASHRAE Standard 90.2-1993: Energy
Efficient Design of New Low-Rise Residential Buildings, American Society of
Heating, Refrigerating, and Air-Conditioning Engineers, 1993. Available from
ASHRAE (see Source List below). 107 pp., $84.00.
Cooling and Heating Load Calculation
Manual, GRP 138, American Society of Heating, Refrigerating, and
Air-Conditioning Engineers, Inc., 1992. Available from ASHRAE (see Source List
below). 209 pp., $80.00.
Heat Loss Calculation Guide No. H-22,
(1st ed.), Hydronics Institute, 1989. Available from Hydronics Institute, 35
Russo Place, P.O. Box 218, Berkeley Heights, NJ 07922, (908) 464-8200. 63 pp.,
$14.00 plus $4.50 shipping.
Residential Equipment Selection Manual,
Manual S, Air Conditioning Contractors of America, (2nd ed). Available from
ACCA (see Source List below). 115 pp., $40.00.
Residential Load Calculation, Manual J,
(7th ed.), Air Conditioning Contractors of America, 1988. Available from ACCA
(see Source List below). 126 pp., $30.00.
Source List
Air Conditioning Contractors of America (ACCA)
1712 New Hampshire Avenue, NW
Washington, DC 20009
Phone: (202) 483-9370
Internet: (E-mail) webmastr@acca.org;
(World Wide Web) http://www.acca.org
American Society of Heating,
Refrigerating, and Air-Conditioning Engineers (ASHRAE)
1791 Tullie Circle, NE
Atlanta, GA 30329-2305
Phone: (800) 527-4723; Fax: (404)321-5478
Internet: (E-mail) ashrae@ashrae.org;
(World Wide Web) http://www.ashrae.org
EREC is operated by NCI Information
Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of
Energy. The statements contained herein are based on information known to EREC
at the time of printing. No recommendations or endorsement of any product or
service is implied if mentioned by EREC.
Energy Efficiency and Renewable Energy
Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com
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