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F. LOUVER AND COIL DESIGN DATA
PROVISIONS FOR TESTING,
ADJUSTING AND BALANCING
The need for accurate balancing of air and water
systems is essential in today's construction industry.
With continuing emphasis on energy conservation,
the heating, ventilating and air conditioning engineer
has been called upon to design more efficient systems.
The mystical "10 percent Safety Factor" to cover all
errors in both design and installation can no longer
be afforded as either a first cost or an operating cost.
Therefore, systems now must be designed with minimum air quantities and fluid flow that will ensure the
proper distribution of air and water to meet the design
loads. Comfort can then be attained by balancing the
systems to these design criteria.
While systems will vary considerably in design, size
or extent of duct distribution, the same general procedure for testing, adjusting and balancing should be
employed on most projects.
The system air must flow to the occupied space with
the least possible losses from leakage and resistance
with proper mixing of tempered air, and with a minimal temperature change from heat gain or loss. Once
having arrived, the air must be distributed in the most
efficient, draftless, noiseless manner available for
each job requirement.
The means to accomplish these requirements are the
ductwork and outlets. Because of aesthetics and
available space within the building, the selection of
the type and size of ducts and outlets is often difficult
and compromises are sometimes made which make
the design and installation a matter of some ingenuity.
The designer should give special consideration to the
balancing and adjusting process during the design.
The TAB technician must be able to test and analyze
the particular installation so that he can properly balance it with the least effort and yet obtain the greatest
system efficiency and comfort level. Therefore, it is
necessary that the balancing capability be designed
into the system initially. The following are some considerations to use when designing duct systems.
1. Ductwork to and from air conditioning equipment should be designed very carefully so that
stratified air will be mixed properly before entering branch ducts or equipment.
2. Splitter-type dampers offer little or no control
of air volume in ducts. They should be regarded as air diverters only, with maximum
effectiveness when present on duct systems
exhibiting low resistance to air flow.
3. Manually operated, opposed blade or single
blade, quadrant-type volume dampers should
be installed in each branch duct takeoff after
leaving the main duct to control the amount of
air entering or leaving the branch (see Figure
4. Turning vanes should be installed so that the
air leaving the vanes is parallel to the downstream duct walls. Turning vanes should be
utilized in all rectangular elbows (return systems as well as supply and exhaust systemssee Figures 5-14 and 5-15).
5. Manual volume dampers should be provided
in branch duct takeoffs to control the total air
to the face dampers of the registers or diffusers. The use of extractors is not recommended
because they can cause turbulence in the
main trunk duct thereby increasing the system
total pressure and affecting the performance
of other branch outlets downstream. Register
or diffuser dampers cannot be used for reducing high air volumes without inducing objectionable air noise levels (see Figure 10-2).
6. Do not use extractors at branch or main duct
takeoffs to provide volume control. Branch
duct tap-in fittings with a 450 entry throat pro-
vide the most efficient airflow of all tap-in type
7. The application of single blade, quadrant volume dampers immediately behind diffusers
and grilles may tend to throw air to one side of
the outlet, preventing uniform airflow across
the outlet face or cones.
8. A slight opening of an opposed blade volume
damper will generate a relatively high noise
level as the air passes through the damper
opening under system pressure.
Figure 10-1 DESIGN CONSIDERATIONS FOR DIFFUSER LAYOUTS
AND BALANCING DAMPER LOCATIONS
9. To minimize generated duct noise at volume
dampers, indicate damper locations at least
two diameters from a fitting and as far as possible from an outlet.
10. All portions of the main return air duct system
require manual balancing dampers at each
branch duct inlet.
11. Avoid placing a return air opening directly in
or adjacent to the return air plenum. Lining of
the duct behind the opening normally will not
reduce the transmitted noise to acceptable
levels (see Chapter 11).
12. Terminal boxes or volume control assemblies
should be located so that the discharge ductwork will minimize air turbulence and stratification (see Figure 10-3).
13. Provide the necessary space around components of the duct system to allow the TAB technician to take proper readings. Allow straight
duct sections of 7-1/2 duct diameters from fan
outlets, elbows, or open duct ends for accurate
traverse readings. (See Figure 6-2 for velocity
profiles at fan discharges.)
14. Adequate size access doors should be installed within a normal working distance of all
volume dampers, fire dampers, pressure reducing valves, reheat coils, volume control assemblies (boxes), blenders, constant volume
regulators, etc., that require adjustments
within the ductwork. Coordinate locations with
15. Provide for test wells, plugged openings, etc.,
normally used in TAB procedures.
Figure 10-2 DUCT DESIGN CONSIDERATIONS FOR SUGGESTED
BALANCING DAMPER LOCATIONS
B AIR MEASUREMENT
Before 1960, there was no established procedure and
few attempts were made to measure airflow in HVAC
systems. Total volumetric airflow measurements were
attempted by making traverses with anemometers at
the central station equipment. Airflow volumes at air
terminals were determined by using instruments that
measured jet velocities of the discharge or intake air
pattern and then applying laboratory developed empirical area factors published by the terminal manufacturer.
Figure 10-3 DESIGN CONSIDERATIONS TO MINIMIZE AIRFLOW TURBULENCE & TO AVOID
STRATIFICATION FROM TERMINAL BOXES
In recent years, a test procedure involving Pitot tube
traverses as the primary means of determining volumetric flows through air distribution systems came
into wide useage. Consequently, the systems approached designed performance. Today, we have a
selection of factory fabricated volumetric airflow
measuring and control devices which may be used in
areas requiring critical air control. A complete list of
these instruments, their accuracy and use may be
found in the National Environmental Balancing Bureau (NEBB) "Procedural Standards for Testing Adjusting and Balancing of Environmental Systems" or
the SMACNA "HVAC Systems-Testing, Adjusting
and Balancing" manual.
The use of sharp-edged orifice plates to balance airflow to outlets or branches induces a high level of
accuracy, but loses the flexibility inherent in dampers.
Where the flow can be determined in advance, procedures can be used to accurately determine the
airflow and the total pressure loss. For duct design
purposes, Table 14-17B may be used.
The sharp-edged orifice has more resistance to flow
but is easily constructed. It can also be made readily
interchangeable for several orifice sizes. The orifice
can be mounted between two flanged sections
sealed with rubber gaskets. Three orifice sizes, 1.400
in., 2.625 in. and 4,900 in. (36.6mm, 66.7mm and
124.5mm) diameters, can be used to meter velocities
from 50 to 8000 fpm (0.25 to 40 m/s). If the orifice
and pipe taps are made to exact dimensions, the
calculated air volume will be within one percent of
actual flow for standard air. The necessary equations
and charts may be found in the SMACNA "HVAC
Systems-Testing, Adjusting and Balancing" manual. Orifices for larger ducts can be sized using data
found in Chapter 2 of the Eighth Edition of the "Fan
Engineering" handbook published by the Buffalo
The orifice can be calibrated with a standard Pitot
tube. A micromanometer is needed to read velocities
below 600 fpm (3 m/s). At 1000 to 3000 fpm (5 to 15
m/s), with a 10:1 inclined manometer, an accuracy of
+ 0.3 to 1.0 percent can be expected; at 3000 to
4000 fpm (15 to 20 m/s), an accuracy of + 0.25 to
0.3 percent can be expected. If the orifice is made to
precise dimensions, no calibration is needed and the
tabulated calculation can be used.
IN SYSTEM DESIGN
1. General Procedures
In Chapter 4, a suggestion was made that a schematic diagram of each duct system be prepared in
order to test and balance the systems after the installation work has been completed. It would also
help the system designer to develop these schematic
diagrams when designing the systems in order to
determine if all necessary balancing devices have
Where there is more than one system, make a separate diagram for each system. All dampers, regulating devices, terminal units, outlets and inlets
should be indicated. Also, show the sizes, velocities
and airflow for main and branch ducts. Include the
sizes and airflow ratings of all terminal outlets and
inlets, including outside air intakes, and return air and
relief air ducts and louvers where applicable. For rapid
identification and reporting purposes, number all outlets. Add general notes indicating thermostat locations, i.e. room thermostat, thermostat integral with
unit or in ductwork, etc.
2. "HVAC Systems-Testing,
Adjusting and Balancing"
The SMACNA "HVAC Systems-Testing, Adjusting
and Balancing" manual presents the basic fundamentals, methods, and procedures, including the
necessary tables and charts, that a SMACNA Contractor, with a reasonable technical background in
HVAC systems, could use to adequately balance
most HVAC systems that the firm installs.
In addition to the fundamentals and procedures for
balancing air systems, this manual includes hydronic
piping system balancing fundamentals and procedures, because many SMACNA Contractors do install the complete mechanical system. Even if only
duct systems are installed and balanced, it is necessary for the SMACNA Contractor to know how to
balance the hydronic portion of the system. Additional
information on testing, adjusting and balancing can
be found in other SMACNA and NEBB manuals listed
on the publications page of this manual.
If a SMACNA Contractor wants to become more proficient and more involved in the testing, adjusting and
balancing of environmental or HVAC systems, it is
recommended that consideration be given to becom-
ing a Certified TAB Contractor of the National Environmental Balancing Bureau (NEBB). NEBB has a
comprehensive home study course designed to educate qualified personnel, particularly those in management positions, to direct and be responsible for
the TAB operations of the firm. Information about the
study course and NEBB membership can be obtained from the SMACNA or NEBB National Offices
or local chapters.