application considerations application VAV

Application
Considerations
VAV-PRC008-EN AC 1
Introduction AC1
Systems AC2–4
Parallel vs. Series AC5
Low-Temperature Air AC6–7
Energy Savings and System Control AC8
Agency Certifications AC9
ControlTypes AC10–12
Flow Measurement and Control AC13–14
Reheat Options AC15
Insulation AC16
Acoustics AC17–18
Duct Design AC19
Selection Program AC20
Best Practices AC21
Unit Conversions AC22
Additional References AC23
Table of
Contents
Introduction
TheVariTrane line of variable-air-
volume (VAV) products has been an
industry leader in performance and
quality for many years.TheVariTrane
line includes single-ductVAV units,
dual-duct VAV units, fan-poweredVAV
units (series, parallel, and low-height
series and parallel), direct digital
controls, pneumatic controls, analog-
electronic controls, direct digital control
retrofit kits and diffusers.This
application section will focus onVAV
units.

Application
Considerations
VAV-PRC008-EN
AC 2
Systems
VAV Systems
There are two primary types ofVAV systems—single-duct and dual-duct.
Single-Duct Systems
Single-duct systems include one supply fan and a single supply duct, which is
attached to each zone.The supply fan delivers cooled air to theVAV zones in variable
volumes, depending upon the cooling requirements.The supply fan is usually
designed to modulate airflow delivered to theVAV zones.
ManyVAV zones require heating as well as cooling.The supply air-handling unit
provides either no heat (cooling only), morning warm-up heat or occupied
(changeover) heat. In addition, heat may be provided at any individualVAV zone
(within the zone or within theVAV terminal) by reheating cool air provided by the
central air handler.
No Heat
Central Cooling Only—In some systems, the central air handler provides only
cooling and ventilation during zone occupied periods.The supply air is maintained at
a constant temperature and the supply airflow is modulated to match theVAV
airflow rate with the zone cooling requirements.
Central Heat
Central Heat for Morning Warm-up—Many buildings cool down during the night.To
be at a comfortable temperature in the morning when the building is again
occupied, heat must be added to the spaces. Heat provided by the central air
handler for morning warm-up is supplied at constant air volume to the zones, prior
to the time of occupancy. During the morning warm-up period, theVAV terminal
units must open to allow heated air to flow into the zones. In most instances very
little additional heat is needed once the building is occupied.
Variable-Air-Volume (VAV) System
RA
EA
OA PA
supply
fan
cooling
coil
variable-
speed drive
thermostat
VAV
box
SA
Central Occupied Heating-Changeover—Some
buildings use the same air handler to provide both
occupied cooling and occupied heating.This is
commonly referred to as a changeover system.The
system changes between heating and cooling
depending on the need of the zones on the system.
In a changeover system, the operation of theVAV
terminal units must also change over, opening to
provide heat in the heating mode and opening to
provide cooling in the cooling mode.Trane's main
product in this type of application is calledVariTrac™
.
VariTrane products can also be used in these
systems. (These types of systems are beyond the
scope of this manual and are discussed in detail in
theVariTrac II Manual,VAV-PRC003-EN.)
Terminal Heat
Remote Heat—In some zones of a single-ductVAV
system, perimeter heating equipment, remote from
the terminal unit, is used to add heat to the zone
VSD
supply
fan
Single-Fan, Dual-Duct VAV System
central air handler
cooling
coil
coil
heating
105˚F (40.6˚C)
dual-duct
VAV
terminal
units
EA
OA
40˚F
(4.4˚C)
(23.9˚C)
75˚F
(12.8˚C)
55˚F
RA
when the cooling load is lower than the
minimum cooling capacity of theVAV
terminal unit. Heat is added directly to
the zone while cool supply air continues
to enter the zone at a minimum rate for
zone ventilation.
Terminal Reheat—In some zones of a
single-ductVAV system, a minimum
flow of cool supply air is reheated at the
terminal unit before entering the zone.
Terminal reheat can be provided by
electrical resistance heaters or by hot
water coils.
Parallel Fan-Powered Heat—In some
zones of a single-ductVAV system, cool
supply air at minimum flow is mixed
with warm plenum air before entering
the zone at a constant flow rate.A fan in
the terminal unit, in parallel with the
central fan, draws air from the plenum
whenever the zone requires heat.
Series Fan-Powered Heat—In some
zones of a single-ductVAV system, the
airflow to the zone is held constant,
during both heating and cooling, by a
terminal unit fan that is in series with the
central fan.The terminal unit fan runs
continuously.When the zone requires
heat, cool supply air at minimum flow is
mixed with warm, return plenum air
before entering the zone.
Dual-Duct Systems
Dual-duct systems have either one or
two supply fans and two duct systems.
One duct system carries heated air and
the other duct system carries cooled air.
Heated air and cooled air are modulated
and/or mixed at each zone in the proper
proportions to control zone temperature.
Terminal reheat is not required in a dual-
duct system.

Application
Considerations
VAV-PRC008-EN AC 3
Systems
VariTraneVAVTerminal Units
The function of theVariTrane terminal
unit in aVAV control zone is to vary the
volumetric airflow rate to the zone.
VariTrane units are available with either
microprocessor-based DDC controls or
pneumatic or analog electronic controls.
Factory-installed controls are available
with all types of terminal units.
VariTraneVAV
Terminal UnitTypes
Single-Duct
Single-duct terminal units control the
volumetric flow of supply air to the
space to maintain the zone temperature
at setpoint.These units are generally
applied in cooling-onlyVAV zones that
require no heat during occupied hours. If
local zone heat is necessary it can be
provided either remotely (for example,
perimeter heat) or by terminal reheat
(either electric or hot water coils).
Single-Duct Unit with HotWater Coil
Parallel Fan-Powered Unit with
HotWater Coil
Parallel Fan-Powered Unit with
Electric Coil
Single-Duct Cooling Only Unit
Parallel Fan-Powered Unit
Cooling Only
Dual-Duct
Dual-duct terminal units are used in a
special type of air distribution system
where the main system has both warm
air and cold air separately ducted to
each terminal unit.The flow of both
warm air and cool air is modulated,
delivering air to theVAV zone at
variable air volumes as well as variable
temperatures. Since full capacity
occupied heating is always available,
control of additional local heat is
not provided.
Dual-DuctTerminal Unit
Parallel Fan-Powered
Parallel fan-powered units are
commonly used inVAV zones which
require some degree of heat during
occupied hours—when the primary
supply air is cool.The terminal unit fan is
in parallel with the central unit fan; no
primary air from the central fan passes
through the terminal unit fan.The
terminal unit fan draws air from the
space return plenum.
When no heat is needed, the local
parallel fan is off and a backdraft
damper on the fan’s discharge is closed
to prevent cool air entry into the return
plenum.When cool airflow to theVAV
zone is at a minimum and the zone
temperature drops below setpoint, the
local parallel fan is turned on and the
backdraft damper opens.A constant
volume of air is delivered to the zone
because the fan delivers a constant
volume of warm plenum air which is
mixed with cool primary air at a
minimum flow. Remote heat or
terminal reheat can provide additional
local heating.

Application
Considerations
VAV-PRC008-EN
AC 4
Systems
Series Fan-Powered
Series fan-powered terminal units are
used commonly inVAV zones that not
only require heat during occupied
hours, but also desire constant air
volume delivery.The terminal unit fan
is in series with the central fan. Primary
air from the central fan always passes
through the terminal unit fan.
The local series fan within the terminal
unit operates whenever the unit is in
the occupied mode.The volume of air
delivered to theVAV zone is constant,
but the temperature of the delivered air
varies. As the zone requires less
cooling, the primary air damper closes.
As the primary air damper closes, the
air mixture supplied to the zone
contains less cool air and more warm
plenum air. Remote heat or terminal
reheat can provide additional local
heating.
Series fan-powered terminal units are
also useful in low supply air
temperature systems, since the
terminal unit fan can be sized so that
warm plenum air is always mixed with
low temperature supply air.This raises
the supply air temperature to an
acceptable distribution level and
reduces condensation potential.
Series Fan-Powered Unit Cooling Only
Low-Height Fan-Powered
Low-height fan-powered terminal units
are a slightly modified version of a fan-
powered terminal unit. As its name
suggests, the low-height fan-powered
unit has a shorter height dimension to
accommodate applications where
ceiling space is limited.To reduce the
height, shorter terminal unit fans are
integrated into the standard height
series or parallel terminal unit.The
result is a unit with a maximum height
of 11.0"–11.5".
For low-height units with the smaller
fan sizes (sizes 08SQ and 09SQ), a
single low-profile fan is used. Low-
height units with the largest fan size
(size 10SQ) use two low-profile fans.
Each fan operates off a separate motor.
The fans still remain in series or
parallel with the primary system
central fan. Low acoustic levels are
much more challenging in these low
ceiling space applications, due to the
reduced radiated ceiling pleunum
effect.
The operation of the low-height
terminal unit is exactly the same as
that of a series or parallel terminal unit,
as are the options for high-efficiency
ECMs, insulation options, etc. As with
the other fan-powered terminal units,
additional local heating can be
provided by remote heat or terminal
reheat.
Series Fan-Powered Unit with
HotWater Coil

Application
Considerations
VAV-PRC008-EN AC 5
Parallel vs.
Series
AIR
AIR
VALVE
PRIMARY
FAN
AIR
PLENUM
AIRFLOW
FAN
AIRFLOW
PRIMARY
AIR
AIR
VALVE
AIR
PLENUM
PARALLEL FAN-POWERED TERMINAL SERIES FAN-POWERED TERMINAL
Fan-Powered versus Single-
DuctVAVT
erminal Units
In many climates, fan-powered
systems are a lower operating cost
alternative than single-duct systems.
The energy inefficiencies inherent in
reheating cold primary air can be
eliminated with a key design
characteristic of fan-powered terminal
units, plenum air heating. Heating with
warmer plenum air allows for recovery
of heat from lighting and other heat
sources in the building.
Comparison of Parallel and
Series Models
Once it has been determined that a
fan-powered system is to be specified,
the designer must decide between
parallel and series configurations. Each
model carries its own characteristics of
delivered airflow, energy consumption,
and acoustics. For the end user, the
designer might consider three goals: a
comfortable and productive tenant
environment, acceptable installed cost,
and low operating costs.
Parallel and series fan-powered
terminal units offer specific advantages
for particular applications.The
following table compares the key
similarities and differences between
the models that the designer should
consider in performing an engineering
analysis.
TypicalApplication of
Parallel Units:
Parallel intermittent fan-powered
terminal units are very common in
perimeter zones or buildings where
loads vary during occupied hours.
Core zones, which maintain a more
constant cooling requirement, are
better suited for variable airflow
(single-duct) units.Typical jobs
combine parallel fan-powered units
(exterior) and single-duct units
(interior) to provide an efficient system
with lowest first cost. Although the
overall NC of parallel systems is lower
than an equivalent series system, the
intermittent fan is sometimes noticed
when energized.To minimize the
impact of this NC change, an ECM
(Electrically Commutated Motor) can
be used which has soft-start
technology.
TypicalApplication of
Series Units:
Applications requiring constant air
movement or blending utilize series
constant fan-powered terminal units.
Conference rooms, laboratories, and
lobbies are common applications.
Because the series fan also adds to the
system external static pressure, office
buildings take advantage of this design
feature and down size main air
handling equipment. Finally, series
terminals are used in low-temperature
air systems to temper cold primary air
with warm plenum air and deliver it to
the zone.
Parallel Series
Fan Operation Intermittent operation during occupied Continuous operation during the
and unoccupied modes. occupied modes. Intermittent operation
during unoccupied mode.
Operating Sequence Variable-volume, constant-temperature Constant-volume, variable-temperature
device during cooling. Constant-volume, device at all times. Delivers design
variable-temperature during heating. airflow regardless of the load.
Fan Energization Based on zone temperature deviation Interlocked with central system fan to
from setpoint. No interlock with deliver required air to the zone in both
central system fan required. heating and cooling modes
Terminal Fan Fan runs during heating load. Size for Fan runs continually. Fan sizing should
Operating and Size design heating load.Typically this is 40 to meet the greater of design cooling or
60% of design primary cooling airflow. heating airflow to the zone.
Air valve Sizing Design cooling airflow. Design cooling airflow.
Minimum Inlet Static Sufficient to overcome unit, heating Sufficient to overcome air valve
Pressure Required for coil, downstream duct and diffuser pressure loss only.
Central Fan Sizing pressure losses.
Acoustics When operating under cooling loads Produces slightly higher background
the terminal fan does not run, offering sound pressure levels in the occupied
superior acoustic performance similar to space.This sound level remains
single-ductVAV. Under heating loads, the constant and is less noticeable than
fan operates intermittently. Impact can be intermittent fan operation with PSC
minimized by use of a ECM. motors.

Application
Considerations
VAV-PRC008-EN
AC 6
Low-T
emperatureAir System
Benefits of Low-TemperatureAir
The benefits of low-temperature air
systems include reduced first cost,
reduced operating cost and increased
revenue potential. Since low-
temperature air transports more
energy per cubic foot, smaller fans and
ducts can be used. An EarthWise™
system takes that a step farther and
includes optimizing the waterside of
the HVAC system as well with low flow
rates through the chilled water and
condenser loops.
Since low-temperature water can
transport more thermal energy per
gallon, smaller pumps, pipes, and
valves can be used. Smaller HVAC
equipment consumes less energy so
both electrical demand and
consumption are lowered, reducing
operating costs.The amount of
revenue generated by a commercial
building is related to the amount and
quality of rental floor space.The
amount of rental floor space is
increased in a low-temperature air
system, since air handlers, riser ducts,
and equipment rooms are smaller.
Since smaller ducts reduce the
required ceiling plenum, additional
floors may be included without
increasing building height.
The concept of the EarthWise system
is to deliver superior comfort and be
less expensive to install and operate.
The method to do this involves both
waterside optimization and airside
optimization.The waterside is
optimized using techniques of low
water flow through the evaporator and
condenser of the chiller as well as
using chiller-tower optimization control
strategies. For more information on the
waterside of the EarthWise system,
contact your localTrane representative
or visit www.trane.com.
LowTemperatureAir System Layout
Airside savings are obtained using a
combination of lower air temperature
and intelligent control strategies.The
ability of theVAV unit to communicate
information is vital to system
coordination.
System Operation
A low-temperature air system could be
done with chilled water or direct
expansion equipment. A chilled water
system includes a chiller plant,VAV air
handlers, and series or parallel fan-
poweredVAV terminal units.TheVAV
air handlers use cold water, typically
around 40°F (4.4°C), from the chiller
plant, to cool the supply air to 45–50°F
(7
.2–10°C). The volume of supply air is
determined by the airflow needs of the
VAV terminal units. A direct-expansion
system would include aVAV air
handler or rooftop with series or
parallel fan-poweredVAV terminal
units.The supply air would be cooled
to 48–52°F (8.9–11.1°C).
TheVAV terminal units include a
parallel or series fan with the central air
handler or rooftop fan.The terminal
unit fan operates continuously, mixing
45-50°F (7
.2–10°C) supply air with warm
plenum air, to provide 50–55°F
(10–12.8°C) cooling air to the occupied
space at design conditions. As the
cooling load in the space decreases,
theVAV terminal air valve closes to
reduce the flow of cold supply air and
increase the flow of warm plenum air
in the case of series terminal units.The
temperature of air supplied to the
space rises, but the volume flow rate to
the space is constant for the series
unit.
Considerations forVAV products
To achieve the maximum benefit from
the low-temperature air system,
severalVAV considerations must be
addressed.
Insulation
The units must be insulated to ensure
that no condensation occurs on the
units. How much insulation is needed?
Trane has tested its insulation with the
goal of developing a thermal
resistance ratio for each type of
insulation.The thermal resistance (TR)
ratio is discussed on page AC 16.The
TR ratio can be used, along with the
properties of the insulation and the
system operating conditions to
determine the necessary insulation
thickness required.
In the low-temperature air system with
fan-powered units, the ducts and
diffusers downstream from the
terminal unit handle air that is 55°F
(12.8°C) or warmer.Therefore,
condensation considerations are no
different from conventional systems.
Linear slot diffusers are recommended
to take advantage of the Coanda effect
described in the Diffusers section later
in the catalog.
Terminal unit surfaces that are
traditionally not insulated—electric and
hot water reheat coils and the primary
air inlet for example—should be
thoroughly field-insulated.
Low-Temperature
Air
Variable
Volume
Exhaust
Fan
Preheat
Coil
Variable Volume
Supply Fan
Series or Parallel
Fan-powered Unit
Cooling
Coil
Heating Coil
Zone 1
Zone 2
55˚
48˚

Application
Considerations
VAV-PRC008-EN AC 7
Low-Temperature
Air
Fan Airflow +Valve Minimum
Fan Airflow +Valve Design
( )x SP1
= SP2
Leakage
When the terminal unit fan is off, the
air valve will close, and not leak. Ducts
upstream of the terminal unit must
also be thoroughly insulated and
constructed for very low leakage.
Duct and terminal unit insulation can
be internal or external. Keep in mind
that internal insulation has hidden
thermal leaks at joints and seams.
These areas must be located and
insulated externally to avoid
condensation. External Insulation, on
the other hand, allows a complete,
uniform thermal seal.
Minimum settings and IAQ
Indoor air quality is usually best when
a specific quantity of outside
ventilation air reaches each building
occupant. Maintaining a minimum
ventilation rate is a challenge in any
VAV system because the amount of
supply air that reaches a particular
space decreases as the cooling load
decreases.To insure that a minimum
amount of supply air reaches the space
at all times, a minimum flow setting on
the terminal unit is used. In low-
temperature air systems, when the
space needs heating, this minimum
flow setting results in increased
heating load. Therefore, it is important
to include the additional load imposed
by the cold supply air when calculating
heating loads. Reheat may be required
since the ventilation values are
absolute requirements and not
percentage of total airflow
requirements.
EarthWise or Low-TemperatureAir
Distribution Design Considerations
with Parallel Fan-poweredTerminal
Units
The parallel fan-powered unit needs to
be set up to run continuously rather
than intermittently. Since it is in
parallel, the airflow required by the fan
is less than a comparable series unit.
This results in energy savings. Running
the parallel fan continuously will take
some minor control changes. It will,
however, create a better acoustical
installation.
The parallel fan should be large
enough to temper the design cooling
airflow at 45–50°F to 50–55°F (7
.2–10°C
to 10–12.8°C). For instance, if the
design cooling airflow is 1000 cfm at
55°F (472 L/s at 12.8°C), you will need
781 cfm of 48°F (368 L/s of 8.9°C)
supply air and 219 cfm of 80°F (103 L/s
of 26.7°C) plenum air.The parallel fan
can be sized for the 219 cfm (103 L/s)
rather than the total room airflow.
The fan airflow plus the minimum
primary airflow must be checked with
the minimum airflow of the diffusers to
insure that dumping doesn’t occur. If
that is a concern, the minimum could
be adjusted up or the fan airflow could
be adjusted up.
As the valve closes, the downstream
static pressure will decrease because
the pressure is related to the airflow.
The fan will supply more air at the
valve minimum condition than at
design due to the decreased static
pressure.This should be a
consideration when calculating how
much airflow would occur at the
minimum valve plus fan airflow
condition.The new fan airflow would
be found by looking at a fan curve at
the new SP point.The new SP can
be calculated:
The following table can be used to
determine what percentage of the total
airflow should come from the fan to
temper the supply air, assuming 80°F
(26.7°C) plenum air.
If anything other than 80°F (26.7°C),
the following equation can be used to
calculate the percentage:
SupplyTemperature =
(%*primarytemperature) + (1-%)*plenumtemperature
downstream of theVAV terminal unit,
the system designer will have some
concerns related to condensation on
diffusers and other low-pressure
ductwork accessories. For instance, if
the occupied space must receive 1000
cfm of 55°F (472 L/s at 12.8°C) air to
satisfy to design cooling load, 715 cfm
must be 45°F (337 L/s must be at 7
.2°C)
supply air and 285 cfm must be 80°F
(135 L/s must be 26.7°C) plenum air.
Therefore, the series fan-powered
terminal must be sized to have the air
valve deliver 715 cfm (337 L/s) of
supply air at design conditions, but the
fan must be sized to deliver 1000 cfm
(472 L/s).
Airside System Factors
A couple of system related factors
should be noted as they apply to
condensation.The first is the
advantage the colder primary air has
from a humidity standpoint. As noted
in the description above, the low-
temperature system operates at space
relative humidity of 30–45% while a
standard system operates at space
relative humidity of 50–60%.The drier
zone air means that the plenum air
returning to the series terminal unit will
also be drier and, therefore, less of a
problem with
condensation.
The second
condensation factor to
note is related to
systems that shut down
in the evening. Many
people believe that
immediately sending low-temperature
primary air to these boxes that have
been off for some time will cause a
shock to the system and may cause
condensation problems at startup.The
solution to this has been the advent of
gradual pull-down or “soft start”
systems. In this type of system, the
primary air temperature is higher on
initial startup (typically 55°F(12.8°C))
and then gradually reduced to the
normal operating point over the next
30 to 60 minutes.
Low-Temperature Air Distribution
Design Considerations with
Series Fan-poweredTerminal
Units
TheVAV terminal unit includes a
fan that operates continuously.
The series fan should be large
enough to insure that the mixture
of cold supply air and warm
plenum air is 50–55°F (10–12.8°C)
at design cooling flow conditions.
In these types of systems, it is a
good design practice to develop
the system based upon 55°F
(12.8°C) air being provided to the
space from the fan-powered
terminal unit. If a lower
temperature air is used
Percentage ofAirflow from Fan
Supply Air
Temp. PrimaryAirTemperature (deg. F (C))
(deg. F (C) 45 (7.2) 46 (7
.8) 47 (8.3) 48 (8.8) 49 (9.4) 50 (10)
50 (0) 14% 12% 9% 6% 3% 0%
51 (10.6) 17% 15% 12% 9% 6% 3%
52 (11.1) 20% 18% 15% 13% 10% 7%
53 (11.7) 23% 21% 18% 16% 13% 10%
54 (12.2) 26% 24% 21% 19% 16% 13%
55 (12.8) 29% 26% 24% 22% 19% 17%

Application
Considerations
VAV-PRC008-EN
AC 8
Energy Savings &
System Control
Electrically Commutated
Motor (ECM)
Fan-Pressure Optimization
WithTrane's Integrated Comfort
System, the information fromVAV
terminal units can be used for other
energy-saving strategies. Fan-pressure
optimization is the concept of reducing
the supply fan energy usage based on
the position of the terminal unit
dampers.
The control system allows this
scenario.The system polls theVAV
units for the damper position on each
unit.The supply fan is modulated until
the most wide-open damper is
between 85% and 95% open.The
correct airflow is still being sent to the
zones since the controls of theVAV
units are pressure-independent, and
the fan modulates to an optimal speed
and duct static pressure which results
in fan energy savings.
Ventilation Reset
TheVentilation Reset control strategy
enables a building ventilation system
The ECM provides an additional
energy-saving option to the system
designer. Some of the advantages of
the motor include high efficiency, quiet
operation, short payback, and easy
installation.There are several
considerations that need to be
addressed when deciding whether to
use these motors or not.The primary
benefit may be seen as increased
efficiency.
Operating Hours—The added cost of
an ECM can be offset more quickly in
applications which require a relatively
high number of hours of operation.
However, if a space does not require
extensive running time for the unit fan,
then it may not be a good candidate
for this type of motor based solely on
payback.Therefore, the decision about
using the ECM may be based on other
benefits, depending on the needs of
the customer.
Airflow Flexibility—The ECM allows a
greater airflow range per fan size. If a
space is going to change uses and load
components frequently, the ability to
change supply airflow with the ECM
without changing units will be a
benefit.
Airflow Balancing—The ability of the
ECM motor to self-balance to an
airflow regardless of pressure can be
an asset when trying to air balance a
job.This will help eliminate additional
dampers or changes to downstream
ductwork to ensure proper airflow. For
more information, please contact your
localTrane sales engineer.
to bring in an appropriate amount of
outdoor air per ASHRAE Standard
62.1.The basis for the strategy is
measuring airflow at each zone,
calculating current system ventilation
efficiency using the multiple-zone
system equations of the standard, and
communicating a new outdoor airflow
setpoint to the air handler.
This strategy continually monitors the
zone ventilation needs and system
outdoor air intake flow, minimizing the
amount of ventilation air and increasing
the energy efficiency of the system. This
insures that the right amount of air is
brought in at all times and that proper
ventilation can be documented.Trane
has integrated this control ability into
theVAV controls, air-handler controls,
and building controls.
For more detailed information on these
energy-saving strategies, please refer
to the “Additional References” section
on page AC 23 of the catalog for
appropriate material.

Application
Considerations
VAV-PRC008-EN AC 9
Agency Certifications
There are numerous regulations and
standards in the industry that determine
the construction and performance
parameters forVAV terminal units.
Some of the more important of those
standards and regulations are listed
below, along with a brief description of
what each one addresses.
American Society of Heating,
Refrigerating andAir-conditioning
Engineers (ASHRAE) - 41.1
ASHRAE - 41.2
ASHRAE - 41.3
These standards specify methods for
temperature measurement (41.1),
laboratory airflow measurement (41.2),
and pressure measurement (41.3).While
none of these standards specifically
discussesVAV air terminals, they
discuss topics that are aspects of
terminal box systems.Therefore, some
engineers will include these standards
in their specifications as a primer on
accepted measurement techniques.
ASHRAE - 62
This standard specifies the minimum
ventilation rates and indoor air quality
that are acceptable for occupied spaces.
ASHRAE - 111
This standard calls out procedures to be
followed for testing and balancing
HVAC systems. It includes descriptions
of the equipment used, procedures
followed, and field changes that must
be made when a system is balanced.
Air Conditioning and Refrigeration
Institute (ARI)
ARI 880 - 1998
This standard sets forth classifications,
performance testing requirements, and
test results reporting requirements for
air terminal units.The standard
contains very detailed procedures that
are to be followed for the testing and
certification program associated with
this standard.This is one of the most
commonly referenced standards in the
VAV terminal unit industry.The ARI-880
certification program is designed to
police the accuracy of documented
performance for terminal units.The
certification program requires a
sampling of at least four units be tested
annually.The tested units are chosen at
random by ARI and sent to an
independent laboratory for the testing.
The performance is tested at one
specific operating condition.The
operating characteristics tested include
discharge and radiated sound power
(for the damper and, in the case of fan-
powered boxes, the fan), wide-open
damper pressure drop, and fan motor
amp draw. VariTrane terminal units
are certified according to ARI-880.
ARI 885-98-02
This document provides a procedure
to estimate sound pressure levels in an
occupied space.The standard accounts
for the amount of sound pressure in
the space due to theVAV air terminal,
diffusers and their connecting low
pressure ductwork.While sound
generated from the central system fan
and ductwork may be a significant
factor in determining the sound
pressure level in the room, this
standard does not address those
factors. It focuses solely on theVAV
terminal and items downstream of it.
This standard is related to ARI-880 by
using sound power determined using
ARI-880 methodology as a starting
point for the ARI-885 procedure.
Underwriter’s Laboratory (UL) 1995
Underwriter’s Laboratory is an
independent testing agency that
examines products and determines if
those products meet safety
requirements. Equipment
manufacturers strive to meet UL
guidelines and obtain listing and
classifications for their products
because customers recognize UL
approval as a measure of a safely
designed product. VariTrane VAV air
terminals are listed per UL-1995,
Heating and Cooling Equipment.
The terminals are listed as an
entire assembly.
National Fire ProtectionAssociation
(NFPA) 70
This standard is also known as the
National Electrical Code (NEC).The
Code gives standards for installation of
wiring and electrical equipment for
most types of commercial and
residential buildings. It is often referred
to inVAV air terminal specifications
when fan-powered boxes, electric heat
or electric controls are included.
NFPA 90A
This standard does not speak directly
toVAV air terminals but does discuss
central system considerations
pertaining to a fire and/or smoke
condition.The standard discusses
safety requirements in design and
construction that should be followed to
keep the air-handling system from
spreading a fire or smoke.The
standard specifies practices that are
intended to stop fire and smoke from
spreading through a duct system, keep
the fire-resistive properties of certain
building structures (fire walls, etc.)
intact, and minimize fire ignition
sources and combustible materials.
Agency
Certifications

Application
Considerations
VAV-PRC008-EN
AC 10
Control
Types
ControlTypes
VAV terminal units are available with
many different options.These options
fall into three main categories of
controls: direct digital (DDC),
pneumatic, and analog electronic. All
of these control types can be used to
perform the same basic unit control
functions, yet differences exist in
accuracy of performance, versatility,
installed cost, operating cost, and
maintenance cost.
Disadvantages:
Versatility—The communications
protocol between controllers will be
different from one controller
manufacturer to another.
Installed Cost—DDC controls are the
most expensive of the three control
types.
Operating and Maintenance Costs—
Building personnel must be trained to
operate and maintain the system.
Diagnostic capability for analog
electronics is not available.
Pneumatic control systems use
compressed air through simple
mechanical control devices, such as
diaphragms, springs, and levers to
change an output in response to a
change in a monitored variable.With
VAV terminal units, the output is
typically a primary airflow and the
monitored variable is zone temperature.
Benefits:
Performance—Pneumatic controls are a
proven technology that is effective and
has a long life cycle.
Installed Cost—When a source of
compressed air exists at the facility,
pneumatics generally have a lower
installed cost than other types of
controls when only a basic functionality
is required.
Pneumatics are still the most familiar
control technology to many building
designers and maintenance people.
Large Installed Base—Pneumatic
systems are very common in existing
buildings.This eliminates the need to
purchase the most expensive piece of
equipment in a pneumatic control
system—the control air compressor.
Extensions to existing pneumatic
systems are generally very simple and
extremely cost-effective.
Disadvantages:
Performance—Pneumatic controls
provide proportional-only control for
VAV terminal unit systems.This control
scheme is less accurate than the more
advanced control schemes. Improper
calibration of pneumatic controls leads
to poor energy utilization.
Versatility—A central pneumatic control
system, where each of the control zones
can be monitored and adjusted from a
remote location, is extremely costly to
configure and to modify.
Pneumatics easily drift and require
constant upkeep and scheduled
maintenance. Diagnostic capability for
pneumatics is not available. A main
compressor which is not maintained
and becomes contaminated with oil or
water can pump those contaminants
into the compressed-air-distribution
system.This may require costly cleaning
of the system and a possible
replacement of system components.
Direct digital control (DDC)
systems became available as
advances in computer technology
made small microprocessors
available and affordable. Much of the
hardware in DDC systems is similar
to analog electronic systems.The
primary difference is that DDC
controllers allow system integration,
remote monitoring, and adjustment.
The microprocessor is programmed
using software that gives the
controller a higher level of capability
than either the pneumatic or analog
electronic options.
Benefits:
Performance—DDC controls offer PI
control capability.A PI control
scheme is the most accurate and
repeatable control scheme available
in theVAV terminal unit industry.
Versatility—DDC controls accepts
software commands to determine
how its outputs will be controlled.
When a control sequence must be
modified, making changes to the
software instructions is easier and
quicker than changing hardware.
DDC controls can be networked
together to provide system-control
strategies for energy savings.
Multiple controllers can be easily
monitored and adjusted from a
remote location. DDC controls also
have system and individual
diagnostic capability.
Analog electronic control systems
began to be used in the 1970s and
1980s. Cost effective and reliable
transistors, resistors, relays, and triacs
(electronic relays) allowed analog
electronics to become a substitute for
pneumatic controls. Analog electronic
controls use varying voltage signals to
change an output in response to a
monitored variable.
Benefits:
Performance—Analog electronic
controls are a basic technology that
has good repeatability.
Analog electronics have minimal drift
and therefore require much less
recalibration than pneumatics.
Ease of Use—Analog electronic
controls can be modified using tools as
basic as a screwdriver and a voltmeter.
Knowledge and availability of a
personal computer is not required.
Disadvantages:
Performance—Analog electronics
provide proportional-only control for
VAV terminal unit systems.This control
scheme is less accurate than the more
advanced control schemes.
Installed Cost—Analog electronics
have a higher installed cost than
pneumatic controls for systems with
basic functions.

Application
Considerations
VAV-PRC008-EN AC 11
Control
Types
DDC Controls Basic Information
DDC controls have become the
industry standard forVAV terminal unit
control systems. DDC systems use
electronic field devices such as a flow
transducer, a primary air modulating
damper, and an electronic thermostat.
These field devices report software
instructions of how the outputs are
positioned in relation to the inputs to a
controller.TheVariTranesystem uses a
primary air valve and flow transducer
for both DDC systems and analog
electronic systems. However, the DDC
zone sensor is different from the
analog electronic thermostat.
DDC controls provide much flexibility
and considerable diagnostic capability.
DDC controllers can be connected
together to form a network of
controllers. Once the controllers are
networked, they can be monitored for
proper operation from a remote
location. Commands and overrides can
be sent for groups of controllers at one
time to make system-wide changes.
Commands and overrides can be sent
to individual units to allow problem
diagnosis, temporary shutdown,
startup schedules or other specialized
changes.When integrated into a
building management system, the
operation of theVAV terminal unit
system can be modified to do such
things, as coincide with occupancy
schedules and reduce energy charges.
DDC control ofVAV terminal units is a
key element in providing intelligent
and responsive building management.
Precision control, flexible comfort, and
after hours access are all available with
theVariTraneDDC control system for
VAV terminal units.
Key features of the system include:
y An advanced unit controller
y Flexible system design
y User-friendly interaction
Analog Electronic Controls Basic
Information
Analog electronic controls continue to
be useful in specific applications.The
users of analog electronic controls can
benefit from the analog electronic
product without the necessary air
compressor capacity for pneumatic
applications or computer-literate
personnel for DDC applications.
However, as more and more people
become computer literate, DDC
controls have become the standard for
non-pneumaticVAV terminal unit
controls.The analog electronic control
system will control room temperature
by modulating the position of the
electronic air valve in response to zone
temperature changes.VariTraneanalog
electronic controls are only available in
pressure-independent operation.
Therefore, the flow is proportional to
the deviation from the zone setpoint.
The primary airflow through the air
valve is monitored by means of an
electronic pressure transducer
connected to the standardVariTrane
flow ring. The thermostat used with
theVariTrane electronic control system
is a thermistor which completes a
voltage divider circuit when wired back
to the analog control board.The
thermostat is designed to operate
specifically withVariTraneanalog
electronic controls and is not
interchangeable with theVariTraneDDC
zone sensor.
Pneumatic Controls Basic Information
Pneumatic controls modulate air
pressure of a controller to maintain
setpoint. ForVAV systems, there are
two primary types of pneumatic
controllers—the room thermostat and
the pneumatic volume regulator (PVR).
RoomThermostats
The most visible controller to the
customer is the room thermostat.
Pneumatic room thermostats can be
classified by two characteristics: the
tubing connection(s) to the thermostat
and the action of the thermostat output
in response to a change in the input.
Room thermostats are available in
models that require a one-pipe or a
two-pipe configuration.The name is
derived from the number of tubes that
must run to the thermostat location.
The difference is really in the
construction of the thermostats.The
two-pipe thermostats have a constant
pressure supply connected via an air
tube to the thermostat supply air port.
The supply air travels through the
thermostat’s relays, levers, diaphragm,
and bleed port to produce an output.
The output line is connected to the
output port of the thermostat and
extends to the controlled device.The
one-pipe thermostat has, as its name
suggests, only one air line connection.
The thermostat works by opening and
closing an air bleed valve.This will
either decrease or increase the
pressure on the controlled device,
which is connected to the same line
that runs to the thermostat.
Room thermostats also can be
classified by their reaction to a change
in temperature. Room thermostats
classified this way are denoted as
either direct-acting or reverse-acting.
Direct-acting thermostats will increase
their output pressure as the
temperature the thermostat measures
increases.
On the contrary, reverse-acting
thermostats will decrease their output
pressure as the temperature the
thermostat measures increases.
Direct-ActingThermostat Response
Output
Pressure Input Temperat
emperature
Reverse-ActingThermostat Response
Output
Pressure
Input
Input Temperat
emperature

Application
Considerations
VAV-PRC008-EN
AC 12
Control
Types
PneumaticVolume Regulators
These controllers accept the room
thermostat signal and modulate the
VAV terminal unit primary air damper.
The primary air damper is controlled
for an airflow setpoint that is
determined by the room thermostat.
The thermostat increases the PVR’s
airflow setting when the temperature
in the space is warm. On the other
hand, the thermostat decreases the
PVR’s airflow setting when the
temperature in the space is cold.
Currently,VariTrane offers two models
of pneumatic volume regulators in its
controls offering—the 3011 regulator
(used in most applications) and the
3501 model (used in dual-duct
constant- volume applications).The
primary difference is the 3501 PVR’s
ability to change the velocity pressure
linearly with a change in thermostat
pressure, which results in improved
stability at low flows. In contrast, the
3011 PVR resets the velocity pressure
with a change in thermostat pressure.
Reset Control of Minimum and
Maximum Flow—The 3011 PVR and
3501 use fixed reset control of
minimum and maximum flow settings.
The primary benefit of fixed reset in a
pneumatic volume regulator is stable
flow control without excessive damper
movement.
Fixed Reset—A fixed reset controller
operates over a thermostat signal
change of 5 psi between minimum and
maximum flow, regardless of the
differential pressure flow sensor signal.
The thermostat is usually set for a gain
of 2.5; i.e. it produces a 2.5 psi output
change per degree of space
temperature change.This control
strategy provides stable flow control
with the primary air valve throttling
between minimum and maximum
flow over a 2°F space temperature
change.
Example 1: Air valve with a 6" inlet,
Pneumatic thermostat gain = 2.5 psi/
degree:
Minimum Flow = 0 cfm, 0.0 in. wg
flow signal
Maximum Flow = 680 cfm, 2.0 in. wg
flow signal
2.0 in. wg signal range
The damper will modulate from zero to
maximum position over a 2°F
temperature change.
Bleed Port to Atmosphere—
Bleeding air to the atmosphere is a
normal operation for a volume
regulator.The 3011 volume regulator
addresses this function with a
dedicated bleed port.When air is bled
through the flow sensor, the differential
pressure signal from the sensor is
affected.As a result, the flow sensor
signal can be radically altered if the
volume regulator is bleeding air, and
may cause excessive damper
movement.
Calibration—The minimum and
maximum settings are independent of
each other and need to be set only
once during calibration.
Signal Configuration Flexibility—
Both can be configured to work with
both normally-open and normally-
closed pneumatic air valves, and both
direct-acting and reverse-acting
thermostats.
PneumaticVolume
Regulators
PVR 3501
PVR 3011

Application
Considerations
VAV-PRC008-EN AC 13
Air Flow
Total
Pressure
Static
Pressure
“Wake”
Pressure
Flow
Measurement
and Control
Flow Measurement and
Control
One of the most important
characteristics of aVAV terminal unit is
its ability to accurately sense and
control airflow.TheVariTrane terminal
unit was developed with exactly that
goal in mind.The patented, multiple-
point, averaging flow ring measures
the velocity of the air at the unit
primary air inlet.The differential
pressure signal output of the flow ring
provides the terminal unit controller a
measurement of the primary airflow
through the inlet.The terminal unit
controller then opens or closes the
inlet damper to maintain the controller
airflow setpoint
perpendicular to the airflow.The low-
pressure taps on theVariTrane flow
ring measure a pressure that is parallel
to the direction of flow but in the
opposite direction of the flow.This
“wake pressure” that the downstream
ring measures is lower than the actual
duct static pressure.The difference
between the “wake pressure” and the
static pressure can be accounted for so
that the above relationship between
flow and differential pressure remain
valid.The difference also helps create a
larger pressure differential than the
velocity pressure. Since the pressures
being measured inVAV terminal box
applications are small, this larger
differential allows transducers and
controllers to measure and control at
lower flow settings than would
otherwise be possible.
The average velocity of air traveling
through the inlet is expressed in
the equation:
Where:
FPM = Velocity of air in feet per
minute
1096.5 = A constant
VP = The velocity pressure of the
air expressed in inches of
water
DENS = The density of the air
expressed in pounds per
cubic foot
Often, the density is assumed to be a
constant for dry air at standard
conditions (68°F (20°C)) and sea level
pressure of 14.7 psi (101.4 kPa)).These
conditions yield the following
commonly used equation:
The velocity pressure is defined as the
difference between the total pressure
in the duct and the static pressure in
the duct:
VP =TP - SP (All units are expressed in
inches of water)
The amount of air traveling through
the inlet is related to the area of the
inlet and the velocity of the air:
AIRFLOW = AREA (square feet) x
AVERAGEVELOCITY (feet per minute)
Accuracy
The multiple, evenly spaced orifices in
the flow ring of theVariTrane terminal
unit provide quality measurement
accuracy even if ductwork turns or
variations are present before the unit
inlet. For the most accurate readings, a
minimum of 1½ diameters, and
preferably 3 diameters, of straight-run
ductwork is recommended prior to the
inlet connection.The straight-run
ductwork should be of the same
diameter as the air valve inlet
connection. If these recommendations
are followed, and the air density effects
mentioned below are addressed, the
flow ring will measure primary airflow
within ±5% of unit nominal airflow.
Air Density Effects
Changes in air density due to the
conditions listed below sometimes
create situations where the standard
flow sensing calibration parameters
must be modified.These factors must
be accounted for to achieve accuracy
with the flow sensing ring. Designers,
installers, and air balancers should be
aware of these factors and know of the
necessary adjustments to correct for
them.
Air Pressure Measurement Orientations
Flow Measurement
MostVAV terminal units contain a
differential pressure airflow
measurement device, mounted at the
primary air inlet, to provide a signal to
the terminal unit controller. Numerous
names exist for the differential
pressure measurement device—flow
sensor, flow bar, flow ring.The
differential pressure measured at the
inlet varies according to the volumetric
flow rate of primary air entering
the inlet.
The total pressure and the static
pressure are measurable quantities.
The flow measurement device in aVAV
terminal unit is designed to measure
velocity pressure. Most flow sensors
consist of a hollow piece of tubing with
orifices in it.TheVariTrane air valve
contains a flow ring as its flow
measuring device.The flow ring is two
round coils of tubing. Evenly spaced
orifices in the upstream coil are the
high-pressure taps that average the
total pressure of air flowing through
the air valve.The orifices in the
downstream ring are low-pressure taps
that average the air pressure in the
wake of flow around the tube. By
definition, the measurement of static
pressure is to occur at a point
Flow Ring
FPM = 1096.5√ VP
DENS
FPM = 4005√ VP

Application
Considerations
VAV-PRC008-EN
AC 14
Flow
Measurement
and Control
Elevation—At high elevations the air is
less dense.Therefore, when measuring
the same differential pressure at
elevation versus sea level the actual
flow will be greater at elevation than it
would be at sea level.To calculate the
density at an elevation other than
standard conditions (most
manufacturers choose sea level as the
point for their standard conditions),
you must set up a ratio between the
density and differential pressure at
standard conditions and the density
and differential pressure at the
new elevation.
Since the data from the manufacturer
is published at standard conditions,
this equation should be solved for the
differential pressure at standard
conditions and the other quantities
substituted to determine the ratio for
the differential pressure measured at
the new conditions.
∆P Standard Conditions
DENS Standard Conditions
∆P New Conditions
DENS New Conditions
=
Duct Pressure andAirTemperature
Variations—While changes in these
factors certainly affect the density of
air, most operating parameters which
VAV systems need keep these effects
very small.The impact on accuracy due
to these changes is less than one half
of one percent except in very extreme
conditions (extreme conditions are
defined as those systems with static
pressures greater than 5 in. wg
(1245 Pa) and primary air temperatures
greater than 100°F (37
.8°C)). Since
those types of systems occur so
infrequently, we assume the effects of
duct pressure and air temperature
variations to be negligible.
Linearity —With the increase in DDC
controls over pneumatic controls, the
issue of linearity is not as great as it
once was.The important aspect of flow
measurement versus valve position is
the accuracy of the controller in
determining and controlling the flow.
Our units are tested for linearity and
that position versus airflow curve is
downloaded and commissioned in the
factory to insure proper control of
the unit.

Application
Considerations
VAV-PRC008-EN AC 15
The equations for current draw for
these coils are:
1 φ amps =
3 φ amps =
VariTrane three-phase electric heat is
available in balanced configurations.
For example, a 9 kW three-phase coil,
each stage would carry 1/3 or 3 kW of
the load.
It is important to note that these coils
have certain minimum airflow rates for
each amount of kW heat the coil can
supply to operate safely.These airflow
values are based upon a maximum
rise across the electric heat coil of
50°F (28°C).
The equation that relates the airflow
across an electric coil to the
temperature rise and the coil change in
temperature is:
CFM =
HotWater Reheat
Hot water heating coils are generally
applied onVAV terminal units as reheat
devices.When applying these coils it is
important to make sure that they are
operating in the proper air flow and
water flow range. Either a two-way or a
three-way valve controls the coils.
The flow and differential pressure are
generally the known quantities.The
equation is solved for the flow
coefficient.The flow coefficient is then
compared to the published CV
values
for the control valves that are available.
The control valve with the CV
that is the
closest, but greater than, the calculated
flow coefficient is the correct choice for
the control valve.This choice will keep
the valve pressure drop below the
maximum allowable valve pressure
drop.The valve pressure drop should
then be checked against the coil
pressure drop. If the coil pressure drop
is appreciably larger than the valve
pressure drop, a valve with a smaller
CV
should be selected to produce a
larger control valve pressure drop. If
this new valve has a pressure drop that
is much larger than the maximum
allowable pressure drop for valves, the
system designer should be consulted
to make sure that the system hot water
pumps can deliver the water at the
new conditions.
Electric Reheat
Electric heating coils are applied on
VAV terminal units as terminal reheat
devices. Electric heat coil capacity is
rated in kilowatts (kW). Coils are
available with the total capacity divided
into one, two, or three stages.
Electric heat coils are available in
single-phase or three-phase models.
This refers to the type of power source
connected to the coil. Single-phase
models have resistance elements
internally connected in parallel.Three-
phase models have resistance
elements internally connected in a
delta or a wye configuration.
The current draw for the electric coil
will depend upon whether it is a single-
phase coil or a three-phase coil.The
current draw is necessary for
determining what size wire should be
used to power the electric coils and
how big the primary power fusing
should be.
HotWater Coil
HotWaterValves
Reheat
Options
Where:
CFM = Minimum airflow rate across
the coil
kW = The heating capacity of the
electric coil
3145 = A constant
∆T = The maximum rise in air
temperature across the coil
(usually 50°F (28°C))
Electric heat coils are available with
magnetic or mercury contactors.
Magnetic contactors are less
expensive than mercury contactors.
However, mercury contactors can be
cycled at a more rapid rate without
failing. Mercury contactors are rated
for heavier duty use and should be
used in as many applications as
possible. For pneumatic applications
the electric coils are available with
factory-installed pressure-electric
switches.
kW x 1000
PrimaryVoltage
kW x 1000
PrimaryVoltage x√ 3
kW x 3145
∆T
The most important factor when sizing
valves is the coefficient of velocity or
CV
.The CV
is defined as the flow rate, in
gallons of 60°F (15.56°C) water, that will
pass through the valve in one minute
with a one pound pressure drop.The
coefficient of velocity, which is
commonly called the flow coefficient,
is an industry standard rating.Valves
having the same flow coefficient rating,
regardless of manufacturer, will have
the same waterside performance
characteristics.
The equation that governs valve
sizing is:
Cv
=
Where:
CV
= Flow coefficient
GPM = The maximum water flow rate
through the valve in gallons
per minute
∆ P = The maximum allowable
differential pressure across
the valve in psi
GPM
√∆P

Application
Considerations
VAV-PRC008-EN
AC 16
Insulation
Insulation
Insulation in aVAV terminal unit is
used to avoid condensation on the
outside of the unit, to reduce the heat
transfer from the cold primary air
entering the unit, and to reduce the
unit noise.TheVariTrane line offers four
types of unit insulation.The type of
facing classifies the types of insulation.
To enhance IAQ effectiveness, edges of
all insulation types have metal
encapsulated edges.
Matte-Faced
This type of insulation is used for
typical applications. It consists of a
fiberglass core covered by a high-
density skin.The dual-density
construction provides good sound
attenuation and thermal performance.
Foil-Faced
This type of insulation is used in
applications where there is some
concern regarding airborne
contaminants entering the space, or
dirt being trapped in the fibers of the
insulation.The insulation is composed
of a fiberglass core laminated to a foil
sheet. Foil-faced insulation will provide
the same sound attenuation
performance as matte-faced insulation.
Encapsulated Edges
Double-Wall
applications where there is extreme
concern regarding airborne
contaminants entering the space or dirt
being trapped in the fibers of the
insulation. The insulation is the same
as the matte-faced insulation.
However, after the insulation is
installed, a second solid wall of
26-gage steel covers the insulation. All
wire penetrations of this insulation are
covered by a grommet.This type of
insulation will result in higher
discharge and radiated sound power.
Closed-Cell
applications where IAQ and fibers are
of primary concern.The acoustics of
the closed-cell insulation are similar to
double-wall insulation.The thermal
properties are similar to fiberglass
insulation.This insulation contains
no fiberglass.

Application
Considerations
VAV-PRC008-EN AC 17
Acoustics
Series vs. Parallel Fan-Powered Units
Acoustical considerations affect
whether a series or parallel fan-
powered terminal unit is selected. Both
units have their advantages.
The parallel unit has the advantage of
the fan being on and contributing to
the sound levels only when heating is
needed.The fans are usually smaller
because they are sized for 30–60% of
total unit flow.This creates a unit which
is quieter than series units.The
disadvantage of the parallel unit is that
the sound is intermittent.This impact
can be minimized by using an ECM,
which has slow fan ramp-up speed.
The primary acoustic benefit to the
series fan-powered unit is that the fan
runs continuously. Sometimes the unit
can be selected at slightly higher
sound levels due to the constant
nature of the sound.
The primary acoustic disadvantage the
series unit has compared to the parallel
unit is the need to size the unit fan for
the total room airflow. Series units
require a larger, louder fan than
parallel configurations.
Note: Operating parallel units with a
continuously operating fan may be
considered for some applications.This
provides the quietest overall fan-
powered system with the benefit of
continuous fan operation.
Insulation types
Insulation is a factor to consider when
dealing with the acoustics of terminal
units. Most insulation types will provide
similar acoustical results, but there are
exceptions. Double-wall and closed-cell
foam insulation will generally increase
your sound levels because of the
increased reflective surface area that the
solid inner-wall and closed-cell
construction provides.This increase in
sound will have to be balanced with the
IAQ and cleanability considerations of the
dual-wall and closed-cell construction.
Placement of units
Unit placement in a building can have a
significant impact on the acceptable
sound levels. Locating units above non-
critical spaces (hallways, closets, and
storerooms) will help to contain radiated
sound from entering the critical occupied
zones.
UnitAttenuation
Terminal unit-installed attenuators are an
option available to provide path sound
attenuation. Manufacturer-provided
attenuators on the discharge of a
terminal unit are targeted at reducing
discharge path noise and are typically a
simple lined piece of ductwork. It would
often be easier and less expensive to
design the downstream ductwork to be
slightly longer and require the installing
contractor to include lining in it.
Attenuators on the plenum inlet of fan-
powered terminals are targeted at
reducing radiated path noise since the
plenum opening on a fan-powered
terminal unit is typically the critical path
sound source. Significant reduction in
radiated path noise can result from a
well-designed inlet attenuator.The
attenuation from these attenuators is due
to simple absorption from the attenuator
lining and occupant line of sight sound
path obstruction.Therefore, longer
attenuators and attenuators that require
the sound to turn multiple corners before
reaching the occupied space provide
superior results, particularly in the lower
frequency bands.
Octave Band Frequencies
Octave Center Band Edge
Band Frequency Frequencies
1 63 44.6–88.5
2 125 88.5–177
3 250 177–354
4 500 354–707
5 1000 707–1414
6 2000 1414–2830
7 4000 2830–5650
8 8000 5650–11300
Acoustical best practices:
Acoustics with terminal units is
sometimes more confusing than it
needs to be. As we know, lower
velocities within a unit leads to
improved acoustical performance.
Additionally, if theVAV terminal unit
has a fan, a lower RPM provides better
Acoustics performance. It is as simple
as that—there are some catches,
however.
We know that lower velocities and
lower RPMs inVAV terminal units
result in improved acoustical
performance. Additional
considerations will be discussed in
more detail throughout this portion of
Application Considerations that pertain
to unit size and type, appurtenance
affects (due to insulation, attenuation,
etc.) certification, and computer
modeling. Let’s take a look at the first
consideration, sizing of units.
Sizing of units
Before blindly increasing the size of
units, we must first understand what is
setting the acoustics within the space.
In general, over 95% of acoustics in
VAV terminal units, which set the
sound pressure levels and ultimately
the NC within the space, is from
radiated sound. This is readily known
for fan-powered units, but less
commonly known for single- and dual-
duct units. Radiated sound emanates
from the unit and enters the occupied
space via means other than through
the supply ductwork. The most typical
path is through the plenum space, then
through the ceiling, then into the
occupied space. While discharge
sound should never be ignored,
radiated sound is the most dominant
and usually the most critical sound
source.
When increasing air valve sizes, BE
CAREFUL. Oversizing an air valve can
adversely impact the ability to
modulate and properly control
temperature in the space. In extremely
oversized situations, the air valve will
operate like a two-position controlled
device, with air either being “on”
, or
“off”
, and not really much in between.
The best way to avoid this is to
understand that the minimum for most
air valves is 300 FPM. This is a function
of the flow sensing device (see wake
pressures pp. AC 13) and the ability of
the pressure transducer and controller
to properly read and report flow. This
is not manufacturer specific, as physics
applies to all. Therefore, when sizing
air valves, regardless of the max
cooling velocity the minimum velocity
for proper pressure independent flow
is 300 FPM.
Modulation capability and range is
vital for proper operation ofVAV
systems. With grossly oversized units,
the unit will act as a constant volume
system eliminating the energy saving
and individual zone control advantages
ofVAV systems. A good rule of thumb
is to size cooling airflow for around
2000 FPM. VAV systems only operate
at full flow when there is a maximum
call for cooling in the zone. The
greatest portion of the time, an air
valve will be operating at partial flows.
When sizing fan-powered units, the fan
airflow range can be determined by
looking at the fan-curve. Because
parallel and series fan-powered units
operate at a constant fan flow,
selections can be made all the way to
the lowest flow ranges of the fan
curve. A good balance of performance
and cost is to select fans at 70-80% of
maximum fan flow.

Application
Considerations
VAV-PRC008-EN
AC 18
Acoustics
Attenuators that are simple “cups” at
the plenum inlet(s) have been shown
inTrane’s acoustical mock-up to
provide no measurable reduction in
sound pressure in the critical octave
bands which set the occupied space
noise criteria.
Certification andTesting
Terminal units should be submitted
based on the same criteria.There are
several ways to ensure this by
certification and testing.
Raw unit sound data can be good
measurement criteria for evaluation. In
using this as a basis for comparison,
the designer needs to make sure that
the information is based on the ARI
Standard 880 that gives the procedure
for testing.
Specifying NC or RC sound levels is a
possible comparison, but the designer
needs to be sure the comparison is fair.
Two options are to specify the
attenuation effect on which you would
like the units to be evaluated or to
specify that ARI Standard 885-98
transfer functions be used.The
importance of ARI Standard 885-98 is
that it is the first ARI Standard that
specifies exact transfer functions to be
used for evaluation. Previous versions
of the standard gave guidelines, but
the manufacturers could choose their
own set of factors.
By using NC sound levels, it is possible
to express acceptable sound levels for
various types of buildings or
environments. A few examples are:
Concert Hall NC-22
Hospital Room NC-30
School Room NC-35
General Office NC-40
Cafeteria NC-45
Factory NC-65
PathAttenuation
Sound is generated by a terminal unit
can reach the occupied space along
several paths.The terminal unit
generated sound will lose energy—i.e.,
the energy is absorbed by path
obstacles—as it travels to the occupied
space.This acoustical energy
dissipation as it travels to the occupied
space is called path attenuation.The
amount of energy lost along a
particular path can be quantified and
predicted using the procedure outlined
in ARI-885. Each path must be
considered when determining
acceptable sound power generated by
a terminal unit.
The term “transfer function” is often
used to describe the entire path
attenuation value for each octave band
(i.e., the sum of all components of a
particular path).
Examples of path attenuation include
locating the terminal unit away from
the occupied space, increasing the STC
(sound transmission classification) of
the ceiling tile used, internally lining
ductwork, drywall lagging the ceiling
tiles or enclosing the terminal unit in
drywall.All of these choices have costs
associated with them that must be
weighed against the benefits. Some of
these alternatives can be acoustically
evaluated from application data
provided in ARI-885. Others may
require professional analysis from an
acoustical consultant.
Computer Modeling
Computer modeling of acoustical
paths is available to help estimate
sound levels and determine problem
sources.The software used byTrane for
computer modeling is calledTrane
Acoustics Program (TAP™).
TAP can analyze different room
configurations and materials to quickly
determine the estimated total sound
levels (radiated and discharged) in a
space.
TheTrane Official Product Selection
System (TOPSS™) can also be used to
determine sound levels of terminal
units.You can base selections on a
maximum sound level and enter your
own attenuation factors (defaults
based on ARI-885 are also available).
Other Resources
Please refer to "Additional References"
(page 29) of the Applications section to
see a list of publications to help with
the basics of acoustical theory and
modeling.You can also contact your
localTrane salesperson to discuss
the issue.

Application
Considerations
VAV-PRC008-EN AC 19
Duct Design
Duct Design
Designing cost-effectiveVAV duct
systems is challenging. Some duct
design methods result in better
pressure balance than others do. Duct
shape and duct material can influence
duct system design and cost. In
addition, duct layout is properly
designed for optimal duct installation
and operation.
Design Methods
The two most widely used supply duct
design methods—equal friction and
static regain—are discussed below.
Equal Friction – Using this method,
ducts are sized at design flow to have
roughly the same static pressure drop
for every 100 feet of duct. Static
pressures throughout the duct system
can be balanced at design flow using
balancing dampers, but are no longer
balanced at part load flows. For this
reason, equal friction duct designs are
better suited for constant volume
systems than forVAV systems. If the
equal friction method is used for the
VAV supply duct design, the terminal
units usually require pressure-
independent (PI) control capability to
avoid excessive flow rates when duct
pressures are high.
InVAV systems, the ducts located
downstream of the terminal unit are
usually sized for equal friction.The
advantage of this design method is its
simplicity. Often, calculations can be
made using simple tables and duct
calculators. Drawbacks include
increased higher total pressure drops
and higher operating costs.
Static Regain – In the static regain
method, ducts are sized to maintain
constant static pressure in each
section, which is achieved by balancing
the total and velocity pressure drops of
each section. In other words, static
pressure is “regained” by the loss of
velocity pressure. Since the static
pressures throughout the duct system
are roughly balanced at design and
part load flow, static regain duct
designs can be used successfully for
either constant volume orVAV
systems.When the static regain
method is used forVAV systems, the
system is roughly pressure balanced
at design.
Advantages of the static regain method
include reduced total pressure drops,
lower operating costs, and balanced
pressures over a wide range of flows.
The drawback of this design is the
time-consuming, iterative calculation
procedure and for large systems, it is
essential to have a duct design
computer program.
Duct Design Program
Trane has developed a computer
program, VariTrane™ Duct Designer,
to aid in the duct design process.This
program is used to calculate duct sizes,
fitting sizes, terminal unit sizes, and
pressure drops according to the equal
friction or static regain method.
The duct design program can be easily
incorporated into the selection ofVAV
terminal units.The inputs and outputs
for the program enableVariTrane units
to be selected based on the conditions
you require.This makes selecting and
scheduling units much easier. Contact
the local sales office or theTrane
C.D.S.™ department for more details
on this program.

Application
Considerations
VAV-PRC008-EN
AC 20
Selection
Program
Selection Program
The advent of personal computers has
served to automate many processes
that were previously repetitive and
time-consuming. One of those tasks is
the proper scheduling, sizing, and
selection ofVAV terminal units.Trane
has developed a computer program to
perform these tasks.The software is
called theTrane Official Product
Selection System (TOPSS™).
TheTOPSS program will take the input
specifications and output the properly
sizedVariTraneVAV terminal unit along
with the specific performance for that
size unit.
WithTOPSS, the user can integrate
selections of single-duct, dual-duct,
and fan-poweredVAV boxes with other
Trane products allowing you to select
all yourTrane equipment with one
software program.
The program has several required
fields, denoted by red shading in the
TOPSS screen, and many other
optional fields to meet the given
criteria. Required values for selections
include the maximum and minimum
airflows, the control type, and unit
model.When selecting models with
reheat, information regarding the
heating coil is needed for selection. In
addition, the user is given the option to
look at all the information for one
selection on one screen or as a
schedule with the otherVAV units on
the job.
Also,TOPSS will calculate sound-
power data for a selected terminal unit.
The user can enter a maximum
individual sound level for each octave
band or a maximum NC value. The
program will calculate acoustical data
subject to default or user supplied
sound attenuation data.
The program has many time-saving
features such as:
y Copy/Paste from spreadsheets like
Microsoft®
Excel
y Easily arranged fields to match your
schedule
y Time-saving templates to store default
settings
y Several output report options including
schedules
The user can also export the Schedule
View to Excel to modify and put into a
CAD drawing as a schedule.
Specific details regarding the program, its operation, and how to obtain a copy of it
are available from your localTrane sales office.
VariT
rane Quick Select
TheVariTrane Quick Select is a tool used by consulting and contracting firms for
specifying and choosingVariTraneVAV terminal units.The tool has basic information
regarding dimensions, pressure drops, acoustics, electric and hot water reheat, and
fan data. For more information, please contact your localTrane sales office.
Sample screen image fromTOPSS Selection Program

Application
Considerations
VAV-PRC008-EN AC 21
Best
Practices
Common Mistakes
Some of the most common system or
installation errors are discussed below.
Reducers at Unit Inlet
This problem is a very common issue
that is seen in applications ofVariTrane
products. It is often mistaken by those
in the field as an unacceptably large
static pressure drop through the unit. It
is also sometimes mistaken as a
malfunctioning flow ring, pressure
transducer (if DDC or analog electronic
controls are present) or PVR (if
pneumatic controls are present).
This problem is sometimes
unknowingly encountered because of
the capability of theVariTrane unit to
allow greater airflow for a specific size
duct than other terminal units. For
example, a project engineer specifies
an 8" (203 mm) round take off from the
main duct trunk to theVAV terminal
unit.The person supplying theVAV
terminal unit checks the required
airflow and finds that aVariTrane unit
with a 6" (152 mm) inlet will provide
the specified terminal unit
performance.The terminal unit
supplier submits, receives approval,
and orders the 6" (152 mm) inlet unit.
While this is happening, the installing
contractor has run the connecting duct
from the main trunk to the terminal
unit in the specified 8" (152 mm) round.
The unit arrives at the job site, and the
installer notices that the 8" (203 mm)
duct and the 6" (152 mm) terminal unit
inlet do not match.To get the unit
installed, an 8- to 6-inch reducer is
placed at the inlet to the terminal unit
air valve.
The reducer will cause a phenomenon
called flow separation at the unit inlet.
Fluid dynamics analysis can present a
detailed technical explanation of flow
separation, but the characteristics
important to this discussion are the
production of pressure loss and
turbulence. The reducer will have a
significant static pressure drop
associated with it since the air velocity
is increased (i.e., static pressure is
given up for increased velocity
pressure). The pressure loss is
sometimes mistaken as a loss due to
the function of the terminal unit.The
turbulence is at its greatest just
downstream of the reducer.
Unfortunately, this is the location of the
flow ring at the air-valve inlet.The
reducer will cause the flow ring to give
an inaccurate and inconsistent reading
because of the turbulent air.
The solutions to this situation are:
y Locate the reducer upstream of the
terminal unit at least three duct
diameters to eliminate flow separation
and turbulence at the unit inlet and to
improve the airflow measurement
accuracy.
y Consider proper sizing of the terminal
unit in the duct design and account for
the pressure loss of the reducer in the
central fan selection if a reducer is
required. Be cautious of “oversizing” a
VAV terminal. It is good practice to
make sure that the inlet duct velocity at
the minimum airflow setting is no
lower than 500 feet per minute.
Improper Use of Flexible Ductwork
While flexible ductwork has many
benefits, improper use can cause
numerous problems in aVAV system.
Flexible ductwork causes turbulent
airflow and relatively large static
pressure drops. Flexible ductwork at a
primary damper inlet (i.e., the flow
sensor location) may cause flow
accuracy and repeatability problems
due to turbulence.The use of flexible
ductwork should be primarily limited to
the downstream side of the terminal
units in aVAV system. Use of flexible
ductwork upstream of terminal units
should be kept to an absolute
minimum.All runs of flexible ductwork
should be kept as short as possible.
While most know these guidelines, the
ease of installation which flexible
ductwork provides is always an
enticement to push the limits of what
are acceptable practices.
Static Pressure Measurement Errors
Improper measurement techniques for
static pressure can lead many to
mistakenly believe that the terminal
unit is causing a large pressure drop in
the system.The chief error made here
is taking a static pressure
measurement in turbulent locations
such as flexible ductwork or near
transitions.This produces invalid static
pressure readings. Another error
commonly made is trying to read the
static pressure at the same point as the
flow sensing device.The inlets toVAV
terminal units produce turbulence and
will give poor readings. Flow sensors
with their multiple-point averaging
capability are best equipped to deal
with this type of flow, while a single-
point static probe is not. Another
common error is the incorrect
orientation of the static pressure probe.
The static pressure is correctly
measured when the probe is oriented
perpendicular to the direction of
airflow.The probe, or a part of it,
should never be facing the direction of
airflow, because the total pressure will
influence the reading of the probe.

Application
Considerations
VAV-PRC008-EN
AC 22
Unit
Conversions
Conversions ofVelocity, Pressure, and Flow Rate
To convert From To Multiply by
Velocity Ft/min M/s 0.00508
Velocity M/s Ft/min 196.850
Pressure Psi Pa 6894.76
Pressure Ft of water Pa 2988.98
Pressure In. of water Pa 249.082
Pressure Pa Psi 0.000145038
Pressure Pa Ft of water 0.000334562
Pressure Pa In. of water 0.00401474
Flow Rate Cfm L/s 0.4719
Flow Rate Cfm m3
/s 0.000471947
Flow Rate Gpm L/s 0.0630902
Flow Rate m3
/s Cfm 2118.88
Flow Rate L/s Cfm 2.1191
Flow Rate L/s Gpm 15.8503
Conversions of Length andArea
To convert From To Multiply by
Length In. m 0.0254
Length Ft m 0.3048
Length m In. 39.3701
Length m Ft 3.28084
Area In.2
m2
0.00064516
Area Ft2
m2
0.092903
Area m2
In.2
1550
Area m2
Ft2
10.7639

Application
Considerations
VAV-PRC008-EN AC 23
Additional
References
VAV System and Product
References
VAV SystemsAir Conditioning Clinic—
This clinic is designed to explain the
system components, the system
configurations, many of theVAV
system options and applications. A
great resource forVAV system
understanding.
Literature #TRG-TRC014-EN
IndoorAir Quality –A guide to
understandingASHRAE Standard
62-2001—
The guide helps to explain the
ASHRAE Standard as well as the
fundamentals of good indoor air
quality. A great resource for
understanding the standard and
ways of designingVAV systems
around that standard.
Literature # ISS-APG001-EN
Managing OutdoorAir –Traq™
Comfort Systems—
This brochure is a good, quick
reference of the issues of managing
outdoor air for aVAV system.
Literature # CLCH-S-26
Ventilation and Fan Pressure
Optimization forVAV Systems—
An engineering bulletin designed to
how aTrane Integrate Comfort™
system can effectively control building
ventilation and supply fan pressure
for increased comfort and IAQ while
keeping energy costs to the lowest
possible.
Literature # SYS-EB-2
Trane DDC/VAV SystemsApplications
Engineering Manual—
This manual gives detailed
descriptions of theTrane DDC/VAV
system.Topics include system
components, how the system interacts
and specific inputs and outputs of
the system.
Literature # ICS-AM-6
Acoustics inAir Conditioning
Applications Engineering Manual—
This manual describes the basic
fundamentals, behavior,
measurement, and control of sound, all
directed at the design of quiet systems.
Literature # FND-AM-5
VariTrac®
Catalog—
The catalog will help explain features
and benefits ofVariTrac, how the
VariTrac product works, applications for
the product, and selection procedures.
Literature #VAV-PRC003-EN
ASHRAE Handbook of Fundamentals
ASHRAE Handbook of HVAC Systems
and Equipment
ASHRAE Handbook of HVAC
Applications
ASHRAE Handbook of Refrigeration
Web sites:
www.ashrae.org
www.ari.org
www.trane.com

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