Trane VariTrac Dampers Catalogue

Communicating
Bypass Controller

HVAC Unit

Bypass Damper

VariTrac Central Control
Panel with touchscreen

interface

VariTrac Zone Damper

Zone Sensor

Wireless Zone Sensor

Wireless Receiver

Product Catalog

Tracker System — CB
VariTrac™ — Changeover Bypass VAV

March 2012 VAV-PRC0 03- E N

Introduction

Comfort Made Simple

Trane has a long history of innovative leadership in variable air volume (VAV) technology. Trane
introduced the: 1) First fan-powered VAV unit; 2) First factory-commissioned DDC controller; and

3) First preprogrammed VAV controller designed specifically for VAV applications. Trane is now the
leading manufacturer of VAV terminal units and VAV-related products in the world.The introduction
of VariTrac™ in 1989 brought VAV controls expertise into the changeover bypass zoning market.
Trane is committed to continuous product improvement and now introduces a new generation of
VariTrac controls. This latest generation retains the functionality of the original VariTrac system
with exciting new enhancements, utilizing the best of today’s technology.

The Changeover Bypass VAV Comfort Advantage

Figure 1. VariTrac CCP

(a) Maximizes system efficiency and reliability by coordinating the components of the changeover-bypass system.

(a)

Packaged unitary systems offer a popular and cost-effective method of supplying conditioned air
to light commercial buildings. These systems commonly have a constant-volume fan with a fixed
outside air damper and a single thermostat. While a constant volume system may meet the overall
thermal requirements of the space, only a single thermostat is available. This system may be
insufficient in multiple-space applications with independent thermal load requirements.
Changeover bypass systems use the practicality and cost effectiveness of constant volume unitary
components like packaged rooftop units, split systems, or water-source heat pumps, and simply
add dampers and a central control panel to coordinate the components. This allows up to 24
individual sensors (thermostats) for independent temperature control.

VariTrac Product Enhancements

Figure 2. VariTrac CCP with optional touch-screen interface

Selected enhancements of the new VariTrac product are as follows: 1) A new central control panel
(CCP) with improved system temperature and pressure control functions; 2) An optional touch­screen operator display for the CCP with built-in time clock for easier system setup and control;

3) A communicating bypass controller allows duct pressure and duct temperature to communicate
to the system via a twisted shielded wire pair, thus eliminating costly “home-run” wiring; 4) The
next generation UCM zone controller allows CO
display zone sensor for simplified occupant control.

(a) Simplifies system operation with intuitive icon-driven design.

Advanced Control Options. Some VariTrac intelligent system control features are listed below.

•CO

• Zone-based HVAC unit control operates heating and cooling only when zone demand exists.

• Discharge air control to avoid extreme supply air conditions and maximize equipment life and

• A simplified system-balancing process is available via PC software or the touch-screen

• Global zone temperature setpoint limits simplify startup, commissioning, and operator control.

-based demand control ventilation resets the position of the HVAC unit ventilation air

2

damper when zone CO2 levels rise.

occupant comfort.

interface.

Trademarks

Trane, VariTrac and the Trane logo are trademarks of Trane in the United States and other countries.
All trademarks referenced in this document are the trademarks of their respective owners.

(a)

and occupancy sensor inputs; and 5) A digital

2

© 2012 Trane All rights reserved VAV-PRC003-EN

Table of Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Features and Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Application Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Selection Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Model Number Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Electrical Data and Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Acoustics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Dimensions and Weights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

VAV-PRC003-EN 3

Features and Benefits

Communicating
Bypass Controller

HVAC Unit

Bypass Damper

VariTrac Central Control
Panel with touchscreen

interface

VariTrac Zone Damper

Zone Sensor

Wireless Zone Sensor

Wireless Receiver

Figure 3. VariTrac changeover-bypass VAV system components

VariTrac Central Control Panel (CCP)

The CCP is the system level controller which
coordinates and monitors VariTrac system
operation, including HVAC system supply
pressure and airflow, heating/cooling
mode, supply air temperature, all zone
temperatures and setpoints, fan mode,

economizer position (when paired with CO2
demand controlled ventilation), time-of-day scheduling, zone
grouping logic, system override mode (after hours operation),
and much more.

Communicating Bypass Controller

provides power to drive the bypass damper actuator.

A single enclosure with duct
temperature sensor, static
pressure sensor, and
communicating controller (UCM)
which easily mounts on the
supply ductwork. The UCM

Zone Sensor

Zone sensors (sometimes referred to as thermostats) measure space temperature and report it to

the zone damper controller (UCM). Five models are available to satisfy varied aesthetic and

application preferences.

4 VAV-PRC003-EN

Features and Benefits

HVAC Unit

Overview

Bypass Damper with Wire and Quick

VariTrac changeover bypass systems

operate with Trane and non-Trane products,

including split systems, packaged rooftop

units, and water-source heat pumps.

These systems are generically referred to as

HVAC (heating, ventilating, and air

conditioning) units.

When combined with a Trane packaged

rooftop with ReliaTel™ controller, wiring,

installation, and system startup efficiency is

maximized by connecting with a simple

twisted shielded wire pair.

Changeover-bypass VAV is a comfort system developed for light commercial applications. A
changeover-bypass VAV system responds to changing cooling or heating requirements by varying
the quantity or volume of air delivered to each zone. Each zone has a thermostat for individual
comfort control. An HVAC unit delivers a constant volume of air to the system. As the volume of
air required by the zone changes, excess supply air is directed to the return duct via a bypass duct
and damper. (See Figure 3, p. 4 for typical system components.)

A changeover-bypass VAV system combines the comfort benefits of VAV with the cost effectiveness
and simplicity of packaged, constant-volume unitary equipment.

Connect

A round or rectangular
damper ducted between
the HVAC supply and
return ducts. It is easily
connected via a “quick­connector” which
provides quick and

consistent field wiring.
The bypass damper is modulated by the CCP to
maintain required system static pressure.

How the System Works

A changeover-bypass VAV system commonly consists of an HVAC unit with a constant-volume
supply fan, and direct-expansion (DX) cooling. This combined system has the ability to “change”
to the heating mode or cooling mode, depending on individual zone comfort requirements. A
heating coil or a gas-fired heater and an outside air damper are possible options.

A temperature sensor in each zone communicates information to an electronic controller on the
VAV terminal unit. The controller then modulates the zone damper open or closed, supplying
heating or cooling air to the zone.

The HVAC unit delivers a constant volume of supply air to the system. In order to maintain duct
static pressure, a bypass duct and damper are required to bypass (detour) air not required in the
zones.

The VAV terminal unit controller communicates zone temperature information to a central control
panel (CCP). The CCP also gathers information from the system, including duct static pressure and
supply-air temperature. The CCP determines zone heating or cooling needs using voting (or
polling) logic, then requests heating or cooling from the HVAC unit. The CCP directs the HVAC unit
to provide ventilation air to high-occupancy areas (demand control ventilation) or free-cooling
when the outside air temperature falls below the temperature setpoint (economizer control).

Auto Changeover

“Auto changeover” refers to the ability of the system to automatically change between the heating
and cooling modes.

In a changeover-bypass VAV system, the CCP determines whether the HVAC unit should heat or
cool by polling the temperature of the individual zones. It then compares the zone temperatures
to the space temperature setpoints. If the supply air does not meet the criteria for the heat or cool
mode called for, the CCP sends a signal to the HVAC unit to change the system to the opposite
mode.

VAV-PRC003-EN 5

Features and Benefits

Central Control Panel

The VariTrac central control panel (CCP) serves as the central source of communications and
decisionmaking between the individual zones and the HVAC unit. The CCP determines system
heating and cooling modes and coordinates the system supply air temperature and static pressure
to satisfy building thermal load conditions. Inputs to the CCP include 24VAC power and
communication wiring to the zone dampers and bypass control.

Binary inputs consist of priority shutdown and occupied/unoccupied modes. Heating, cooling, and
the HVAC unit fan on split systems and non-Trane HVAC units can be controlled through binary
outputs on an accessory relay board. If a Trane rooftop air conditioner with factory-installed
electronic controls is used, the CCP can control heating, cooling, and the fan with a two-wire
communication link tied to an interface board mounted in the rooftop. It can also display status
information from the electronic controller in the rooftop. (See Figure 4, p. 6.)

CCP Feature Summary

• Communicates with up to 24 VAV unit control modules (UCMs).

• Makes optimal heating and cooling decisions based on setpoint and temperature information
received from individual zones.

• Automatically calibrates all dampers, significantly reducing labor-intensive and costly field
calibration.

• Windows-based PC software simplifies setup and control.

• Provides diagnostic information for all system components via the operator display or PC
software.

• Provides status and diagnostic information for Trane HVAC units equipped with Trane ReliaTel
or UCP electronic controls.

Figure 4. VariTrac central control panel (L) and a screen representation from the central control panel, illustrating

system status (R).

Optional Operator Display

Figure 5. VariTrac central control panel with optional operator display

The optional operator display is a backlit, liquid crystal
display with touch-screen programming capability.

The operator can access system and zone status through
the display and perform basic setup of zone VAV UCMs
and CCP system operating parameters. The display
allows an installer to commission a VariTrac system
without using a PC. The operator display has a seven­day time clock for stand-alone scheduling capability.

6 VAV-PRC003-EN

Operator Display Feature Summary

Quick Connect

Duct Temperature
Sensor

Static Pressure
Sensor

• Backlit LCD touch-screen display for easy operator interface.

• Combination of icon- and menu-based navigation provides intuitive operation.

• Provides a level of control for the daily operator, and a second level for commissioning and
service.

• Three levels of security are available to protect system settings.

• Seven-day time clock for stand-alone, time-of-day scheduling.

Communicating Bypass Controller

The communicating bypass controller is a single control enclosure with the following integrated
devices included:

• Integrated UCM board;

• Static pressure sensor; and

• Discharge air temperature sensor.

The communicating bypass controller directly controls the bypass damper and communicates duct
conditions to the central control panel via a simple twisted shielded wire pair.

Quick Connect

Minimizes field wiring labor and assures wiring consistency

Features and Benefits

Duct Temperature Sensor

The supply air temperature sensor allows the CCP to control heating and cooling stages to maintain
the supply air temperature. Supply air temperature setpoints can be edited through the operator
display or PC software.

Static Pressure Sensor

The static pressure sensor measures duct static pressure and positions the bypass damper(s) to
maintain the static pressure setpoint.

Figure 6. Communicating bypass controller, side (L) and 3D (R) views

Tracker System Integration

The VariTrac system can be fully integrated with the new family of Tracker building controls. A
Tracker building management system can manage multiple VariTrac systems from a single control
point.

VAV-PRC003-EN 7

Tracker System Summary

• Controls up to 10 VariTrac systems from a single Tracker panel for easy building operation.

Features and Benefits

Up to 24
VariTac or
VariTrane
Dampers

• LCD touch-screen operator display or Tracker PC software interface provides single-point

• 365-day scheduling function and the flexibility of up to 10 schedules.

• Assign all systems to a single schedule, if desired, for simplified schedule changes.

• Exception scheduling feature for easy management of vacations and holidays.

• Automatically adjusts for daylight savings time and leap year.

• Remote communications capability via modem for system programming and control.

Figure 7. Tracker system architecture

building management by a local operator.

VariTrac Bypass Dampers

Bypass dampers are non-communicating VariTrac dampers and include an integrated fully­modulating 24 VAC electric actuator.

Field wiring errors are reduced with a quick-connect harness that plugs into the communicating
bypass controller.

Dampers are nominally rated up to 1800–2400 fpm at 1.75" of static pressure, depending on size.

For damper performance information, see Ta bl e 2, p. 2 7, Table 3, p. 28, Tabl e 4 , p. 28, and Tab l e 5 ,

p. 28.

Round Bypass Damper Summary

• Round bypass dampers are available with inlet diameters 6, 8, 10, 12, 14, & 16 inches.

• Heavy gage galvanized steel cylinder with rolled bead for high structural integrity and positive
shut off.

• Sandwich damper blade with elastomeric seal provides quiet operation and tight shutoff for low
leakage.

• Factory-installed, direct-coupled, fully-modulating 24 VAC actuator.

8 VAV-PRC003-EN

• Rated up to 2400 fpm at 1.75" of static pressure.

Rectangular Bypass Damper Summary

• Rectangular bypass dampers are available in sizes 14 x 12, 16 x 16, 20 x 20, and 30 x 20 inches.

• Formed heavy gage galvanized steel frame, mechanically joined with linkage concealed in the
side channel.

• Air leakage is minimized with an opposed blade design.

• Damper casing is 16 inches long and constructed of heavy gage galvanized sheet metal with slip
and drive connections on the inlet and outlet for easy installation.

• Blades are either four-inch or five-inch nominal widths, depending on height of damper,
constructed of 18 gage galvanized steel.

• A blade rotation stop feature prevents over-rotation of the blades in the fully open position.

• Factory-installed, direct-coupled, fully-modulating 24 VAC actuator.

• Rated up to 3000 fpm at 2" of static pressure.

VariTrac Zone Dampers

VariTrac zone dampers are fully-modulating, pressure-dependent VAV devices. The dampers
control zone temperature by varying the volume of air flowing into a space. Each VariTrac damper
has a control box with a VAV control board and actuator enclosed. The dampers are designed to
operate in static pressures up to 1.75 in. wg.

Features and Benefits

Figure 8. VariTrac rectangular (L) and round zone (R) dampers with UCMs

Round Zone Damper

• Round dampers are available in 6, 8, 10, 12, 14, and 16 inch diameters.

• Heavy gage galvanized steel cylinder with rolled bead for high structural integrity and positive
shut off.

• Sandwich damper blade with elastomeric seal provides quiet operation and tight shutoff.

• 90° blade rotation for a wide control range and stable operation.

• Rated up to 2000 fpm at 1.75" of static pressure.

Rectangular Zone Damper

• Rectangular dampers are available in sizes 8 x 12, 8 x 14, 8 x 16, 10 x 16, 10 x 20, and 14 x 18
inches.

• Heavy gage G60 galvanized steel frame assembled by a mechanical joining process.

• Single-ply, heavy gage G60 galvanized steel blades.

• Linkage uses glass fiber filled nylon crank bearings connected by a metal drive link.

• Factory-installed 24 VAC direct-coupled actuator.

• Rated up to 2400 fpm at 2" of static pressure.

VAV-PRC003-EN 9

Features and Benefits

Unit Control Module

A unit control module (UCM) is the individual zone controller for the VariTrac air damper and is
mounted on each zone damper. The unit controller continually monitors the zone temperature to
maintain space temperature. The UCM varies the damper position as needed to meet zone
setpoints and communicates current space requirements and system operating modes to the CCP.

The UCM can also control local heat. Local heat may be duct- or space-mounted, and can be staged
electric, pulse-width modulating electric, and modulating or two-position staged hot water.

Zone Sensors

Figure 9. DDC zone sensor with LCD (L) and DDC zone sensors (R)

DDC Zone Sensor

The direct digital control (DDC) zone sensor is an uncomplicated, reliable electro-mechanical room
sensor. No programming is required and most sensors contain an internal communications jack.
Models are available with combinations of features such as override (on-cancel) buttons and space­mounted setpoint. Four sensor variations are available:

• Sensor only (no communications jack)

• Sensor with override buttons

• Sensor with temperature setpoint only

• Sensor with temperature setpoint and override buttons

DDC Zone Sensor with LCD

The DDC zone sensor with LCD (liquid crystal display or digital) is compatible with VariTrane VAV
and VariTrac controllers.

Digital Zone Sensor Summary

• Displays setpoint adjustment and space temperature in °F or °C.

• Simple, two-button control of space setpoint.

• Setpoint control and room temperature display can be optionally disabled.

• Includes button for timed override and a cancel feature for after-hours system operation.

• An easily accessible communications jack is provided for Trane portable edit terminal devices.

• Nonvolatile memory stores last programmed setpoints.

• For field balancing, maximum and minimum airflow or position can be overridden from the
sensor.

10 VAV-PRC003-EN

CO2 Sensor

Features and Benefits

Figure 10. Duct-mounted CO

Wall- and duct-mounted carbon dioxide (CO2) sensors are designed for demand-controlled
ventilation zone applications. The sensor is compatible with VariTrane VAV and VariTrac
controllers. The Trane CO2 sensors measure carbon dioxide in parts-per-million (ppm) in occupied
building spaces. Carbon dioxide measurements are used to identify under-ventilated building
zones. Outdoor airflow increases beyond design ventilation rates if the CO2 exceeds specified
levels.

sensor (L) and wall-mounted CO2 sensor (R)

2

CO2 Zone Sensor Summary

• Use with the UCM CO2 input for demand control ventilation.

• Silicone-based NDIR sensor technology for long-term stability.

• Measurement range of 2000 ppm CO2 input with an output of 0–10 Vdc.

• Wall-mount transmitter is compact and aesthetic in appearance.

• Optional zone return duct-mount transmitter is available.

Zone Occupancy Sensor

Figure 11. Zone occupancy sensor

The energy-saving zone occupancy sensor is ideal for
zones having intermittent use during the occupied
mode. The sensor sends a signal to the VAV controller
upon detection of movement in the coverage area. The
VAV system then changes the zone from occupied
standby mode to occupied mode.

Occupancy Zone Sensor Summary

• Compatible with VariTrane VAV and VariTrac controllers

• Used with zone damper UCM for controlling the occupied standby function

• Ceiling-mount PIR occupancy sensor detects motion over an adjustable range up to 360
degrees

• Single detector covers up to 1200 square feet. For areas larger than 120 0 square feet, multiple
sensors can be wired in parallel

• Adjustable time delay avoids nuisance change of state on loss of detection

VAV-PRC003-EN 11

Features and Benefits

• Adjustable sensitivity

• SPDT isolated contacts connect to UCM input

Auxiliary Temperature Sensor

Figure 12. Auxiliary temperature sensor

Installed Temperature Sensor

• Thermistor sensing element 10,000 Ohms @ 77°F

• Wiring connection 2 feet, 18 awg

• Sleeving for wire leads is acrylic #5 awg grade C rated @ 155C

The auxiliary temperature sensor is used with any UCM
damper control. The sensor allows the operator to
monitor duct temperature or air temperature leaving a
reheat device at the zone damper. This sensor is used for
automatic changeover of a UCM damper when not using
a CCP. The auxiliary temperature sensor is ideal for
remote monitoring and diagnostics from the CCP
operator display.

12 VAV-PRC003-EN

Application Considerations

Single Zone Building

One thermal and
one comfort zone

Thermal Zoned Building

Multiple thermal zones

each with one

comfort zone

Thermal and

Comfort Zoned Building

Multiple thermal zones

each with multiple

comfort zones

Least

Most

Energy Savings • Comfort • Flexibility

Introduction

The VariTrac system is a changeover-bypass VAV system. One fan supplies either warm air for
heating or cool air for cooling. It is typically applied in small buildings which use unitary heating/
cooling air conditioners. These buildings need the simplicity and low cost of unitary equipment,
but more than one comfort control zone (one zone temperature sensor) for each air conditioner.

When is VariTrac a good HVAC system choice? To help answer this question, several important
application concepts and considerations are discussed below.

Figure 13. System design affects occupancy comfort

Zoning Considerations

Consider the following two questions when evaluating your HVAC system design:

Will the building occupants be comfortable?

A system designed with a single-zone HVAC unit and one zone sensor provides comfort to
occupants near the zone sensor. However, occupants in perimeter areas or interior rooms may be
too hot or too cold.

VAV-PRC003-EN 13

Will comfort be consistent from room to room and area by area?

A building is normally divided into thermal zones for increased comfort control and energy
savings. Each thermal zone should have a dedicated HVAC unit. For optimum comfort, each thermal
zone should be further divided into comfort zones.

Choosing the number and location of thermal and comfort zones is critical in planning an effective
system. Some things to consider in the design process include:

• Geographic location

• Orientation of the building to the sun

• Prevailing winds

• Wall construction (glass, insulation, building materials)

Application Considerations

Step 1. Define Occupant

Comfort Needs

Step 2. Define Thermal Zones

St ep 4 . Si ze Hea ting /

Cooling Equipment

Step 3. Determine

Comfor t Zo nes

St ep 6. De sig n t h e Du ct syst em

St ep 5 . Si ze Zon e an d
Byp ass D am per Un its

Involves archit ect,

engineer(s), and buildi ng owner

Involves engineers

an d cont ractor s

Involves engineers,

contract ors, and building owner

Involves engineer(s) and cont ractors

Involves engineer(s) and cont ractors

Involves engineer(s) and cont ractors

Step 7. Air Diffuser

Selection and Placement

Involves engineer(s) and cont ractors

• Building layout, design, occupancy and occupancy pattern throughout the day and year

• Activities in each zone

Zoned unitary systems, such as changeover-bypass VAV, divide thermal zones into smaller comfort
zones. Each comfort zone has a damper and zone sensor that controls the amount of heated or
cooled air delivered to the zone. A central system controller monitors the status of each zone
damper and zone sensor. The controller then makes the decision to heat or cool for the HVAC unit.

Individual comfort zones served by a common HVAC unit (part of the same thermal zone) can
require heating and cooling at the same time. In a changeover-bypass VAV system, the unit
alternately provides warm and cool air in an attempt to satisfy the needs of all comfort zones. This
is effective if the simultaneous calls for heating and cooling exist for short time periods only. Wide
temperature variations may occur if some comfort zones need heating for extended periods of time
while others need cooling.

Some comfort zones require special consideration because of their use or location. An example is
the foyer or reception area of an office building. These areas often have wide variations in thermal
load because of glass (relative to other areas of the building) and frequently-opened exterior doors.
Another example is an interior storage room with the need for ventilation but little or no heating
or cooling. These zones can significantly influence efficient operation and comfort levels
throughout the building.

Preferably, areas such as these are designed as separate thermal zones with dedicated HVAC units.
However, this may be impractical or costly. Instead, use fan-powered variable-volume terminal
units, or units with local reheat.

Effective Changeover Bypass VAV System Design

14 VAV-PRC003-EN

Unitary zoning systems feature low first cost and quick, easy system design and equipment
selection. The system is simple, but it is essential that key elements are considered during the
design process. This section offers a system design sequence and discusses application
considerations that, when followed, help avoid system control and operational instabilities.

Suggested design steps for unitary zoning systems are summarized in Figure 14, p. 14.

Figure 14. Suggested design steps

Step 1. Define occupant comfort needs

The design process begins by considering the needs of building occupants and intended building
use.

What is the intended use of the building?

Is the building usage primarily office space? Is there a manufacturing operation? Are there areas
that have special requirements such as computer or electronic rooms, video/television production,
training facilities, etc.?

What physical activity level is expected of the occupants?

Seated occupants require different indoor temperatures for comfort than continuously moving
occupants. An example may be a building with a mix of office space and light assembly or
manufacturing.

Where will the occupants be located and at what times?

Pay particular attention to areas with intermittent use, such as conference, training, and
lunchrooms.

How are the occupants expected to dress?

Give consideration to how the building occupants will dress. Will they dress in traditional business
attire, such as long-sleeved shirts or blouses, ties, and jackets? Or, will they dress in cooler, casual
attire, such as golf shirts, light slacks, skirts, or shorts?

Gather as much usage information as possible before designing a system. This can be challenging,
particularly when finishing out tenant spaces. However, usage information is crucial to the
selection of heating and cooling equipment, building zoning, and duct layout.

Several publications provide guidance for properly assessing indoor space comfort. An example
is ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) Standard
55, Thermal Environmental Conditions for Human Occupancy. This standard specifies the
combinations of indoor space environments and personal factors (activity and clothing) that will
produce thermal environmental conditions acceptable to 80 percent or more of the occupants
within a space. Standard 55 addresses temperature, thermal radiation, humidity, and air speed.

ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality, is another source for occupant
comfort and safety issues regarding indoor air quality. The standard recommends that relative
humidity be maintained between 30 and 60 percent. This maximizes comfort and reduces the
potential for microbial growth.

Application Considerations

Step 2. Define the Thermal Zones

A thermal zone is an area with similar load profiles and occupant comfort requirements. A thermal
zone can be a single room, an area, a group of rooms or an entire building. Defining the thermal
zones within a building is crucial to designing a comfortable indoor environment. Each thermal
zone is conditioned by a single heating and/or cooling unit. The load of the thermal zone
determines the size of the heating and cooling unit.

Cost vs. Comfort

First cost can be reduced by limiting the number of thermal zones. Unfortunately, this may impact
the thermal flexibility of the system, and result in zone comfort issues. Let’s take a closer look at
this important system decision known as “thermal zoning.”

Characteristics of a building which can influence thermal load are:

• Orientation of the building (North, South, East, West)

• Amount and thermal resistance (R-value) of glass (walls, skylights, etc.)

• Expected occupancy within the area

VAV-PRC003-EN 15

Application Considerations

Men's
Restroom

Women's
Restroom

ThermostatT

Glass windows

with no shading

Minimal wall

insulation

Poor Design Elements

• One thermostat for space

• Glass windows with no shading

• Minimal wall insulation

• Interior partitions and doors

• Varying loads from equipment or processes

Let’s examine a few building examples and discuss the zoning criteria of each.

Building Example 1 (See Figure 15, p. 16.)

Consider an existing single-story office building which is small, poorly insulated, with many large
windows and few interior partitions. On a clear, cool spring day, the entire building is cool in the
morning so heating is required. By afternoon, however, the south side of the building being
influenced by the solar load, is warm and requires cooling. The north side remains shaded and
continues to require heating. This situation results in a simultaneous requirement for heating and
cooling for extended periods. Due to the varying loads throughout the building, controlling the
building as a single thermal zone (with a single HVAC unit) cannot satisfy the comfort needs of all
areas. It also is not a good candidate for a zoning system because of the simultaneous need for
heating and cooling.

A similar building with good insulation and fewer shaded windows, on the other hand, may be a
good candidate for a single thermal zone with individual comfort zones. The reduction in wall glass
reduces the solar effect on the building resulting in all areas of the building having similar load
profiles throughout the day. In this case, the building has a single thermal zone and is a good
candidate for one HVAC unit. Individual comfort zones (zone dampers) will be needed to assure
comfortable conditions throughout the zone.

Figure 15. Building Example 1 illustrates a small, poorly insulated office on the left, and improved design on the right.

ThermostatT

Shaded

Windows

Men's
Restroom

Women's
Restroom

ThermostatT

Improved Design Elements

• Multiple zone thermostats

• Shaded windows

• Insulated walls

Insulated

Walls

ThermostatT

Building Example 2 (See Figure 16, p. 17.)

Consider a strip mall in the spring or fall with stores that face both east and west. In the morning,
the east side of the building gets full sun and warms up while the west side is shaded and requires
heating. In the afternoon, the east side of the building may need heat and the west side cooling.
Because of the thermal load variation throughout the day, this building will not remain comfortable
if designed with a single heating and cooling unit.

On the other hand, comfort in this building could be improved by dividing the building into two
thermal zones (two HVAC units), one serving the east exposure and the other serving the west.
Even with the two systems, individual occupant comfort is not necessarily assured. Interior
partitioning, varying schedules and number of occupants within the thermal zone will drive
differing amounts of heating and cooling. The issues related to comfort zoning are addressed in
the next section.

16 VAV-PRC003-EN