Trane Dedicated Outdoor Air Systems Catalogue

Application Guide

Dedicated Outdoor Air Systems

Trane DX Outdoor Air Unit

August 2012 SYS-APG001-EN

Preface

For more information on dedicated
outdoor air systems (DOAS), refer to the
following:

• Dehumidification in HVAC Systems,
Trane application manual
(SYS-APM004-EN)

• “Dedicated Outdoor Air Equipment,”
Tra ne Engineers Newsletter Live
program (DVD; APP-CMC043-EN)

• Water-Source and Ground-Source
Heat Pump Systems, Trane
application manual (SYS-APM010-EN)

• “Dedicated Outdoor Air Systems”
ASHRAE webcast, 2012
(www.ashrae.org)

Trane, in proposing these system design and application concepts, assumes no
responsibility for the performance or desirability of any resulting system design. Design of
the HVAC system is the prerogative and responsibility of the engineering professional.

Trademarks

Trane 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.

As a leading HVAC manufacturer, we deem it our responsibility to serve the
building industry by regularly disseminating information that promotes the
effective application of building comfort systems. For that reason, we regularly
publish educational materials, such as this one, to share information gathered
from laboratory research, testing programs, and practical experience.

This guide discusses HVAC systems that use:

• a dedicated outdoor air unit to treat all of the outdoor air brought into the
building for ventilation, and …

• zone-mounted terminal units to treat the indoor air.

Treating the outdoor air separately from recirculated return air makes it easy to
verify sufficient ventilation airflow and enables enforcement of a maximum
humidity limit in occupied zones.

© 2012 Trane All rights reserved Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001-EN

Table of Contents

Defining the Dehumidification Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Dedicated OA System Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Comparison of Different Dedicated OA System Configurations . . . . . . . . . . 3

Cold or Neutral Air? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Designing a Dedicated OA System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Selecting the Dedicated OA Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Control of the Dedicated OA Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Dehumidification mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Sensible cooling mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Sensible heating mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Ventilation only mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

ii Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001-EN

Defining the

Figure 1. Cooling loads at different

outdoor conditions *

* Based on an example classroom, which is

located in Jacksonville, Fla., and has a
target space condition of 74°F dry bulb and
50% relative humidity

Dehumidification

Building professionals expend much time and effort to design HVAC systems
that handle both ventilation and dehumidification. High-occupancy spaces,
such as classrooms, pose a particular challenge — especially when the system
of choice delivers a constant-volume mixture of outdoor and recirculated
return air. Why? The answer lies in the fact that the sensible- and latent-cooling
loads on the HVAC equipment do not peak at the same time.

When it’s hot outside, the sensible-cooling load often far exceeds the latent­cooling load (Figure 1). By contrast, when it’s cooler but humid outside, the
latent-cooling load can approach or even exceed the sensible-cooling load.

Constant-volume, mixed-air HVAC equipment traditionally is selected with
sufficient cooling capacity to handle the design load at the peak outdoor dry­bulb condition and controlled by a thermostat that matches the sensible­cooling capacity of the coil with the sensible-cooling load in the space.
Therefore, as the sensible-cooling load in the space decreases, the cooling
capacity (both sensible and latent) provided by the HVAC equipment also
decreases. In most climates, the combination of less latent-cooling capacity
and a lower SHR (sensible-heat ratio) in the space elevates the indoor humidity
level at part-load conditions.

An “off-the-shelf,” packaged unitary air conditioner may further aggravate this
situation. Such equipment is designed to operate with a supply-airflow-to­cooling-capacity ratio of 350 to 400 cfm/ton. In hot, humid climates, offsetting
the ventilation load for high-occupancy spaces may require that the unit
delivers no more than 200 to 250 cfm/ton in order to achieve the dew point
needed for adequate dehumidification.

Challenge

SYS-APG001-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 1

Selecting a packaged unitary air conditioner with enough cooling capacity
(tonnage) to meet the high ventilation load results in “excess” supply airflow,
that is, more supply airflow than is necessary to meet the sensible-cooling
load. To avoid overcooling the space, the air conditioner must deliver the
supply air at a warmer dry-bulb temperature. Unfortunately, this reduces the
dehumidification capacity of the coil and raises the humidity level in the space,
especially at part load. The right combination of cooling capacity and supply
airflow (large compressors, small fan) simply may not exist in packaged air
conditioners with prematched refrigeration and air-handling components.

Dedicated OA System Configurations

One way to overcome the challenges imposed by a constant-volume, mixed­air system is to design it as a dedicated outdoor air system (DOAS). The
design approach outlined in this guide permits each component of the HVAC
system to do what it does best: Zone-level heating-and-cooling equipment
provides occupants with air circulation and thermal comfort by modulating the
cooling-coil capacity to match the sensible-cooling load in the space. Any local
latent cooling occurs coincidentally; the latent-cooling load does not affect the
selection of zone-level HVAC equipment. Meanwhile, a central, dedicated
outdoor air unit sufficiently dehumidifies and tempers the outdoor air to meet
both the latent-cooling load and the ventilation requirements for all spaces
served by the system.

Dividing the building’s cooling load in this fashion can make it easier to
effectively ventilate and dehumidify occupied spaces. Key concepts to
remember when undertaking such a design include the following:

• Always provide conditioned air that is drier than the air in the
space. This practice minimizes the cooling capacity required from the local
HVAC terminals and adequately controls the indoor humidity without
additional, zone-level dehumidification enhancements.

• Deliver “cold” conditioned air whenever possible, and use
recovered energy to reheat during mild weather. Providing “cold”
conditioned air from the DOAS minimizes the cooling loads at the local
HVAC terminals. During mild weather (spring and fall), modulate the
amount of recovered energy used by the DOAS for reheat; only warm the
conditioned air enough to minimize inefficient reheat at the local HVAC
terminals.

“Neutral”-temperature conditioned air (which has a dry-bulb temperature
approximating that of the air in the space) increases the cooling capacity
required from the local HVAC terminals and requires more reheat at the
dedicated outdoor air unit.

• Deliver the conditioned outdoor air directly to each occupied
space, whenever possible. This helps ensure that the required amount
of outdoor airflow reaches each occupied space, allows the conditioned OA
to be delivered at a “cold” temperature (rather than reheated to neutral),
simplifies the application of demand-controlled ventilation (when desired),
and allows the fans in the local HVAC equipment to cycle off without
affecting ventilation performance.

Dedicated outdoor air systems can be designed to deliver conditioned outdoor
air either directly to each occupied space or to the individual HVAC terminals
or air handlers serving those spaces. Evaluate the advantages and
disadvantages of each configuration when designing a DOAS application.

Table 1 summarizes the advantages and drawbacks of each configuration.

2 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001-EN

Table 1. Comparison of different dedicated OA system configurations

dedicated
outdoor air unit

local HVAC
unit

dedicated
outdoor air unit

local HVAC unit

Conditioned OA delivered directly to each space

The DOAS in Figure 2 consists of a dedicated outdoor air unit, which
delivers conditioned outdoor air (CA) to each occupied space via
separate ductwork and diffusers. The local HVAC equipment
conditions only recirculated air (RA). This configuration
accommodates a wide variety of local equipment, including water­source heat pumps, vertical or horizontal fan–coils, unit ventilators,
DX (direct-expansion) rooftop units, split systems, blower–coils,
through-the-wall air conditioners (PTACs), variable-refrigerant-flow
(VRF) terminals, passive chilled beams, and radiant cooling surfaces.

Advantages: Disadvantages:

• Makes it easier to ensure the required amount of outdoor air
reaches each zone, because separate ventilation diffusers allow
easy airflow measurement and balancing

• Affords opportunity to cycle off the fan inside the local unit
(reducing fan energy use), because outdoor air is not distributed
to the zone by the local fan

• Allows the dedicated OA system to operate during unoccupied
periods (for after-hours humidity control or preoccupancy purge,
for example) without needing to operate the fans inside the local
units

• Affords the opportunity to downsize local heat pumps (reducing
installed cost and energy use) if the conditioned outdoor air is
delivered at a cold temperature (rather than reheated to “neutral”)

Conditioned OA delivered to the intake of each local HVAC unit

The DOAS in Figure 3 also uses a dedicated outdoor air unit to handle
the ventilation load. Ductwork carries the conditioned outdoor air
(CA) to each local HVAC terminal or air handler (typically blower–
coils, horizontal fan–coils, or water-source heat pumps), discharging
it near or directly into the inlet. The conditioned outdoor air then
mixes with recirculated return air (RA) and passes through the
cooling coil of the local terminal (or air handler), which delivers the
mixed supply air (SA) to the space.

Figure 2.

• Requires installation of additional ductwork and separate diffusers

• May require multiple diffusers to ensure that outdoor air is
adequately dispersed throughout the zone

Figure 3.

Dedicated OA System Configurations

SYS-APG001-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 3

Advantages: Disadvantages:

• Helps ensure the required amount of outdoor air reaches each local
unit, because the OA is ducted directly to each intake

• Avoids the cost and space needed to install additional ductwork and
separate diffusers

• Easier to ensure that outdoor air is adequately dispersed
throughout the zone, because outdoor air is distributed by the
local fan

• Measurement and balancing is more difficult than if the OA was
delivered directly to the zone via separate diffusers

• May re qu ir e a fi el d- fa br ica te d p le nu m or se ct io n o f duc t t o c on n ect
the outdoor air duct and mix it with recirculated air prior to entering
the local HVAC unit

• Fans inside the local units must operate continuously to provide
ventilation during scheduled occupancy, rather than cycling off

• If the dedicated OA system operates during unoccupied periods
(for after-hours humidity control or preoccupancy purge, for
example), the fans inside the local units typically must operate also

Dedicated OA System Configurations

local HVAC
unit

dedicated
outdoor air unit

OA

SA

SA

top of
dividing wall

return air

inlet to ceiling plenum

WSHP

WSHP

air balancing means

Source: ASHRAE 62.1-2010 User’s Manual, Figure 5-D ©American Society of
Heating, Refrigerating and Air-Conditioning Engineers, Inc, www.ashrae.org.

Table 1. Comparison of different dedicated OA system configurations

Conditioned OA delivered to the supply-side of each local HVAC unit

The DOAS in Figure 4 delivers the conditioned outdoor air (CA)
directly to the supply-side of each local HVAC terminal, where it
mixes with supply air from the local HVAC terminal before being
delivered to the occupied space. The local equipment conditions only
recirculated air (RA).

Advantages: Disadvantages:

• Helps ensure the required amount of outdoor air reaches each unit,
because the OA is ducted directly to the supply-side of each unit

• Avoids the cost and space needed to install additional ductwork and
separate diffusers

• Affords the opportunity to downsize local units (reducing installed
cost and energy use) if the conditioned outdoor air is delivered at
a cold temperature (rather than reheated to “neutral”)

• Easier to ensure that outdoor air is adequately dispersed
throughout the zone, because outdoor air is distributed by the
local fan

Conditioned OA delivered to the open ceiling plenum, near each local HVAC unit

The DOAS in Figure 5 delivers the conditioned outdoor air (CA) to the
ceiling plenum, near the intake of each local HVAC terminal. The
outdoor air mixes with recirculated air (RA) in the plenum before
being drawn in through the intake of the unit. The local unit
conditions this mixture of outdoor and recirculated air, and delivers
it to the occupied space through a shared duct system and diffusers.

Figure 4.

• Measurement and balancing is more difficult than if the OA was
delivered directly to the zone via separate diffusers

• Fans inside the local units typically must operate continuously to
provide ventilation during scheduled occupancy, rather than
cycling off (unless a pressure-independent VAV terminal is used to
maintain outdoor airflow)

Figure 5.

(continued)

Advantages: Disadvantages:

• Avoids the cost and space needed to install additional ductwork,
separate diffusers, or field-fabricated mixing plenums

4 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001-EN

• More difficult to ensure the required amount of outdoor air reaches
each unit, since the OA is not ducted directly to each local unit
(refer to the ASHRAE 62.1 User's Manual for further guidance)

• Conditioned outdoor air may not be able to be delivered at a cold
temperature, due to concerns over condensation within the ceiling
plenu m (rather, it must t ypi cally be reh eated closer to a “neu tra l”
temperature)

• Fans inside the local units must operate continuously to provide
ventilation during scheduled occupancy, rather than cycling off

• If the dedicated OA system operates during unoccupied periods
(for after-hours humidity control or preoccupancy purge, for
example), the fans inside the local units typically must operate also