Trane DX User guide

Dedicated Outdoor Air Systems
Trane DX Outdoor Air Unit
October 2019 SYS-APG001A-EN
Application Guide

© 2019 Trane All rights reserved Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN
Preface
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.
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,”
Trane 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.

ii Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-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
Dehumidification mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Sensible cooling mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Sensible heating mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Ventilation only mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 1
Defining the
Dehumidification Challenge
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 latent-
cooling 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.
Conventional HVAC equipment traditionally is selected with sufficient cooling
capacity to handle the design load at the peak outdoor dry-bulb condition and
controlled by a thermostat that matches the sensible-cooling 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-to-
cooling-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.
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

2 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN
Dedicated OA System
Configurations
One way to successfully limit indoor humidity levels is to use 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 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
dehumidified air enough to avoid overcooling the zones.
“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.

SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 3
Dedicated OA System Configurations
Table 1. Comparison of different dedicated OA system configurations
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 water-
source 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.
Figure 2.
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, or vary the speed of, the fan inside
the local unit (reducing fan energy use) when no cooling or heating
is required, 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 units (reducing installed
cost and energy use) if the conditioned outdoor air is delivered at
a cold temperature (rather than reheated to “neutral”)
• Requires installation of additional ductwork and separate diffusers
• May require multiple diffusers to ensure that outdoor air is
adequately dispersed throughout the zone
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 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 require a field-fabricated plenum or section of duct to connect
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
outdoor air unit
local HVAC
unit
dedicated
outdoor air unit
local HVAC unit

4 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN
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).
Figure 4.
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
•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)
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 5.
Advantages: Disadvantages:
• Avoids the cost and space needed to install additional ductwork,
separate diffusers, or field-fabricated mixing plenums
• 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
plenum (rather, it must typically be reheated closer to a “neutral”
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
Table 1. Comparison of different 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 62.1-2016 User’s Manual, Figure 5-D ©American Society of
Heating, Refrigerating and Air-Conditioning Engineers, Inc, www.ashrae.org.
(continued)

SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 5
Dedicated OA System Configurations
Cold or Neutral Air?
Regardless of where the conditioned outdoor air is delivered, the dedicated
OA unit should dehumidify the outdoor air so that it is drier than the zone.
This offsets the latent load associated with ventilation and, if the dew-point
temperature of the conditioned outdoor air is lower than the dew point in the
zone (Figure 6), also offsets some (or all) of the zone latent loads. This
approach can adequately limit indoor humidity levels, at both full- and part-
load conditions, without the need for additional dehumidification
enhancements in the local HVAC equipment.
Many dedicated OA systems are designed to dehumidify the outdoor air and
then reheat it to approximately zone temperature (neutral). Delivering the
dehumidified outdoor air at a neutral dry-bulb temperature can simplify
control because it has no impact on the zone sensible cooling or heating
loads.
However, when a chilled-water or DX cooling coil is used for
dehumidification, a by-product of that process is that the dry-bulb
temperature of the air leaving the coil is colder than the zone (Figure 6). If the
dehumidified outdoor air (DH) is reheated to neutral (CA), most of the
sensible cooling performed by the dedicated OA unit is wasted.
If the dedicated OA system delivers air directly to each zone (see Figure 2,
p. 3) or to the supply-side of each local HVAC unit (see Figure 4, p. 4), the
dehumidified outdoor air (DH) can be delivered “cold,” rather than reheated
to neutral. The low dry-bulb temperature of the conditioned OA offsets part of
the sensible cooling load in the zone, reducing the energy used by the local
unit. At design conditions, this means that the local unit can be sized for less
airflow and less cooling capacity than in a neutral-air system.
Figure 6. Sensible cooling is a by-product of 'cold-coil' dehumidification
CA
DH
space
sensible cooling
dehumidification
30 35 40 45
50
55 65 70 80 90 100 11075 85 95 105
30 35 40
45
50
55
65
70
80
75
85
60
60
wet-bulb temperature, °F
dry-bulb temperature, °F
coil curves
relative humidity, %
humidity ratio, grains/lb of dry air
0
20
40
60
80
100
120
140
160
180
20
40
60
80

6 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN
Dedicated OA System Configurations
Compared to a neutral-air system, a dedicated OA system that delivers cold
air directly to each zone or to the supply-side of each local HVAC unit:
•Requires less overall cooling capacity
The required capacity of the dedicated OA unit is the same for both
configurations, but the required cooling capacity of each local unit is less
in a cold-air system than in a neutral-air system.
•Requires less overall cooling energy for much ofthe year
By taking advantage of the sensible cooling already done by the
dedicated OA unit, the cold-air system requires less cooling energy at
each local unit. The neutral-air system throws away this sensible cooling
benefit by reheating the air to approximately zone temperature.
•Requires less overall fan airflow and, therefore, less fan energy
The airflow delivered by the dedicated OA unit is the same for both
configurations, but for those zones that require seasonal cooling and
heating, the supply airflow delivered by the local HVAC unit is less in a
cold-air system than in a neutral-air system. (For zones that require year-
round cooling, the local HVAC equipment may not be able to be
downsized as much, since it may need to be sized based on the warmest
temperature expected to be delivered by the dedicated OA unit.)
While the conditioned outdoor air should be delivered cold whenever
possible, there are situations when the dedicated OA unit should reheat the
dehumidified outdoor air:
•To avoid overcooling at part-load conditions
As explained earlier, delivering the conditioned OA at a dry-bulb
temperature colder than the zone temperature offsets part of the sensible
cooling load in the zone. As the zone sensible cooling load decreases—
due to changes in outdoor conditions, solar heat gain, and/or internal
loads—it is possible that the cold, conditioned OA may provide more
sensible cooling than the zone requires. As a result, the temperature in
the zone begins to drop. At these conditions, depending on the type of
local HVAC equipment being used, it may be desirable to heat (or reheat)
the outdoor air before delivering it directly to the zones.
For many applications, a better approach to avoid overcooling is to
implement demand-controlled ventilation. This control strategy reduces
the quantity of outdoor air delivered to a zone when there are fewer
people in that zone. This often avoids overcooling altogether, and reduces
the energy used to condition and deliver that air.
•In applications where zone sensible cooling loads differ greatly
at any given time
In hotel guest rooms or dormitories, the sensible cooling loads can be
drastically different from zone to zone. The result is that, if the
conditioned OA is delivered cold, it may be more likely that some zones
will experience overcooling. For these applications, it may be simpler to
deliver the conditioned OA at a neutral dry-bulb temperature because the
benefit of delivering the air cold occurs less frequently.
In classrooms or offices, however, sensible cooling loads in the zones are
relatively high during daytime hours. In fact, for some climates,

SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 7
Dedicated OA System Configurations
classrooms may never reach the point when overcooling occurs during
occupied hours, especially if demand-controlled ventilation is used to
reduce outdoor airflow when zone population decreases. These
applications are typically well-suited for delivering the conditioned OA at
a cold temperature.
•In applications that require low dew points
If an application has very high indoor latent loads the outdoor air may
need to be dehumidified to a very low dew point. In this case, the
corresponding dry-bulb temperature of the air leaving the cooling coil
may be colder than the HVAC design engineer is willing to discharge
directly into an occupied zone—below 45°F (7°C), for example. In this
case, the dehumidified OA could be reheated to a more traditional supply-
air temperature—55°F (13°C), for example—but not reheated all the way
to neutral.
•To avoid condensation when conditioned OA is delivered to the
ceiling plenum
In some applications, the dedicated OA system delivers the conditioned
outdoor air (CA) to the ceiling plenum, near the intake of each local HVAC
terminal (see Figure 5, p. 4). The outdoor air mixes with recirculated air
(RA) in the plenum before being drawn in through the intake of the local
unit. In this configuration, the dedicated OA unit should reheat the
dehumidified OA to a dry-bulb temperature that is above the expected
dew-point temperature of the air within the ceiling plenum. If cold air is
dumped into the ceiling plenum, it could cool surfaces (structural beams,
electrical conduit, ceiling framework). At night, when the dedicated OA
unit is off, wind or operating exhaust fans may cause humid outdoor air
to leak into the plenum, which may lead to condensation on these cold
surfaces.