Dedicated outdoor-air systems
A dedicated outdoor-air system (DOAS) (dedicated ventilation
unit) uses a separate air handler to dehumidify, cool, heat, and
sometimes humidify the incoming outdoor air before delivering it
to other HVAC units or directly to the occupied zones. In either
case, the local HVAC units handle the zone cooling and heating
loads. 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.
Figure 4–15 Dedicated outdoor-air systems
conditioned air to
local terminal units
Application considerations
■ 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
relative 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
CDS-PRM001-EN • TRACE 700 User’s Manual
System Design Options
Dedicated outdoor-air systems
4–45
capacity required from the local HVAC terminals and requires
more reheat at the dedicated outdoor-air unit.
■ Select equipment to limit indoor relative humidity to
60–65 percent. Targeting a drier condition in the space will
require larger equipment and consume more energy.
■ Employ communicating controls to optimize energy use.
Dynamically resetting the dew point or dry-bulb temperature
delivered by the DOAS can minimize the energy use by the
overall system.
Related reading
■ “Dedicated Ventilation Systems,” Engineers Newsletter (volume
30, number 3)
■ Dehumidification in HVAC Systems Applications Engineering
Manual (Trane literature number SYS-APM004-EN)
■ Designing Dedicated Outdoor-Air Systems Application Guide
(Trane literature number SYS-APG001-EN.
Sample scenarios
Table 4–5 Sample Scenarios
Topic
Scenario 1 (page 46) Cool/Heat (Fixed Setpoints)
Scenario 2 (page 51) Dehumidify and Reheat/Heat (Fixed Setpoints)
Scenario 3 (page 56) Dehumidify (Priority) or Cool/Heat (Optimized SADP)
Scenario 4 (page 62) Dehumidify and Reheat/Heat (Optimized SADB))
Scenario 1. A building is served by a fan-coil system and a
dedicated outdoor-air unit that cools the outdoor air to 70°F dry
bulb during the cooling season, or heats the outdoor air to 68°F
dry bulb during the heating season, before delivering the
conditioned outdoor air directly to the individual spaces. (Note: In
non-arid climates, this approach may not adequately dehumidify
the outdoor air.)
■ When the outdoor dry-bulb temperature is greater than the
cooling supply-air dry-bulb setpoint, the dedicated outdoor-air unit
will cool the entering outdoor air to the cooling supply-air dry-bulb
setpoint.
4–46
System Design Options
Dedicated outdoor-air systems
TRACE 700 User’s Manual • CDS-PRM001-EN
■ When the outdoor dry-bulb temperature is less than the cooling
supply-air dry-bulb setpoint, the dedicated outdoor-air unit will
heat the entering outdoor air to the heating supply-air dry-bulb
setpoint.
This example will cover how to model the air-distribution system
(steps 1–8), model the cooling and heating equipment (steps 9–
11), and assign the loads from the coils to the equipment (step
12).
1 Start by selecting Fan
Coil as the system type.
2 On the Dedicated OA
tab, select the type of
dedicated ventilation unit
to be modeled. In this
example, the dedicated
ventilation unit will supply
air to spaces at a specific
dry bulb. Therefore,
Cool / Heat will be
selected.
None – no dedicated ventilation unit
Cool / heat – a dedicated ventilation unit is used to cool the outdoor air to the
user-input cooling dry bulb during the cooling season or heat the outdoor air to
the user-input heating dry bulb during the heating season.
Dehumidify (priority) or Cool / Heat – a dedicated ventilation unit is used to
dehumidify the outdoor air to the user-input cooling dew point (not reheated)
during the cooling season or heat the outdoor air to the user-input heating dry
bulb during the heating season.
Dehumidify and Reheat / Heat – a dedicated ventilation unit is used to
dehumidify the outdoor air to the user-input cooling dew point (then reheat it
to the user-input cooling dry bulb) during the cooling season or heat the
outdoor air to the user-input heating dry bulb during the heating season.
CDS-PRM001-EN • TRACE 700 User’s Manual
System Design Options
Dedicated outdoor-air systems
4–47
3 Select the control method
for the unit. Because the
dedicated ventilation
cooling coil cools the
outdoor air to the cooling
supply-air dry bulb, or the
dedicated ventilation
heating coil heats the
outdoor air to the heating
supply-air dry bulb, Fixed
Setpoints will be used.
Note: For additional
information related to the other
Control Methods, press F1 and
click Control Method.
4 Input the Cooling and
Heating Design
Setpoints. In this
scenario, 70°F should be
input for the cooling
supply-air dry bulb, and
68°F for the heating
supply-air dry bulb.
If the ventilation air is to be precooled, the
Dedicated Ventilation Cooling Coil
schedule defines the hours of available
operation of the ventilation cooling coil. The
optional ventilation cooling coil is available
for any hour that this schedule reads
1 percent or greater.
5 Select the schedule for
the coils and fan in the
dedicated outdoor-air unit.
If the ventilation air is to be preheated, the
Dedicated Ventilation Heating Coil
schedule defines the hours of available
operation of the ventilation heating coil. The
optional ventilation heating coil is available
for any hour that this schedule reads
1 percent or greater.
The Optional ventilation fan schedule
defines the hours that the ventilation unit
can operate. If the Optional ventilation fan
schedule reads 1 percent or greater, the fan
is available that hour.
Note: If a custom schedule is created for
the fan or coils, it is recommended to use
values of 0 percent for hours off and
100 percent for hours available to run to
avoid confusion.
4–48
System Design Options
Dedicated outdoor-air systems
TRACE 700 User’s Manual • CDS-PRM001-EN
Return/Outdoor Deck – conditioned
outdoor air is mixed with the return air in the
main unit(s).
Plenum – conditioned outdoor air is
delivered to the plenum above the spaces.
Room Direct – conditioned outdoor air is
delivered directly to the spaces. This can
also be used to model the delivery of
conditioned outdoor air to dual-duct VAV
boxes where it mixes locally with
recirculated return air.
6 Select the dedicated
ventilation-deck location.
This tells TRACE 700
where to supply the
preconditioned outdoor
air. For this scenario, the
outdoor air will be
delivered directly to the
spaces. As a result,
Room Direct will be
selected.
7 Select the level of the
ventilation-deck location.
For this scenario, one
large dedicated
ventilation unit will
condition the outdoor air
for all of the rooms
assigned to the fan-coil
system, so the coils
should be set at the
System level.
System – The dedicated ventilation unit will be placed at the system level. It
will pretreat the outdoor air for all of the rooms and zones assigned to the
airside system. This is the most common choice for dedicated ventilation
systems. The System Checksums or System Component Selection reports
should be used to size the dedicated ventilation unit.
Zone – Each zone (or room assigned directly to the system) will get a separate
dedicated ventilation system. The Zone Checksums or System Component
Selection reports should be used to size the dedicated ventilation unit.
Room – Each room will get a separate dedicated ventilation system. The Room
Checksums or System Component Selection reports should be used to size
the dedicated ventilation unit.
Same level as main cooling coil – The dedicated ventilation unit will be placed
at the same level as the main cooling coil. The System Component Selection
report should be used to size the dedicated ventilation unit.
Same level as main heating coil – The dedicated ventilation unit will be
placed at the same level as the main heating coil. The System Component
Selection report should be used to size the dedicated ventilation unit.
CDS-PRM001-EN • TRACE 700 User’s Manual
System Design Options
Dedicated outdoor-air systems
4–49
8 On the Fans tab, input
the fan type and the total
static pressure for the
primary fan (the fans in
the fan-coil units) and the
optional ventilation fan
(the fan in the dedicated
ventilation unit).
Next, define the
cooling and heating plants
represented by the fan-coil
system and the rooftop air
conditioner:
9 Add two new cooling
plants and two new
heating plants to the
Configuration pane.
10Drag the appropriate
icons from the
Equipment Category
section to define each
plant.
For cooling, use water-cooled chiller
and air-cooled unitary.
For heating, use boiler and gas-fired
heat exchanger.
11To finish defining this
equipment, enter the
information from
Table 4–6 on page 4–51
on the appropriate
equipment tab.
Finally, assign the coil
loads of the air-distribution
system to the proper
cooling and heating plants:
12Drag each coil in the
Unassigned Systems/
Coils pane to the
appropriate plant in the
Systems and Plants
pane.
Before…
After…
The illustration labeled
After shows the
completed assignments.
4–50
System Design Options
Dedicated outdoor-air systems
TRACE 700 User’s Manual • CDS-PRM001-EN
Table 4–6 Equipment types
Cooling Equipment tab
Heating Equipment tab
Water-cooled chiller
2-stage centrifugal
gas-fired hot water
boiler
Air-cooled unitary
split system RAUC20+ ton scroll
rooftop gas heat
Scenario 2. A building is served by a water-source heat pump
(WSHP) system and a dedicated outdoor-air unit that delivers
conditioned outdoor air to the individual heat pumps. During the
cooling season, the dedicated outdoor-air unit dehumidifies the
outdoor air to 55°F dew point and then reheats it to a “neutral”
70°F dry bulb. During the heating season, the dedicated unit
heats the outdoor air to 68°F dry bulb.
■ When the outdoor dew point is greater than the cooling supply-
air dew point setpoint, the dedicated outdoor-air unit will
dehumidify the entering outdoor air to the cooling supply-air dew
point setpoint, then reheat it to the cooling supply-air dry-bulb
setpoint.
■ When the outdoor dew point is less than the cooling supply-air
dew point setpoint and the outdoor dry-bulb temperature is
greater than the cooling supply-air dry-bulb setpoint, the
dedicated outdoor-air unit will cool the entering outdoor air to the
cooling supply-air dry-bulb setpoint.
■ When the outdoor dew point is less than the cooling supply-air
dew point setpoint and the outdoor dry-bulb temperature is less
than the cooling supply-air dry-bulb setpoint, the dedicated
outdoor-air unit will heat the entering outdoor air to the heating
supply-air dry-bulb setpoint.
This example will cover how to model the air-distribution system
(steps 1–8), model the cooling and heating equipment (steps 9–
11), and assign the loads from the coils to the equipment (step
12)
.
CDS-PRM001-EN • TRACE 700 User’s Manual
System Design Options
Dedicated outdoor-air systems
4–51
1 Start by selecting Water
source heat pump as the
system type.
2 On the Dedicated OA
tab, select the type of
dedicated ventilation unit
to be modeled. In this
example, the dedicated
ventilation unit will supply
low dew-point air
reheated to a roomneutral dry bulb in cooling
mode, and air at a specific
dry bulb in heating mode.
Therefore, Dehumidify
and Reheat / Heat will
be selected.
4–52
None – no dedicated ventilation unit
Cool / heat – a dedicated ventilation unit is used to cool the outdoor air to the
user-input cooling dry bulb during the cooling season, or heat the outdoor air to
the user-input heating dry bulb during the heating season.
Dehumidify (priority) or Cool / Heat – a dedicated ventilation unit is used to
dehumidify the outdoor air to the user-input cooling dew point (not reheated)
during the cooling season, or heat the outdoor air to the user-input heating dry
bulb during the heating season.
Dehumidify and Reheat / Heat – a dedicated ventilation unit is used to
dehumidify the outdoor air to the user-input cooling dew point (then reheat it to
the user-input cooling dry bulb) during the cooling season, or heat the outdoor air
to the user-input heating dry bulb during the heating season.
System Design Options
Dedicated outdoor-air systems
TRACE 700 User’s Manual • CDS-PRM001-EN
3 Select the control method
for the unit. Because the
dedicated ventilation
cooling coil dehumidifies
the outdoor air to cooling
supply-air dew point and
reheats it to room neutral,
or the dedicated
ventilation heating coil
heats the outdoor air to
heating supply-air dry
bulb, Fixed Setpoints
will be used.
4 Input the Cooling and
Heating Design
Setpoints. In this
scenario, 55°F should be
input for the cooling
supply-air dew point, 70°F
for the cooling supply-air
dry bulb, and 68°F for the
heating supply-air dry
bulb.
5 Select the schedule for
the coils and fan in the
dedicated outdoor-air unit.
Note: If a custom schedule
is created for the fan or
coils, it is recommended to
use values of 0 percent for
hours off and 100 percent
for hours available to run to
avoid confusion.
CDS-PRM001-EN • TRACE 700 User’s Manual
Note: For additional
information related to the other
Control Methods, press the F1
key and click Control Method.
If the ventilation air is to be precooled, the
Dedicated Ventilation Cooling Coil schedule
defines the hours of available operation of the
ventilation cooling coil. The optional ventilation
cooling coil is available for any hour that this
schedule reads 1 percent or greater.
If the ventilation air is to be preheated, the
Dedicated Ventilation Heating Coil
schedule defines the hours of available
operation of the ventilation heating coil. The
optional ventilation heating coil is available for
any hour that this schedule reads 1 percent or
greater.
The Optional ventilation fan schedule
defines the hours that the ventilation unit can
operate. If the Optional ventilation fan
schedule reads 1 percent or greater, the fan is
available that hour.
System Design Options
Dedicated outdoor-air systems
4–53
Return/Outdoor Deck – conditioned outdoor air is
mixed with the return air in the main unit(s).
Plenum – conditioned outdoor air is delivered to the
plenum above the spaces.
Room Direct – conditioned outdoor air is delivered
directly to the spaces. This can also be used to model
the delivery of conditioned outdoor air to dual-duct
VAV boxes where it mixes with conditioned
recirculated return air.
6 Select the dedicated
ventilation deck location.
This tells TRACE 700
where to supply the
preconditioned outdoor
air. For this scenario, the
outdoor air will be
delivered to the heat
pumps. As a result,
Return/Outdoor Deck
will be selected.
7 Select the level of the
ventilation deck location.
For this scenario, one
large dedicated
ventilation unit will
condition the outdoor air
for all of the rooms
assigned to the WSHP
airside system, so the
coils should be set at the
System level.
Note: For additional details
on Systems, refer to the
System Type:
Characteristics in online
Help.
4–54
System – The dedicated ventilation unit will be placed at the system level. It
will pretreat the outdoor air for all of the rooms and zones assigned to the
airside system. This is the most common choice for dedicated ventilation
systems. The System Checksums or System Component Selection reports
should be used to size the dedicated ventilation unit.
Zone – Each zone (or room assigned directly to the system) will get a separate
dedicated ventilation system. The Zone Checksums or System Component
Selection reports should be used to size the dedicated ventilation unit.
Room – Each room will get a separate dedicated ventilation system. The Room
Checksums or System Component Selection reports should be used to size
the dedicated ventilation unit.
Same level as main cooling coil – The dedicated ventilation unit will be placed
at the same level as the main cooling coil. The System Component Selection
report should be used to size the dedicated ventilation unit.
Same level as main heating coil – The dedicated ventilation unit will be placed
at the same level as the main heating coil. The System Component Selection
report should be used to size the dedicated ventilation unit.
System Design Options
Dedicated outdoor-air systems
TRACE 700 User’s Manual • CDS-PRM001-EN
8 On the Fans tab, input
the fan type and the total
static pressure for the
primary fan (the fans in
the WSHP units) and the
optional ventilation fan
(the fan in the dedicated
ventilation unit).
Next, define the
cooling and heating plants
represented by the WSHP
system and the rooftop air
conditioner.
9 Add two new cooling
plants and two new
heating plants to the
Configuration pane.
For cooling, use water-source heat
pump and air-cooled unitary.
For heating, use boiler (for the WSHP
system) and gas-fired heat
exchanger.
10Drag the appropriate
icons from the
Equipment Category
section to define each
plant.
11To finish defining this
equipment, enter the
information from
Table 4–7 on page 4–56
on the appropriate
equipment tab.
Before…
Finally, assign the coil
loads of the air distribution
system to the proper
cooling and heating plants.
12Drag each coil in the
Unassigned Systems/
Coils pane to the
appropriate plant in the
Systems and Plants pane.
After…
The illustration labeled
“After” shows the
completed assignments.
CDS-PRM001-EN • TRACE 700 User’s Manual
System Design Options
Dedicated outdoor-air systems
4–55
Table 4–7 Equipment types
Cooling Equipment tab
Heating Equipment tab
Water-source heat
pump
water source HP–
high efficiency
gas-fired hot water
boiler
Air-cooled unitary
split system RAUC20+ ton scroll
rooftop gas heat
Scenario 3. A building is served by a VAV air-handling system
on each floor. A dedicated outdoor-air unit delivers conditioned
outdoor air to the intake of each air handler. During the cooling
season, the dedicated outdoor-air unit dehumidifies the outdoor
air to 55°F dew point, but does not reheat it. During the heating
season, the dedicated unit heats the outdoor air to 68°F dry bulb.
In addition, the system-level controller polls the humidity sensors
located in each room and resets the dew point leaving the
dedicated outdoor-air unit, to prevent the relative humidity in the
worst-case room from rising above 60% while minimizing energy
use.
■ The system controller determines the highest possible cooling
supply-air dew point setpoint that still avoids the relative humidity
from rising above the maximum upper limit (60%, in our
example) in any room.
■ When the outdoor dew point is greater than the current cooling
supply-air dew point setpoint, the dedicated outdoor-air unit will
dehumidify the entering outdoor air to the cooling supply-air dew
point setpoint.
■ When the outdoor dew point is less than the current supply-air
dew point setpoint and the outdoor dry-bulb temperature is
greater than the cooling supply-air dry-bulb setpoint, the
dedicated outdoor-air unit will cool the entering outdoor air to the
cooling supply-air dry-bulb setpoint.
■ When the outdoor dew point is less than the current cooling
supply-air dew point setpoint and the outdoor dry-bulb
temperature is less than the cooling supply-air dry-bulb setpoint,
the dedicated outdoor-air unit will heat the entering outdoor air to
the heating supply-air dry-bulb setpoint.
4–56
System Design Options
Dedicated outdoor-air systems
TRACE 700 User’s Manual • CDS-PRM001-EN
This example will cover how to model the air-distribution system
(steps 1–8), model the cooling and heating equipment (steps 9–
11), and assign the loads from the coils to the equipment (step
12).
1 Start by selecting
Variable Volume Reheat
as the system type.
2 On the Dedicated OA
tab, select the type of
dedicated ventilation unit
to be modeled. In this
example, the dedicated
ventilation unit will supply
low dew-point air (not
reheated) in cooling
mode, and air at a specific
dry bulb in heating mode.
Therefore, Dehumidify
(priority) or Cool / Heat
will be selected.
None – no dedicated ventilation unit
Cool / heat – a dedicated ventilation unit is used to cool the outdoor air to the
user-input cooling dry bulb during the cooling season, or heat the outdoor air to
the user-input heating dry bulb during the heating season.
Dehumidify (priority) or Cool / Heat – a dedicated ventilation unit is used to
dehumidify the outdoor air to the user-input cooling dew point (not reheated)
during the cooling season, or heat the outdoor air to the user-input heating dry
bulb during the heating season.
Dehumidify and Reheat / Heat – a dedicated ventilation unit is used to
dehumidify the outdoor air to the user-input cooling dew point (then reheat it to
the user-input cooling dry bulb) during the cooling season, or heat the outdoor air
to the user-input heating dry bulb during the heating season.
CDS-PRM001-EN • TRACE 700 User’s Manual
System Design Options
Dedicated outdoor-air systems
4–57
3 Select the control method
for the unit. Because the
dew point delivered by
the dedicated ventilation
cooling coil is reset based
on the worst-case room,
Optimize Supply Air
Dew Point will be used.
4 Input the Cooling and
Heating Design
Setpoints. In this
scenario, 65°F should be
input for the cooling
supply-air dry bulb, 68°F
for the heating supply-air
dry bulb, 65°F for the
cooling supply-air dew
point high limit, and 55°F
for the cooling supply-air
dew point low limit.
Note: For additional
information related to the other
Control Methods, press the F1
key and click Control Method.
5 Select the schedule for
the coils and fan in the
dedicated outdoor-air unit.
If the ventilation air is to be precooled, the
Dedicated Ventilation Cooling Coil
schedule defines the hours of available
operation of the ventilation cooling coil. The
optional ventilation cooling coil is available for
any hour that this schedule reads 1 percent or
greater.
Note: If a custom schedule
is created for the fan or
coils, it is recommended to
use values of 0 percent for
hours off and 100 percent
for hours available to run to
avoid confusion.
If the ventilation air is to be preheated, the
Dedicated Ventilation Heating Coil
schedule defines the hours of available
operation of the ventilation heating coil. The
optional ventilation heating coil is available for
any hour that this schedule reads 1 percent or
greater.
The Optional ventilation fan schedule
defines the hours that the ventilation unit can
operate. If the Optional ventilation fan
schedule reads 1 percent or greater, the fan is
available that hour.
4–58
System Design Options
Dedicated outdoor-air systems
TRACE 700 User’s Manual • CDS-PRM001-EN
6 Select the dedicated
ventilation deck location.
This tells TRACE 700
where to supply the
preconditioned outdoor
air. For this scenario, the
outdoor air will be
delivered to the mixing
box of the VAV air handler.
As a result, Return /
Outdoor Deck will be
selected.
7 Select the level of the
ventilation deck location.
For this scenario, one
large dedicated
ventilation unit will
condition the outdoor air
for all of the rooms
assigned to the VAV
airside system, so the
coils should be set at the
System level.
Note: For additional details
on Systems, refer to the
System Type:
Characteristics in online
Help.
Return/Outdoor Deck – conditioned
outdoor air is mixed with the return air in the
main unit(s).
Plenum – conditioned outdoor air is
delivered to the plenum above the spaces.
Room Direct – conditioned outdoor air is
delivered directly to the spaces. This can
also be used to model the delivery of
conditioned outdoor air to dual-duct VAV
boxes where it mixes with conditioned
recirculated return air.
System – The dedicated ventilation unit will be placed at the system level. It
will pretreat the outdoor air for all of the rooms and zones assigned to the
airside system. This is the most common choice for dedicated ventilation
systems. The System Checksums or System Component Selection reports
should be used to size the dedicated ventilation unit.
Zone – Each zone (or room assigned directly to the system) will get a separate
dedicated ventilation system. The Zone Checksums or System Component
Selection reports should be used to size the dedicated ventilation unit.
Room – Each room will get a separate dedicated ventilation system. The Room
Checksums or System Component Selection reports should be used to size
the dedicated ventilation unit.
Same level as main cooling coil – The dedicated ventilation unit will be placed
at the same level as the main cooling coil. The System Component Selection
report should be used to size the dedicated ventilation unit.
Same level as main heating coil – The dedicated ventilation unit will be placed
at the same level as the main heating coil. The System Component Selection
report should be used to size the dedicated ventilation unit.
CDS-PRM001-EN • TRACE 700 User’s Manual
System Design Options
Dedicated outdoor-air systems
4–59
8 On the Temp/Humidity
tab, specify a Maximum
room relative humidity
of 60%.
9 On the Fans tab, input
the fan type and the total
static pressure for the
primary fan (the fans in
the VAV air handler) and
the optional ventilation
fan (the fan in the
dedicated ventilation
unit).
Next, define the
cooling and heating plants
represented by the VAV
system and the rooftop air
conditioner.
10Add two new cooling
plants and two new
heating plants to the
Configuration pane.
11Drag the appropriate
icons from the
Equipment Category
section to define each
plant.
12To finish defining this
equipment, enter the
information from
Table 4–8 on page 4–61
on the appropriate
equipment tab.
4–60
System Design Options
Dedicated outdoor-air systems
For cooling, use air-cooled chiller
and air-cooled unitary.
For heating, use boiler (for the WSHP
system) and gas-fired heat
exchanger.
TRACE 700 User’s Manual • CDS-PRM001-EN
Finally, assign the coil
loads of the air distribution
system to the proper
cooling and heating plants.
13Drag each coil in the
Unassigned Systems/
Coils pane to the
appropriate plant in the
Systems and Plants pane.
Before…
The illustration labeled
“After” shows the
completed assignments.
After…
Table 4–8 Equipment types
Cooling Equipment tab
Heating Equipment tab
Air-cooled chiller
air-cooled helicalrotary chiller
gas-fired hot water
boiler
Air-cooled unitary
split system RAUC20+ ton scroll
rooftop gas heat
CDS-PRM001-EN • TRACE 700 User’s Manual
System Design Options
Dedicated outdoor-air systems
4–61
Scenario 4. A building is served by a fan-coil system and a
dedicated outdoor-air unit that delivers conditioned outdoor air
directly to the individual spaces. The system-level controller polls
the fan-coils and resets the dry-bulb temperature leaving the
dedicated outdoor-air unit to avoid overcooling or overheating any
room.
During the cooling season, the dedicated outdoor-air unit
dehumidifies the outdoor air to 55°F dew point and then, if
necessary, reheats it just enough to avoid overcooling the room
with the lowest sensible cooling load. During the heating season,
the dedicated unit heats the outdoor air as much as possible,
without overheating the room with the lowest sensible heating
load.
■ The system controller determines the current supply-air dry-bulb
setpoint needed to avoid overcooling or overheating any room.
■ When the outdoor dew point is greater than the cooling supply-
air dew point setpoint, the dedicated outdoor-air unit will
dehumidify the entering outdoor air to the cooling supply-air dew
point setpoint, then reheat it to the current supply-air dry-bulb
setpoint.
■ When the outdoor dew point is less than the cooling supply-air
dew point setpoint and the outdoor dry-bulb temperature is
greater than the current supply-air dry-bulb setpoint, the
dedicated outdoor-air unit will cool the entering outdoor air to the
current supply-air dry-bulb setpoint.
■ When the outdoor dew point is less than the cooling supply-air
dew point setpoint and the outdoor dry-bulb temperature is less
than the current supply-air dry-bulb setpoint, the dedicated
outdoor-air unit will heat the entering outdoor air to the current
supply-air dry-bulb setpoint.
This example will cover how to model the air-distribution system
(steps 1–8), model the cooling and heating equipment (steps 9–
11), and assign the loads from the coils to the equipment (step
12).
4–62
System Design Options
Dedicated outdoor-air systems
TRACE 700 User’s Manual • CDS-PRM001-EN
1 Start by selecting Fan
Coil as the system type.
2 On the Dedicated OA
tab, select the type of
dedicated ventilation unit
to be modeled. In this
example, the dedicated
ventilation unit will supply
low dew-point air (and
possibly reheated) in
cooling mode, and air at a
specific dry bulb in
heating mode. Therefore,
Dehumidify and Reheat
/ Heat will be selected.
3 Select the control method
for the unit. Because the
dedicated ventilation
cooling coil dehumidifies
the outdoor air to cooling
supply-air dew point and
reheats it based on the
worst-case room, or the
dedicated ventilation
heating coil heats the
outdoor air to heating
supply-air dry bulb,
Optimize SADB will be
used.
CDS-PRM001-EN • TRACE 700 User’s Manual
None – no dedicated ventilation unit
Cool / heat – a dedicated ventilation unit is
used to cool the outdoor air to the user-input
cooling dry bulb during the cooling season, or
heat the outdoor air to the user-input heating
dry bulb during the heating season.
Dehumidify (priority) or Cool / Heat – a
dedicated ventilation unit is used to dehumidify
the outdoor air to the user-input cooling dew
point (not reheated) during the cooling season,
or heat the outdoor air to the user-input heating
dry bulb during the heating season.
Dehumidify and Reheat / Heat – a dedicated
ventilation unit is used to dehumidify the
outdoor air to the user-input cooling dew point
(then reheat it to the user-input cooling dry
bulb) during the cooling season, or heat the
outdoor air to the user-input heating dry bulb
during the heating season.
Note: For additional
information related to the other
Control Methods, press the F1
key and click Control Method.
System Design Options
Dedicated outdoor-air systems
4–63
4 Input the Cooling and
Heating Design
Setpoints. In this
scenario, 55°F should be
input for the cooling
supply-air dew point, 55°F
for the supply-air dry bulb
low limit, and 70°F for the
supply-air dry bulb high
limit.
5 Select the schedule for
the coils and fan in the
dedicated outdoor-air unit.
Note: If a custom schedule
is created for the fan or
coils, it is recommended to
use values of 0 percent for
hours off and 100 percent
for hours available to run to
avoid confusion.
6 Select the dedicated
ventilation deck location.
This tells TRACE 700
where to supply the
preconditioned outdoor
air. For this scenario, the
outdoor air will be
delivered directly to the
spaces. As a result,
Room Direct will be
selected.
4–64
System Design Options
Dedicated outdoor-air systems
If the ventilation air is to be precooled, the
Dedicated Ventilation Cooling Coil schedule
defines the hours of available operation of the
ventilation cooling coil. The optional ventilation
cooling coil is available for any hour that this
schedule reads 1 percent or greater.
If the ventilation air is to be preheated, the
Dedicated Ventilation Heating Coil schedule
defines the hours of available operation of the
ventilation heating coil. The optional ventilation
heating coil is available for any hour that this
schedule reads 1 percent or greater. The
Optional ventilation fan schedule defines the
hours that the ventilation unit can operate.
If the Optional ventilation fan schedule reads
1 percent or greater, the fan is available that
hour.
Return/Outdoor Deck – conditioned
outdoor air is mixed with the return air in the
main unit(s).
Plenum – conditioned outdoor air is
delivered to the plenum above the spaces.
Room Direct – conditioned outdoor air is
delivered directly to the spaces. This can
also be used to model the delivery of
conditioned outdoor air to dual-duct VAV
boxes where it mixes with conditioned
recirculated return air.
TRACE 700 User’s Manual • CDS-PRM001-EN
7 Select the level of the
ventilation deck location.
For this scenario, one
large dedicated
ventilation unit will
condition the outdoor air
for all of the rooms
assigned to the Fan Coil
system, so the coils
should be set at the
System level.
Note: For additional details
on Systems, refer to the
System Type:
Characteristics in online
Help.
System – The dedicated ventilation unit will be placed at the system level. It
will pretreat the outdoor air for all of the rooms and zones assigned to the
airside system. This is the most common choice for dedicated ventilation
systems. The System Checksums or System Component Selection reports
should be used to size the dedicated ventilation unit.
Zone – Each zone (or room assigned directly to the system) will get a separate
dedicated ventilation system. The Zone Checksums or System Component
Selection reports should be used to size the dedicated ventilation unit.
Room – Each room will get a separate dedicated ventilation system. The Room
Checksums or System Component Selection reports should be used to size
the dedicated ventilation unit.
Same level as main cooling coil – The dedicated ventilation unit will be placed
at the same level as the main cooling coil. The System Component Selection
report should be used to size the dedicated ventilation unit.
Same level as main heating coil – The dedicated ventilation unit will be placed
at the same level as the main heating coil. The System Component Selection
report should be used to size the dedicated ventilation unit.
8 On the Fans tab, input
the fan type and the total
static pressure for the
primary fan (the fans in
the Fan Coil units) and the
optional ventilation fan
(the fan in the dedicated
ventilation unit).
CDS-PRM001-EN • TRACE 700 User’s Manual
System Design Options
Dedicated outdoor-air systems
4–65
Next, define the
cooling and heating plants
represented by the fan coil
system and the rooftop air
conditioner.
9 Add two new cooling
plants and two new
heating plants to the
Configuration pane.
For cooling, use water-source hea
pump and air-cooled unitary.
For heating, use boiler (for the WS
system) and gas-fired heat
exchanger.
10Drag the appropriate
icons from the
Equipment Category
section to define each
plant.
11To finish defining this
equipment, enter the
information from
Table 4–9 on page 4–66
on the appropriate
equipment tab.
Before…
Finally, assign the coil
loads of the air distribution
system to the proper
cooling and heating plants.
After…
12Drag each coil in the
Unassigned Systems/
Coils pane to the
appropriate plant in the
Systems and Plants pane.
The illustration labeled
“After” shows the
completed assignments.
Table 4–9 Equipment types
4–66
Cooling Equipment tab
Heating Equipment tab
Water-source heat
pump
3-stage centrifugal
chiller
gas-fired hot water
boiler
Air-cooled unitary
split system RAUC20+ ton scroll
rooftop gas heat
System Design Options
Dedicated outdoor-air systems
TRACE 700 User’s Manual • CDS-PRM001-EN