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4–45CDS-PRM001-EN • TRACE 700 User’s Manual System Design Options

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.

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

conditioned air to

local terminal units

Figure 4–15 Dedicated outdoor-air systems

4–46 System Design Options TRACE 700 User’s Manual • CDS-PRM001-EN


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

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.

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))



� 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.



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


heating coil heats the

outdoor air to the heating

supply-air dry bulb, Fixed

Setpoints will be used.

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.

5 Select the schedule for

the coils and fan in the

dedicated outdoor-air unit.

Note: For additional

information related to the other

Control Methods, press F1 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


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.



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.

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.

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).

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.

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.

The illustration labeled

After shows the

completed assignments.

For cooling, use water-cooled chiller

and air-cooled unitary.

For heating, use boiler and gas-fired

heat exchanger.

Before…

After…



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.


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.




12)

.

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 RAUC-20+ ton scroll

rooftop gas heat



1 Start by selecting Water

source heat pump as the

system type.







low dew-point air

reheated to a room-

neutral dry bulb in cooling

mode, and air at a specific

dry bulb in heating mode.

Therefore, Dehumidify

and Reheat / Heat will

be selected.None – no dedicated ventilation unit


user-input cooling dry bulb during the cooling season, or heat the outdoor air to




during the cooling season, or heat the outdoor air to the user-input heating dry



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.






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.


Heating Design

Setpoints. In this



supply-air dew point, 70°F

for the cooling supply-air

dry bulb, and 68°F for the

heating supply-air dry

bulb.




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.



Control Methods, press the F1

key and click Control Method.


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.





optional ventilation heating coil is available for

any hour that this schedule reads 1 percent or

greater.


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.




ventilation deck location.


where to supply the



outdoor air will be

delivered to the heat

pumps. As a result,

Return/Outdoor Deck

will be selected.




large dedicated




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.

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
















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








the WSHP units) and the



ventilation unit).

Next, define the


represented by the WSHP


conditioner.


plants and two new


Configuration pane.


icons from the

Equipment Category


plant.



information from


on the appropriate

equipment tab.


loads of the air distribution


cooling and heating plants.


Unassigned Systems/

Coils pane to the


Systems and Plants pane.


“After” shows the


For cooling, use water-source heat

pump and air-cooled unitary.

For heating, use boiler (for the WSHP

system) and gas-fired heat

exchanger.

Before…

After…



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


point setpoint.

� When the outdoor dew point is less than the current supply-air


greater than the cooling supply-air dry-bulb setpoint, 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.



Water-source heat pump

water source HP–high efficiency

gas-fired hot water boiler


rooftop gas heat






12).

1 Start by selecting

Variable Volume Reheat

as the system type.







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


user-input cooling dry bulb during the cooling season, or heat the outdoor air to




during the cooling season, or heat the outdoor air to the user-input heating dry



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.





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.


Heating Design

Setpoints. In this



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.










avoid confusion.






Dedicated Ventilation Cooling Coil


operation of the ventilation cooling coil. The

optional ventilation cooling coil is available for


greater.





optional ventilation heating coil is available for


greater.


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.






where to supply the



outdoor air will be

delivered to the mixing

box of the VAV air handler.

As a result, Return /

Outdoor Deck will be

selected.




large dedicated




assigned to the VAV

airside system, so the

coils should be set at the

System level.



System Type:


Help.



main unit(s).







boxes where it mixes with conditioned





















8 On the Temp/Humidity

tab, specify a Maximum

room relative humidity

of 60%.





the VAV air handler) and

the optional ventilation

fan (the fan in the


unit).

Next, define the


represented by the VAV


conditioner.

10Add two new cooling

plants and two new


Configuration pane.


icons from the

Equipment Category


plant.



information from


on the appropriate

equipment tab.

For cooling, use air-cooled chiller

and air-cooled unitary.

For heating, use boiler (for the WSHP


exchanger.



Before…

After…






Unassigned Systems/

Coils pane to the








Air-cooled chiller air-cooled helical-rotary chiller



rooftop gas heat



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


point setpoint, then reheat it to the current supply-air dry-bulb

setpoint.



greater than the current supply-air dry-bulb setpoint, the


current supply-air dry-bulb setpoint.


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.




12).



1 Start by selecting Fan

Coil as the system type.







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.




cooling coil dehumidifies

the outdoor air to cooling

supply-air dew point and

reheats it based on the

worst-case room, or the


heating coil heats the

outdoor air to heating

supply-air dry bulb,

Optimize SADB will be

used.





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.




Heating Design

Setpoints. In this



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.










avoid confusion.




where to supply the



outdoor air will be

delivered directly to the

spaces. As a result,

Room Direct will be

selected.



main unit(s).







boxes where it mixes with conditioned



Dedicated Ventilation Cooling Coil schedule


ventilation cooling coil. The optional ventilation

cooling coil is available for any hour that this

schedule reads 1 percent or greater.


Dedicated Ventilation Heating Coil schedule


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.






large dedicated




assigned to the Fan Coil

system, so the coils

should be set at the

System level.



System Type:


Help.





the Fan Coil units) and the



ventilation unit).




















Next, define the


represented by the fan coil


conditioner.


plants and two new


Configuration pane.


icons from the

Equipment Category


plant.



information from


on the appropriate

equipment tab.






Unassigned Systems/

Coils pane to the






For cooling, use water-source hea

pump and air-cooled unitary.

For heating, use boiler (for the WS


exchanger.

Before…

After…



Water-source heat pump

3-stage centrifugal chiller



rooftop gas heat

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