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Dedicated outdoor-air systemsA dedicated outdoor-air system (DOAS) (dedicated ventilationunit) uses a separate air handler to dehumidify, cool, heat, andsometimes humidify the incoming outdoor air before delivering itto other HVAC units or directly to the occupied zones. In eithercase, the local HVAC units handle the zone cooling and heatingloads. Treating the outdoor air separately from recirculated returnair makes it easy to verify sufficient ventilation airflow andenables enforcement of a maximum humidity limit in occupiedzones.Figure 4–15 Dedicated outdoor-air systemsconditioned air tolocal terminal unitsApplication considerations Always provide conditioned air that is drier than the air in thespace. This practice minimizes the cooling capacity required fromthe local HVAC terminals and adequately controls the indoorrelative humidity without additional, zone-level dehumidificationenhancements. Deliver “cold” conditioned air whenever possible, and userecovered energy to reheat during mild weather. Providing “cold”conditioned air from the DOAS minimizes the cooling loads at thelocal HVAC terminals. During mild weather (spring and fall),modulate the amount of recovered energy used by the DOAS forreheat; only warm the conditioned air enough to minimizeinefficient reheat at the local HVAC terminals. “Neutral”temperature conditioned air (which has a dry-bulb temperatureapproximating that of the air in the space) increases the coolingCDS-PRM001-EN TRACE 700 User’s ManualSystem Design OptionsDedicated outdoor-air systems4–45

capacity required from the local HVAC terminals and requiresmore reheat at the dedicated outdoor-air unit. Select equipment to limit indoor relative humidity to60–65 percent. Targeting a drier condition in the space willrequire larger equipment and consume more energy. Employ communicating controls to optimize energy use.Dynamically resetting the dew point or dry-bulb temperaturedelivered by the DOAS can minimize the energy use by theoverall system.Related reading “Dedicated Ventilation Systems,” Engineers Newsletter (volume30, number 3) Dehumidification in HVAC Systems Applications EngineeringManual (Trane literature number SYS-APM004-EN) Designing Dedicated Outdoor-Air Systems Application Guide(Trane literature number SYS-APG001-EN.Sample scenariosTable 4–5 Sample ScenariosTopicScenario 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 adedicated outdoor-air unit that cools the outdoor air to 70 F drybulb during the cooling season, or heats the outdoor air to 68 Fdry bulb during the heating season, before delivering theconditioned outdoor air directly to the individual spaces. (Note: Innon-arid climates, this approach may not adequately dehumidifythe outdoor air.) When the outdoor dry-bulb temperature is greater than thecooling supply-air dry-bulb setpoint, the dedicated outdoor-air unitwill cool the entering outdoor air to the cooling supply-air dry-bulbsetpoint.4–46System Design OptionsDedicated outdoor-air systemsTRACE 700 User’s Manual CDS-PRM001-EN

When the outdoor dry-bulb temperature is less than the coolingsupply-air dry-bulb setpoint, the dedicated outdoor-air unit willheat the entering outdoor air to the heating supply-air dry-bulbsetpoint.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 (step12).1 Start by selecting FanCoil as the system type.2 On the Dedicated OAtab, select the type ofdedicated ventilation unitto be modeled. In thisexample, the dedicatedventilation unit will supplyair to spaces at a specificdry bulb. Therefore,Cool / Heat will beselected.None – no dedicated ventilation unitCool / heat – a dedicated ventilation unit is used to cool the outdoor air to theuser-input cooling dry bulb during the cooling season or heat the outdoor air tothe user-input heating dry bulb during the heating season.Dehumidify (priority) or Cool / Heat – a dedicated ventilation unit is used todehumidify 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 drybulb during the heating season.Dehumidify and Reheat / Heat – a dedicated ventilation unit is used todehumidify the outdoor air to the user-input cooling dew point (then reheat itto the user-input cooling dry bulb) during the cooling season or heat theoutdoor air to the user-input heating dry bulb during the heating season.CDS-PRM001-EN TRACE 700 User’s ManualSystem Design OptionsDedicated outdoor-air systems4–47

3 Select the control methodfor the unit. Because thededicated ventilationcooling coil cools theoutdoor air to the coolingsupply-air dry bulb, or thededicated ventilationheating coil heats theoutdoor air to the heatingsupply-air dry bulb, FixedSetpoints will be used.Note: For additionalinformation related to the otherControl Methods, press F1 andclick Control Method.4 Input the Cooling andHeating DesignSetpoints. In thisscenario, 70 F should beinput for the coolingsupply-air dry bulb, and68 F for the heatingsupply-air dry bulb.If the ventilation air is to be precooled, theDedicated Ventilation Cooling Coilschedule defines the hours of availableoperation of the ventilation cooling coil. Theoptional ventilation cooling coil is availablefor any hour that this schedule reads1 percent or greater.5 Select the schedule forthe coils and fan in thededicated outdoor-air unit.If the ventilation air is to be preheated, theDedicated Ventilation Heating Coilschedule defines the hours of availableoperation of the ventilation heating coil. Theoptional ventilation heating coil is availablefor any hour that this schedule reads1 percent or greater.The Optional ventilation fan scheduledefines the hours that the ventilation unitcan operate. If the Optional ventilation fanschedule reads 1 percent or greater, the fanis available that hour.Note: If a custom schedule is created forthe fan or coils, it is recommended to usevalues of 0 percent for hours off and100 percent for hours available to run toavoid confusion.4–48System Design OptionsDedicated outdoor-air systemsTRACE 700 User’s Manual CDS-PRM001-EN

Return/Outdoor Deck – conditionedoutdoor air is mixed with the return air in themain unit(s).Plenum – conditioned outdoor air isdelivered to the plenum above the spaces.Room Direct – conditioned outdoor air isdelivered directly to the spaces. This canalso be used to model the delivery ofconditioned outdoor air to dual-duct VAVboxes where it mixes locally withrecirculated return air.6 Select the dedicatedventilation-deck location.This tells TRACE 700where to supply thepreconditioned outdoorair. For this scenario, theoutdoor air will bedelivered directly to thespaces. As a result,Room Direct will beselected.7 Select the level of theventilation-deck location.For this scenario, onelarge dedicatedventilation unit willcondition the outdoor airfor all of the roomsassigned to the fan-coilsystem, so the coilsshould be set at theSystem level.System – The dedicated ventilation unit will be placed at the system level. Itwill pretreat the outdoor air for all of the rooms and zones assigned to theairside system. This is the most common choice for dedicated ventilationsystems. The System Checksums or System Component Selection reportsshould be used to size the dedicated ventilation unit.Zone – Each zone (or room assigned directly to the system) will get a separatededicated ventilation system. The Zone Checksums or System ComponentSelection reports should be used to size the dedicated ventilation unit.Room – Each room will get a separate dedicated ventilation system. The RoomChecksums or System Component Selection reports should be used to sizethe dedicated ventilation unit.Same level as main cooling coil – The dedicated ventilation unit will be placedat the same level as the main cooling coil. The System Component Selectionreport should be used to size the dedicated ventilation unit.Same level as main heating coil – The dedicated ventilation unit will beplaced at the same level as the main heating coil. The System ComponentSelection report should be used to size the dedicated ventilation unit.CDS-PRM001-EN TRACE 700 User’s ManualSystem Design OptionsDedicated outdoor-air systems4–49

8 On the Fans tab, inputthe fan type and the totalstatic pressure for theprimary fan (the fans inthe fan-coil units) and theoptional ventilation fan(the fan in the dedicatedventilation unit).Next, define thecooling and heating plantsrepresented by the fan-coilsystem and the rooftop airconditioner:9 Add two new coolingplants and two newheating plants to theConfiguration pane.10Drag the appropriateicons from theEquipment Categorysection to define eachplant.For cooling, use water-cooled chillerand air-cooled unitary.For heating, use boiler and gas-firedheat exchanger.11To finish defining thisequipment, enter theinformation fromTable 4–6 on page 4–51on the appropriateequipment tab.Finally, assign the coilloads of the air-distributionsystem to the propercooling and heating plants:12Drag each coil in theUnassigned Systems/Coils pane to theappropriate plant in theSystems and Plantspane.Before After The illustration labeledAfter shows thecompleted assignments.4–50System Design OptionsDedicated outdoor-air systemsTRACE 700 User’s Manual CDS-PRM001-EN

Table 4–6 Equipment typesCooling Equipment tabHeating Equipment tabWater-cooled chiller2-stage centrifugalgas-fired hot waterboilerAir-cooled unitarysplit system RAUC20 ton scrollrooftop gas heatScenario 2. A building is served by a water-source heat pump(WSHP) system and a dedicated outdoor-air unit that deliversconditioned outdoor air to the individual heat pumps. During thecooling season, the dedicated outdoor-air unit dehumidifies theoutdoor air to 55 F dew point and then reheats it to a “neutral”70 F dry bulb. During the heating season, the dedicated unitheats 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 willdehumidify the entering outdoor air to the cooling supply-air dewpoint setpoint, then reheat it to the cooling supply-air dry-bulbsetpoint. When the outdoor dew point is less than the cooling supply-airdew point setpoint and the outdoor dry-bulb temperature isgreater than the cooling supply-air dry-bulb setpoint, thededicated outdoor-air unit will cool the entering outdoor air to thecooling supply-air dry-bulb setpoint. When the outdoor dew point is less than the cooling supply-airdew point setpoint and the outdoor dry-bulb temperature is lessthan the cooling supply-air dry-bulb setpoint, the dedicatedoutdoor-air unit will heat the entering outdoor air to the heatingsupply-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 (step12).CDS-PRM001-EN TRACE 700 User’s ManualSystem Design OptionsDedicated outdoor-air systems4–51