Application Table of Considerations Contents - Trane

1 ApplicationConsiderationsVAV-PRC008-ENAC 1 IntroductionAC 1 SystemsAC 2 4 Parallel vs. SeriesAC 5 Low-Temperature AirAC 6 7 Energy Savings and system ControlAC 8 Agency CertificationsAC 9 Control TypesAC 10 12 Flow Measurement and ControlAC 13 14 Reheat OptionsAC 15 InsulationAC 16 AcousticsAC 17 18 duct DesignAC 19 Selection ProgramAC 20 Best PracticesAC 21 Unit ConversionsAC 22 Additional ReferencesAC 23 Table ofContentsIntroductionThe VariTrane line of variable-air-volume (VAV) products has been anindustry leader in performance andquality for many years. The VariTraneline includes single- duct VAV units,dual- duct VAV units, fan-powered VAVunits (series, parallel, and low-heightseries and parallel), direct digitalcontrols, pneumatic controls, analog-electronic controls, direct digital controlretrofit kits and diffusers.

2 Thisapplication section will focus on 2 SystemsVAV SystemsThere are two primary types of VAV systems single- duct and SystemsSingle- duct systems include one supply fan and a single supply duct , which isattached to each zone. The supply fan delivers cooled air to the VAV zones in variablevolumes, depending upon the cooling requirements. The supply fan is usuallydesigned to modulate airflow delivered to the VAV VAV zones require heating as well as cooling. The supply air-handling unitprovides either no heat (cooling only), morning warm-up heat or occupied(changeover) heat. In addition, heat may be provided at any individual VAV zone(within the zone or within the VAV terminal) by reheating cool air provided by thecentral air HeatCentral Cooling Only In some systems, the central air handler provides onlycooling and ventilation during zone occupied periods.

3 The supply air is maintained ata constant temperature and the supply airflow is modulated to match the VAVairflow rate with the zone cooling HeatCentral Heat for Morning Warm-up Many buildings cool down during the night. Tobe at a comfortable temperature in the morning when the building is againoccupied, heat must be added to the spaces. Heat provided by the central airhandler for morning warm-up is supplied at constant air volume to the zones, priorto the time of occupancy. During the morning warm-up period, the VAV terminalunits must open to allow heated air to flow into the zones. In most instances verylittle additional heat is needed once the building is (VAV) SystemRAEAOAPA supplyfancoolingcoilvariable-speed drivethermostatVAVboxSACentral Occupied Heating-Changeover Somebuildings use the same air handler to provide bothoccupied cooling and occupied heating.

4 This iscommonly referred to as a changeover system . Thesystem changes between heating and coolingdepending on the need of the zones on the a changeover system , the operation of the VAVterminal units must also change over, opening toprovide heat in the heating mode and opening toprovide cooling in the cooling mode. Trane 's mainproduct in this type of Application is called VariTrac .VariTrane products can also be used in thesesystems. (These types of systems are beyond thescope of this manual and are discussed in detail inthe VariTrac II Manual, VAV-PRC003-EN.)Terminal HeatRemote Heat In some zones of a single- duct VAVsystem, perimeter heating equipment, remote fromthe terminal unit, is used to add heat to the zoneVSDsupplyfanSingle-Fan, Dual- duct VAV Systemcentral air handlercoolingcoilcoilheating105 F ( C)dual-ductVAVterminalunitsEAOA40 F( C)( C)75 F( C)55 FRAwhen the cooling load is lower than theminimum cooling capacity of the VAVterminal unit.

5 Heat is added directly tothe zone while cool supply air continuesto enter the zone at a minimum rate forzone Reheat In some zones of asingle- duct VAV system , a minimumflow of cool supply air is reheated at theterminal unit before entering the reheat can be provided byelectrical resistance heaters or by hotwater Fan-Powered Heat In somezones of a single- duct VAV system , coolsupply air at minimum flow is mixedwith warm plenum air before enteringthe zone at a constant flow rate. A fan inthe terminal unit, in parallel with thecentral fan, draws air from the plenumwhenever the zone requires Fan-Powered Heat In somezones of a single- duct VAV system , theairflow to the zone is held constant,during both heating and cooling, by aterminal unit fan that is in series with thecentral fan.

6 The terminal unit fan runscontinuously. When the zone requiresheat, cool supply air at minimum flow ismixed with warm, return plenum airbefore entering the SystemsDual- duct systems have either one ortwo supply fans and two duct duct system carries heated air andthe other duct system carries cooled air and cooled air are modulatedand/or mixed at each zone in the properproportions to control zone reheat is not required in a dual- duct 3 SystemsVariTrane VAV Terminal UnitsThe function of the VariTrane terminalunit in a VAV control zone is to vary thevolumetric airflow rate to the units are available with eithermicroprocessor-based DDC controls orpneumatic or analog electronic controls are availablewith all types of terminal VAVT erminal Unit TypesSingle-DuctSingle- duct terminal units control thevolumetric flow of supply air to

7 Thespace to maintain the zone temperatureat setpoint. These units are generallyapplied in cooling-only VAV zones thatrequire no heat during occupied hours. Iflocal zone heat is necessary it can beprovided either remotely (for example,perimeter heat) or by terminal reheat(either electric or hot water coils).Single- duct Unit with Hot Water CoilParallel Fan-Powered Unit withHot Water CoilParallel Fan-Powered Unit withElectric CoilSingle- duct Cooling Only UnitParallel Fan-Powered UnitCooling OnlyDual-DuctDual- duct terminal units are used in aspecial type of air distribution systemwhere the main system has both warmair and cold air separately ducted toeach terminal unit. The flow of bothwarm air and cool air is modulated,delivering air to the VAV zone atvariable air volumes as well as variabletemperatures.

8 Since full capacityoccupied heating is always available,control of additional local heat isnot Terminal UnitParallel Fan-PoweredParallel fan-powered units arecommonly used in VAV zones whichrequire some degree of heat duringoccupied hours when the primarysupply air is cool. The terminal unit fan isin parallel with the central unit fan; noprimary air from the central fan passesthrough the terminal unit fan. Theterminal unit fan draws air from thespace return no heat is needed, the localparallel fan is off and a backdraftdamper on the fan s discharge is closedto prevent cool air entry into the returnplenum. When cool airflow to the VAVzone is at a minimum and the zonetemperature drops below setpoint, thelocal parallel fan is turned on and thebackdraft damper opens.

9 A constantvolume of air is delivered to the zonebecause the fan delivers a constantvolume of warm plenum air which ismixed with cool primary air at aminimum flow. Remote heat orterminal reheat can provide additionallocal 4 SystemsSeries Fan-PoweredSeries fan-powered terminal units areused commonly in VAV zones that notonly require heat during occupiedhours, but also desire constant airvolume delivery. The terminal unit fanis in series with the central fan. Primaryair from the central fan always passesthrough the terminal unit local series fan within the terminalunit operates whenever the unit is inthe occupied mode. The volume of airdelivered to the VAV zone is constant,but the temperature of the delivered airvaries. As the zone requires lesscooling, the primary air damper the primary air damper closes, theair mixture supplied to the zonecontains less cool air and more warmplenum air.

10 Remote heat or terminalreheat can provide additional fan-powered terminal units arealso useful in low supply airtemperature systems, since theterminal unit fan can be sized so thatwarm plenum air is always mixed withlow temperature supply air. This raisesthe supply air temperature to anacceptable distribution level andreduces condensation Fan-Powered Unit Cooling OnlyLow-Height Fan-PoweredLow-height fan-powered terminal unitsare a slightly modified version of a fan-powered terminal unit. As its namesuggests, the low-height fan-poweredunit has a shorter height dimension toaccommodate applications whereceiling space is limited. To reduce theheight, shorter terminal unit fans areintegrated into the standard heightseries or parallel terminal unit.