Prolon VC2000 Series - HVAC System Principles

VAV System Principle
VAV systems (Variable Air Volume) typically have a central A/C unit (rooftop, split system, heat pump, etc.) providing conditioned
airtoanumberofzonesthroughanetworkofventilationducts.Eachzoneisindependentandequippedwithadamperthat
opens or closes to satisfy space temperature demand. These systems generally supply cooling only to the zones at all times
whenthebuildingisoccupied.ThetargetdischargetemperatureforthecoolairsuppliedbytheA/Cunitiseitherxedorreset
by another variable, such as outdoor or return air temperature. With a VAV system, as a zone calls for cooling, its damper opens
toletcoolairenterthespace.Iteventuallyclosesasthezonebecomessatised.Ifthecallisforheating,thedamperremains
closedsinceitisdesignedtostrictlyprovidecooling.Inthisinstance,theVAVControllersequenceisconsideredtobe“Cooling
Only”.Shouldheatingberequiredinthespace,thezonemustbeequippedwithaterminalductheater,usedtoreheatthe
incoming air into the zone. When such a zone calls for heating, the damper opens to an intermediate position, giving proper air
owtotheductheatersoitcanreheattheincomingcoolairfromthecentralA/Cunitandprovidewarm/hotairtothespace.In
this instance, the VAV Controller is considered to be “Cooling Only with Reheat”. VAV systems are very common in applications
where cooling loads are prevalent year round.
Changeover Bypass System Principle (also known as “VVT”, or Variable Volume and Temperature)
Changeover Bypass systems (or “VVT”) are very similar to VAV systems as they too have a central A/C unit providing conditioned
airtozonesequippedwithdampers.TheydierhoweverbyusingA/Cunitsthatprovideeithercoolingorheatingtothezones.
Since the A/C unit changes its supply over from heating to cooling and vice-versa to satisfy various heating and cooling
demands, its supply temperature is always variable. Each zone calculates its own demand and factors in a voting weight which
willdeterminehowmuchofaninuenceitwillhaveonthesystem(ifeveryzonehasthesamevotingweight,everyzonehas
equal inuence).TheA/CUnitController receives andanalyzes thesedemands,andwill then decidetoprovide heatingor
coolingbasedonauser-denedstrategy(highestheat/coolcall,weightedaverage,etc.).Thisisknownasademand-based
system.Additionally,thezoneshaveachangeoverlogicoftheirown;eachZoneController“knows”whetherthesupplyair
temperature fed is cool or hot, and therefore will only modulate its damper to open if the supply air available matches its own
demand.Eachzonemayalsobeequippedwithaductheaterforadditionalheatingcapabilities.“VVT”systemsarecommonin
applications where heating and cooling loads change year round.
The Prolon Control System can be congured for 2 kinds of HVAC zoning system principles, which are commonly
known as “VAV” systems and “Changeover Bypass” systems.
The following principles are general descriptions of HVAC zoning applications. Although common, these principles
must not be interpreted as absolute guidelines when doing system design. Each HVAC installation is dierent and
unique, and there are endless variations based on these principles.
Setting up a Zoning System with Prolon
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