Download - VAV System Theory
VAV System TheoryVAV System Theory
Air Side System EvaluationAir Side System EvaluationGeneral temperature controlGeneral temperature control
Zone temperature selectionZone temperature selection
Flexibility to adapt to changeFlexibility to adapt to change
Energy efficiencyEnergy efficiency
Ease of maintenanceEase of maintenance
Fire / smoke controlFire / smoke control
Indoor air qualityIndoor air quality
Vibration / noiseVibration / noise
VAV System TheoryVAV System Theory
General Temperature ControlGeneral Temperature Control
VAV System TheoryVAV System Theory
General Temperature General Temperature ControlControl
Issue #1 : Control of CoolingIssue #1 : Control of Cooling
Modulating control of the valve Modulating control of the valve provides good general temperature provides good general temperature
control.control.
Multiple stages of cooling provide fair Multiple stages of cooling provide fair general temperature control.general temperature control.
On-off control of a valve provides poor On-off control of a valve provides poor general temperature control.general temperature control.
VAV System TheoryVAV System Theory
General Temperature ControlGeneral Temperature ControlIssue #2 : Sensor PlacementIssue #2 : Sensor Placement
The system is controlled to achieve the The system is controlled to achieve the setpoint only at the sensor location.setpoint only at the sensor location.
Sensor in the room ensures comfort in that Sensor in the room ensures comfort in that specific room.specific room.
Sensor in return air duct reacts to Sensor in return air duct reacts to averageaverage load; concentrated loads cause problems.load; concentrated loads cause problems.
DDC controllers can do averaging, high DDC controllers can do averaging, high select or low select of multiple sensors.select or low select of multiple sensors.
VAV System TheoryVAV System Theory
General Temperature General Temperature ControlControl
Ratings of SystemsRatings of Systems
Split UnitSplit Unit FairFair
WCPUWCPU PoorPoor
CAVCAV GoodGood
VAVVAV ExcellentExcellent
VAV System TheoryVAV System Theory
Zone Temperature SelectionZone Temperature Selection
VAV System TheoryVAV System Theory
Zone Temperature Zone Temperature SelectionSelection
Different zones have different loads Different zones have different loads requiring different setpoints and requiring different setpoints and
different amounts of cooling.different amounts of cooling.
VAV System TheoryVAV System Theory
Zone Temperature Zone Temperature SelectionSelection
The biggest contributors to air The biggest contributors to air conditioning loads are lighting load conditioning loads are lighting load
and solar load. People and and solar load. People and equipment loads are smaller.equipment loads are smaller.
Clear Plate GlassClear Plate Glass
Incoming solarIncoming solarradiation - 100%radiation - 100%
Solar heatSolar heatexcluded - 16%excluded - 16% Solar heatSolar heat
admitted - 84%admitted - 84%
VAV System TheoryVAV System Theory
Zone Temperature Zone Temperature SelectionSelection
The cooling load in a zone varies The cooling load in a zone varies through the day. Because of thermal through the day. Because of thermal
inertia, the greatest building load inertia, the greatest building load occurs about 15:00.occurs about 15:00.
Interior ZoneInterior Zone
SS
WWEE
VAV System TheoryVAV System Theory
Zone Temperature SelectionZone Temperature SelectionLighting :Lighting : Relatively FixedRelatively Fixed
Solar : Solar : Varies through dayVaries through day
Conduction :Conduction : Varies through dayVaries through day
Ventilation :Ventilation : Varies with other loadsVaries with other loads
People : People : Varies with occupancyVaries with occupancy
Equipment :Equipment : Varies with use of Varies with use of spacespace
VAV System TheoryVAV System Theory
Zone Temperature SelectionZone Temperature SelectionAny air conditioning system must deal Any air conditioning system must deal
with varying loads.with varying loads.
Distributed systems such as fan coil Distributed systems such as fan coil units and package units allow localized units and package units allow localized
control but are energy inefficient.control but are energy inefficient.
Not all zones will call for maximum Not all zones will call for maximum cooling at the same time. A centralized cooling at the same time. A centralized
system can be designed with a system can be designed with a diversity factor (20%).diversity factor (20%).
VAV System TheoryVAV System Theory
Zone Temperature Selection Zone Temperature Selection A centralized Constant Air Volume (CAV) A centralized Constant Air Volume (CAV)
system is energy efficient but only system is energy efficient but only responds to the average load, and does responds to the average load, and does
not allow individual zones to be not allow individual zones to be controlled. For constant loads, a CAV controlled. For constant loads, a CAV
system is best.system is best.
A centralized Variable Air Volume (VAV) A centralized Variable Air Volume (VAV) system is the most economical way to system is the most economical way to provide zone control, both in terms of provide zone control, both in terms of
first cost and operating cost.first cost and operating cost.
VAV System TheoryVAV System Theory
Zone Temperature Zone Temperature SelectionSelection
Ratings of SystemsRatings of Systems
Split UnitSplit Unit FairFair
WCPUWCPU FairFair
CAVCAV PoorPoor
VAVVAV ExcellentExcellent
VAV System TheoryVAV System Theory
Flexibility to Adapt to ChangeFlexibility to Adapt to Change
VAV System TheoryVAV System Theory
Flexibility to Adapt to ChangeFlexibility to Adapt to ChangeOne of the key criteria of an “Intelligent One of the key criteria of an “Intelligent
Building” is the flexibility to adapt to Building” is the flexibility to adapt to change. This is because as they way in change. This is because as they way in which we do business changes, so too which we do business changes, so too
the way in which we use the office the way in which we use the office changes.changes.
For example, increasing computerization For example, increasing computerization changes the heat load, power, lighting, changes the heat load, power, lighting,
communication and LAN wiring communication and LAN wiring requirements in the office.requirements in the office.
VAV System TheoryVAV System Theory
Flexibility to Adapt to Flexibility to Adapt to ChangeChange
As partitioning layout changes, the air As partitioning layout changes, the air conditioning system must be capable conditioning system must be capable of cost effectively change the airflow of cost effectively change the airflow
patterns and temperature control patterns and temperature control requirements in each area.requirements in each area.
VAV System TheoryVAV System Theory
Flexibility to Adapt to Flexibility to Adapt to ChangeChange
Ratings of SystemsRatings of Systems
Split UnitSplit Unit FairFair
WCPUWCPU PoorPoor
CAVCAV FairFair
VAVVAV ExcellentExcellent
VAV System TheoryVAV System Theory
Energy EfficiencyEnergy Efficiency
VAV System TheoryVAV System Theory
Energy EfficiencyEnergy EfficiencyCentralized systems are far more Centralized systems are far more energy efficient than distributed energy efficient than distributed
systems.systems.
Variable Air Volume systems are more Variable Air Volume systems are more energy efficient than Constant energy efficient than Constant
Volume Systems as they deliver the Volume Systems as they deliver the minimum amount of cooling required minimum amount of cooling required
and do not overcool.and do not overcool.
VAV System TheoryVAV System Theory
Energy EfficiencyEnergy EfficiencyRatings of SystemsRatings of Systems
Split UnitSplit Unit PoorPoor
WCPUWCPU PoorPoor
CAVCAV FairFair
VAVVAV ExcellentExcellent
VAV System TheoryVAV System Theory
Ease of MaintenanceEase of Maintenance
VAV System TheoryVAV System Theory
Ease of MaintenanceEase of MaintenanceThe ease of maintenance of a system The ease of maintenance of a system
relates to the availability of skilled relates to the availability of skilled labour to service the equipment.labour to service the equipment.
The increased complexity associated The increased complexity associated with the centralized systems is with the centralized systems is
partially offset by the diagnostic partially offset by the diagnostic information available from a BAS.information available from a BAS.
VAV System TheoryVAV System Theory
Ease of MaintenanceEase of MaintenanceRatings of SystemsRatings of Systems
Split UnitSplit Unit ExcellentExcellent
WCPUWCPU GoodGood
CAVCAV FairFair
VAVVAV GoodGood
VAV System TheoryVAV System Theory
Fire / Smoke ControlFire / Smoke Control
VAV System TheoryVAV System Theory
Fire / Smoke ControlFire / Smoke ControlOnly centralized systems can help Only centralized systems can help
control the egress of smoke during a control the egress of smoke during a fire.fire.
If properly designed, the air If properly designed, the air conditioning system can work conditioning system can work
together with the pressurization together with the pressurization system to provide a smoke spill system to provide a smoke spill
system or supplementary system or supplementary pressurization.pressurization.
VAV System TheoryVAV System Theory
Fire / Smoke ControlFire / Smoke ControlRatings of SystemsRatings of Systems
Split UnitSplit Unit PoorPoor
WCPUWCPU PoorPoor
CAVCAV ExcellentExcellent
VAVVAV ExcellentExcellent
VAV System TheoryVAV System Theory
Indoor Air QualityIndoor Air Quality
VAV System TheoryVAV System Theory
Indoor Air QualityIndoor Air QualityThe issue of Indoor Air Quality (IAQ) The issue of Indoor Air Quality (IAQ) and Sick Building Syndrome started and Sick Building Syndrome started
to become an issue in the west in the to become an issue in the west in the early 1980s.early 1980s.
The energy crisis of the 1970s The energy crisis of the 1970s resulted in energy efficient office resulted in energy efficient office
buildings with sealed envelopes to buildings with sealed envelopes to minimize fresh air intake.minimize fresh air intake.
VAV System TheoryVAV System Theory
Indoor Air QualityIndoor Air QualityAt the same time, new types of At the same time, new types of
building materials (carpets, building materials (carpets, partitions, etc.) became popular. The partitions, etc.) became popular. The
new building materials used new building materials used formaldehyde-based adhesives which formaldehyde-based adhesives which
gave off a irritating odour.gave off a irritating odour.
Chlorinated Hydrocarbons and Volatile Chlorinated Hydrocarbons and Volatile Organic Compounds (VOCs) were also Organic Compounds (VOCs) were also
introduced into the space from introduced into the space from cleaning solutions and other sources.cleaning solutions and other sources.
VAV System TheoryVAV System Theory
Indoor Air QualityIndoor Air QualityLegionnaire's disease increased Legionnaire's disease increased
awareness of bacteria, fungus and awareness of bacteria, fungus and mould which lives in air conditioning mould which lives in air conditioning
systems.systems.
Peoples’ jobs became more sedentary Peoples’ jobs became more sedentary as personal computers became more as personal computers became more
common in the workplace.common in the workplace.
VAV System TheoryVAV System Theory
Indoor Air QualityIndoor Air QualityReduced fresh air ventilationReduced fresh air ventilation
++Increased number of pollutants in the Increased number of pollutants in the
officeoffice++
Less activity in the jobLess activity in the job==
Complaints (and litigation!)Complaints (and litigation!)
Complaints are of headaches, irritated Complaints are of headaches, irritated eyes, nose and throat and flu-like eyes, nose and throat and flu-like
symptoms.symptoms.
VAV System TheoryVAV System Theory
Indoor Air QualityIndoor Air QualityASHRAE increased ventilation ASHRAE increased ventilation
requirements in an office requirements in an office environment.environment.
ASHRAE recommends measuring COASHRAE recommends measuring CO2 2
as an indicator of ventilation as an indicator of ventilation effectiveness. People generate COeffectiveness. People generate CO22
and the ventilation system disperses and the ventilation system disperses it. Office COit. Office CO22 should be <1000 ppm should be <1000 ppm
(ambient is 400 ppm).(ambient is 400 ppm).
VAV System TheoryVAV System Theory
Indoor Air QualityIndoor Air QualityVAV systems have been linked to VAV systems have been linked to indoor air quality problems. With a indoor air quality problems. With a
conventional VAV system, at part load conventional VAV system, at part load the amount of fresh air introduced is the amount of fresh air introduced is
reduced.reduced.
This can be countered by modulating This can be countered by modulating the fresh air dampers or increasing the fresh air dampers or increasing
the off-coil temperature setpoint but the off-coil temperature setpoint but these approaches add to first cost or these approaches add to first cost or
to operating cost.to operating cost.
VAV System TheoryVAV System Theory
Indoor Air QualityIndoor Air QualityIndoor air quality has not become an Indoor air quality has not become an issue with local owners or tenants. It issue with local owners or tenants. It is unlikely to become a serious issue is unlikely to become a serious issue
locally. locally.
Local “air quality” problems are Local “air quality” problems are usually related to improper usually related to improper
placement of fresh air intakes (i.e. placement of fresh air intakes (i.e. next to a durian stand or beside a next to a durian stand or beside a
toilet exhaust).toilet exhaust).
VAV System TheoryVAV System Theory
Indoor Air QualityIndoor Air QualityRatings of SystemsRatings of Systems
Split UnitSplit Unit PoorPoor
WCPUWCPU PoorPoor
CAVCAV ExcellentExcellent
VAVVAV GoodGood
VAV System TheoryVAV System Theory
Vibration / NoiseVibration / Noise
VAV System TheoryVAV System Theory
Vibration / NoiseVibration / NoiseLocalized units are inherently more Localized units are inherently more
noisy than centralized system.noisy than centralized system.
Conversely, the ductwork can Conversely, the ductwork can distribute noise from the fan or noise distribute noise from the fan or noise
resulting from airflow.resulting from airflow.
The box in a VAV system provides The box in a VAV system provides additional acoustic buffering.additional acoustic buffering.
VAV System TheoryVAV System Theory
Vibration / NoiseVibration / NoiseRatings of SystemsRatings of Systems
Split UnitSplit Unit PoorPoor
WCPUWCPU PoorPoor
CAVCAV FairFair
VAVVAV ExcellentExcellent
VAV System TheoryVAV System Theory
VAV System ComponentsVAV System Components
VAV System TheoryVAV System Theory
Return AirReturn Air
OutdoorOutdoorAirAir
Cooling CoilCooling Coil
Chilled WaterChilled WaterValveValve
VAVVAVBoxBox
Ceiling DiffusersCeiling Diffusers
FanFan
Mechanical ComponentsMechanical ComponentsMechanical ComponentsMechanical Components
VAV System TheoryVAV System Theory
VAV System ComponentsVAV System ComponentsReturn AirReturn Air
Typically from ceiling plenumTypically from ceiling plenumRepresents average of all zonesRepresents average of all zones
Outdoor AirOutdoor AirUsed to dilute contaminants in Return Used to dilute contaminants in Return
AirAir
Cooling CoilCooling CoilCools air stream by exposing it to coil Cools air stream by exposing it to coil
with chilled water passing throughwith chilled water passing through
VAV System TheoryVAV System Theory
VAV System ComponentsVAV System ComponentsChilled Water ValveChilled Water Valve
Controls the water passing through Controls the water passing through coil and thus controls off-coil air coil and thus controls off-coil air
temperaturetemperature
FanFanControls the amount of air in the Controls the amount of air in the
systemsystem
VAV BoxVAV BoxControls the amount of air going to a Controls the amount of air going to a
zonezone
VAV System TheoryVAV System Theory
Control ComponentsControl ComponentsControl ComponentsControl Components
VariableVariableSpeedSpeedDriveDrive
ControlControl
Off-coil AirOff-coil AirTemperatureTemperature
SensorSensor
Valve ActuatorValve Actuator
StaticStaticPressurePressureSensorSensor
ActuatorActuator
P SensorP Sensor
TRoom Temperature SensorRoom Temperature Sensor
VAV System TheoryVAV System Theory
VAV System ComponentsVAV System ComponentsValve ActuatorValve Actuator
Controls Chilled Water ValveControls Chilled Water Valve
Off - Coil TemperatureOff - Coil TemperatureUsed to control valve actuatorUsed to control valve actuator
Variable Speed Drive ControlVariable Speed Drive ControlControls fan speedControls fan speed
Static Pressure SensorStatic Pressure SensorUsed to control fan speedUsed to control fan speed
VAV System TheoryVAV System Theory
VAV System ComponentsVAV System ComponentsVAV Box ActuatorVAV Box Actuator
Controls air flow through boxControls air flow through box
P SensorP SensorUsed to measure air flow through Used to measure air flow through
boxbox
Temperature SensorTemperature SensorUsed to determine amount of air Used to determine amount of air
requiredrequired
VAV System TheoryVAV System Theory
VAV Control TheoryVAV Control Theory
VAV System TheoryVAV System Theory
VAV Control TheoryVAV Control TheoryRoom temperature sensor determines Room temperature sensor determines
amount of cold air requiredamount of cold air required
P sensor measures actual amountP sensor measures actual amountof cold airof cold air
Actuator controls cold air to matchActuator controls cold air to matchrequired amountrequired amount
VAV System TheoryVAV System Theory
VAV Control TheoryVAV Control TheoryWhen VAV box actuators open or When VAV box actuators open or close, the static pressure sensor close, the static pressure sensor
detects the changedetects the change
The variable speed drive control The variable speed drive control changes the fan speed to maintain a changes the fan speed to maintain a
static pressure setpoint static pressure setpoint
VAV System TheoryVAV System Theory
VAV Control TheoryVAV Control TheoryThe off-coil sensor measures the air The off-coil sensor measures the air
temperaturetemperature
The chilled water valve is controlled to The chilled water valve is controlled to achieve a desired off-coil achieve a desired off-coil
temperaturetemperature
VAV System TheoryVAV System Theory
VAV / CAV ComparisonVAV / CAV Comparison
VAV System TheoryVAV System Theory
As compared to a CAV System:As compared to a CAV System: VAV System allows precise control ofVAV System allows precise control of each zone each zone
VAV System automatically compensatesVAV System automatically compensates for shifting solar load during day for shifting solar load during day
VAV System allows each occupant toVAV System allows each occupant to establish their own setpoint establish their own setpoint
VAV System saves energy as fan onlyVAV System saves energy as fan only supplies as much air as is required supplies as much air as is required
VAV System has improved acousticalVAV System has improved acoustical performance as box baffles duct noise performance as box baffles duct noise
VAV System TheoryVAV System Theory
As compared to a CAV As compared to a CAV System:System:
VAV System saves energy when VAV System saves energy when connected to BAS as individual zones connected to BAS as individual zones can be set to occupied modecan be set to occupied mode
VAV System allows improved VAV System allows improved diagnostics when connected to BAS as diagnostics when connected to BAS as individual zone information is availableindividual zone information is available
VAV System has slightly higher first VAV System has slightly higher first cost and slightly higher maintenance cost and slightly higher maintenance cost due to boxes and controlscost due to boxes and controls