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


General Temperature ControlGeneral Temperature Control


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.


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.


General Temperature General Temperature ControlControl

Ratings of SystemsRatings of Systems

Split UnitSplit Unit FairFair

WCPUWCPU PoorPoor

CAVCAV GoodGood

VAVVAV ExcellentExcellent


Zone Temperature SelectionZone Temperature Selection


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.



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%



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


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


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%).


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.





WCPUWCPU FairFair

CAVCAV PoorPoor



Flexibility to Adapt to ChangeFlexibility to Adapt to Change


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.


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.


Flexibility to Adapt to Flexibility to Adapt to ChangeChange



WCPUWCPU PoorPoor

CAVCAV FairFair



Energy EfficiencyEnergy Efficiency


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.


Energy EfficiencyEnergy EfficiencyRatings of SystemsRatings of Systems

Split UnitSplit Unit PoorPoor

WCPUWCPU PoorPoor

CAVCAV FairFair



Ease of MaintenanceEase of Maintenance


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.


Ease of MaintenanceEase of MaintenanceRatings of SystemsRatings of Systems

Split UnitSplit Unit ExcellentExcellent

WCPUWCPU GoodGood

CAVCAV FairFair

VAVVAV GoodGood


Fire / Smoke ControlFire / Smoke Control


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.


Fire / Smoke ControlFire / Smoke ControlRatings of SystemsRatings of Systems


WCPUWCPU PoorPoor

CAVCAV ExcellentExcellent



Indoor Air QualityIndoor Air Quality


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.


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.


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.


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.


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).


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.


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).


Indoor Air QualityIndoor Air QualityRatings of SystemsRatings of Systems


WCPUWCPU PoorPoor

CAVCAV ExcellentExcellent

VAVVAV GoodGood


Vibration / NoiseVibration / Noise


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.


Vibration / NoiseVibration / NoiseRatings of SystemsRatings of Systems


WCPUWCPU PoorPoor

CAVCAV FairFair



VAV System ComponentsVAV System Components


Return AirReturn Air

OutdoorOutdoorAirAir

Cooling CoilCooling Coil

Chilled WaterChilled WaterValveValve

VAVVAVBoxBox

Ceiling DiffusersCeiling Diffusers

FanFan

Mechanical ComponentsMechanical ComponentsMechanical ComponentsMechanical Components


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 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


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 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 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 Control TheoryVAV Control 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 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 Control TheoryVAV Control TheoryThe off-coil sensor measures the air The off-coil sensor measures the air


The chilled water valve is controlled to The chilled water valve is controlled to achieve a desired off-coil achieve a desired off-coil



VAV / CAV ComparisonVAV / CAV Comparison


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


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

vav system theory

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vav system theoryair

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air conditioning loads