metasys ventilation controller application (mvca

M-Applications 653Metasys Ventilation Controller Section

Issue Date 0101

© 2001 Johnson Controls, Inc. www.johnsoncontrols.comPart No. 24-8394-13 Rev. —Code No. LIT-653435

Metasys Ventilation Controller Application (MVCA) ..........................3

Introduction........................................................................................................*3

Key Concepts......................................................................................................5

ASHRAE Standard 62-1999 Ventilation Rate Procedure ............................................... *5

MVCA Strategy for Air Handling Units ............................................................................ 6

Product Offerings........................................................................................................... *6

Overview of MVCA Strategy .......................................................................................... *9

System Applicability..................................................................................................... *19

Implementation of Outdoor Air Flow Control (MVCA) Strategy .................................... *24

MVCA Software ........................................................................................................... *29

Procedure Overview........................................................................................*43

Detailed Procedures .........................................................................................44

Evaluating the HVAC System Integrity......................................................................... *44

Checking the MVCA System Stability .......................................................................... *45

Accessing MVCA Modules Via GX-9100 Software ...................................................... *46

APPLICATION NOTE

Metasys Ventilation Controller Application (MVCA)

* Indicates those sections where changes have occurred since the last printing.

Metasys Ventilation Controller2

Metasys Ventilation Controller Application (MVCA) 3

Metasys Ventilation ControllerApplication (MVCA)

IntroductionThe American Society of Heating, Refrigeration, and Air ConditioningEngineers (ASHRAE) Standard 62-1999 defines acceptable indoor airquality as “indoor air in which there are no known contaminants atharmful concentrations, as determined by cognizant authorities, andwith which a substantial majority (80% or more) of the people exposeddo not express dissatisfaction.” Indoor air quality is influenced by amultitude of factors, many of which (poor design of a building and itssystems, lack of equipment maintenance, indoor sources of aircontaminants, etc.) are outside the realm of correction by controlstrategies. A fundamental means of maintaining acceptable indoor airquality is ventilation: the introduction of clean outdoor air at a ratesufficient to dilute the indoor air contaminant concentrations to anacceptable level. Control strategies can play a major role inimplementing required ventilation levels while minimizing energy use.

This application note details the Metasys Ventilation ControllerApplication (MVCA) for the DX-9100 Extended Digital Controlleras a means to achieve acceptable indoor air quality. It provides a basicunderstanding of the timers, comparators, and calculators that sample,measure, and maintain air quality. MVCA is based on the VentilationRate Procedure (VRP) and demand-based overrides using the MetasysVentilation Controller (MVC) 2000-1.


This application note also describes how to:

• evaluate the HVAC (Heating, Ventilating, and Air Conditioning)system integrity

• check the MVCA system stability

• access MVCA modules via GX-9100 software

Use this document with the Metasys Ventilation ControllerApplication for the DX-9100 Technical Bulletin (LIT-653425), whichprovides information on MVC-2000-1 installation procedures, locationconsiderations, and other important information. For specific details onthe DX-9100 and its software (GX-9100), refer to the System 9100Technical Manual (FAN 636.4).


Key Concepts

ASHRAE Standard 62-1999 Ventilation Rate Procedure

ASHRAE (American Society of Heating, Refrigeration, and AirConditioning Engineers) Standard 62-1999: Ventilation for AcceptableIndoor Air Quality provides two procedures to determine Outdoor Air(OA) flow rates for buildings: the VRP and the Indoor Air Quality(IAQ) procedure. Only the VRP implementation is described in detailin this application note.

The VRP specifies the OA flow rate as a function of occupancy andbuilding use. The specified OA flow rates are “derived fromphysiological considerations, subjective evaluations and professionaljudgments.” The ventilation (outdoor) air must have acceptablequality, as specified in ASHRAE Standard 62-1999. The contaminantconcentrations in the indoor air are not directly measured under thisprocedure, but are expected to be at typical levels for the given types ofoccupied space. The prescribed OA flow rates are then expected todilute the indoor air contaminant concentrations to acceptable levels.

The IAQ procedure provides a methodology to measure and control theconcentrations of indoor air contaminants below safe levels. However,consistent guidelines do not exist for acceptable concentration levels ofmost contaminants. The IAQ procedure is extremely difficult to applywith current technology and is not the subject of this application note.

Continuous measurement of the OA flow rate provides the basis forimplementing the ASHRAE Standard 62-1999 VRP. It regulates theventilation air flow to the specified rate under all applicationconditions. Common practice has been to simply set the OA damper toa fixed minimum position, based on design conditions. For VariableAir Volume (VAV) systems, this common practice does not meet therequirements of ASHRAE Standard 62-1999. The minimumprescriptive OA flow rates are now 15 cfm per person, with 20 cfmper person required for office spaces (refer to Table 2 ofStandard 62-1999).


MVCA Strategy for Air Handling Units

The MVCA strategy focuses on compliance with the VRP, withsecondary attention to minimizing energy costs. The increase in overallbuilding energy cost of raising the minimum OA flow rates from 5 cfmto 20 cfm per person has been estimated (Lawrence BerkeleyLaboratory) to be less than 5% in most cases. This small increase iswell worth the cost if it can avoid loss of occupant productivity.

The requirements of the VRP of ASHRAE Standard 62-1999 are mosteffectively implemented with the use of an online OA flowmeasurement and closed-loop control arrangement. Closed-loopcontrol of ventilation air flow rate can ensure that the OA flow neverdrops below the minimum required by ASHRAE Standard 62-1999.Closed-loop control also enhances the reliability of space staticpressure control by causing outdoor makeup air flow for spacepressurization to be independent of system load.

The control strategies necessary for conformance with the ASHRAEStandard 62-1999 prescribed ventilation rates have been implementedin hardware and DX-9100 Programmable Modules. A comprehensiveset of tools is provided to achieve compliance with the VRP from theVAV air handling system viewpoint. Implementation of additionalstrategies (not covered in this document) from the zone viewpoint mayalso be necessary.

Product Offerings

The hardware and software for implementing the MVCA strategy havebeen developed and tested for use on air handling units controlled byMetasys Application Specific Controllers (ASCs). For documentationof ventilation performance, this strategy requires networking to aworkstation and printer.


Hardware

Outdoor air flow is measured indirectly, using the CO2 ConcentrationBalance method, which is implemented with the following device:

Table 1: Metasys Ventilation Control Multipoint CO2

SamplerDevice Description

MVC-2000-1 Includes one DX-9100 controller for ventilation control logic,used for ASC applications. Installed in a Universal PackagingModule (UPM) enclosure.

This device functions to sample CO2 concentrations in three remotelocations and incorporate one carbon dioxide sensor, two V11 solenoidair valves, and an air pump. The MVC-2000-1 samples the CO2

concentrations in outdoor, supply, and return air ducts. Standard6.35 mm (1/4 in) tubing is used for CO2 sampling probes. Applicationof this multipoint CO2 sampler is independent of the Air Handling Unit(AHU) capacity or size. See Figure 1 and Figure 2 for typicalmeasurement configuration and application examples.

Return AirTemperature

Outdoor AirSample

AirSampling

Pump

CarbonDioxideSensor

Supply Air

Supply AirSample

Supply AirFlow Measuring

StationMixed Air

Temperature

Outdoor Air

ExhaustAir

CoupledMixed AirDampers Mixed Air

Plenum

SupplyFan

ReturnFan

3-WayValve

3-WayValve

Outdoor AirTemperature

system

Return AirSample

Figure 1: Multiplexed Method of CO2 Measurement


SupervisoryController

does data trending.

MVC-2000-1

VAV Air Handling System

Supply Air FlowMeasuring Station

CoupledDampers

Workstation

Printerarchives and prints

trend data.

MVCAPB5

Calculatesoutdoor air flow

and controlsthe dampers.

Supply Air Flow

Multiplexed Samplesof Supply, Return,

Outdoor Air

Discharge Air Temperature

Mixed Air Damper Control

SupervisoryController

N2 Bus

Air Handling UnitController

N2 Bus

Figure 2: Sample System Using MVC-2000-1 Connected to aMetasys Supervisory Controller


Ventilation Control Software

The DX-9100 uses GX-9100 software to implement the followingcontrol modules (see the MVCA Software section of this document fordetails about these modules):

• CO2 Multiplexer

• Outdoor Air Flow Calculator

• CO2 Sensor Autozero Function

• Sequencer for the CO2 Multiplexer

• Denominator Check

• Outdoor Air Flow Controller

• Outdoor Air Flow Setpoint Selector

• Supply Air Flow Calculator

• Square Root of Velocity Pressure Calculator

• CO2 High Limit/MALL (Mixed Air Low Limit) Selector

• Concentration Viewer

• CO2 Concentration Filter

Overview of MVCA Strategy

This strategy, when applied in conjunction with communication to aworkstation, provides the ability to document compliance withventilation rate codes.

The MVCA strategy measures the quantity of outdoor air flowing intothe conditioned space, applies closed loop PI control, and enforcesthree (automatically selected) low setpoint limit values for the OAflow rate. The OA flow rate is normally determined by theconventional discharge air temperature and economizer controls, withlow setpoint limits imposed to conform to ASHRAE Standard 62-1999under all operating circumstances. The new low setpoint limits for theOA Flow Controller setpoint are:

• Low setpoint limit for OA flow requirements based on expectedoccupancy.

• Low setpoint limit for building pressurization requirements.


Note: If the pressurization strategy employed is the fan volumematching method, then this low limit would be equal to thecubic feet per minute (cfm) setpoint of the volume matchingcontroller. Alternatively this limit can be determined in thesystem commissioning process or calculated to make up forthe sum of exhaust fan volumes.

• Low setpoint limit for OA flow requirements based on theCO2 concentration level in the conditioned space.

Note: DX-9100 logic uses differential (indoor - outdoor) CO2

concentrations.

Online Measurement of Outdoor Air Flow Rate

ASHRAE Standard 62-1999 states: “When mechanical ventilation isused, provision for air flow measurement should be included” and“sufficient ventilation shall be demonstrable.” The HVAC industryinterprets this to mean that for VAV systems, measurement of the OAflow is required to meet the standard. An additional benefit of OA flowmeasurement is to improve the operation of space pressurization andmixed air controls.

Outdoor air flow is measured indirectly, using the CO2 ConcentrationBalance measurement method. In this method, the OA flow iscalculated from Supply Air (SA) flow (which is relatively easy tomeasure directly), and from three CO2 concentrations. OA, SA, andReturn Air (RA) CO2 concentrations are used to compute the fractionof OA in the SA stream. This provides a calculated OA flow value as acontrolled variable input for the Outdoor Air Flow Controller.

The volumetric concentration balance for the outdoor and RA streamsbeing mixed can be calculated for any tracer gas injected into the airstreams. Since human respiration generates significant amounts of CO2

in the RA stream and economical CO2 sensors are available, CO2 is agood tracer gas for this method. CO2 has been used in some HVACapplications as an indicator of occupancy.

Note: In the CO2 Concentration Balance method of indirect OAflow measurement, CO2 is not used as an occupancyindicator, but as a tracer gas. This is a novel method foronline OA flow measurement, presently offered only byJohnson Controls.


Closed Loop Control of Ventilation Air Flow Rate

When online flow measurement is used in conjunction with closedloop control, response to the varying demands for building makeup airis possible. This hardware and software provide a method ofimplementing an Outdoor Air Flow Controller that performsclosed-loop PI control of OA flow for ventilation per ASHRAEStandard 62-1999. The Outdoor Air Flow Controller functions areintegrated with the conventional discharge air temperature control,economizer control and space pressurization (fan volume matching)control such that the OA flow for ventilation and pressurizationrequirements represent low setpoint limits for the range of OA flowcontrolled by these conventional controls.

Time Scheduled Ventilation

To ensure sufficient ventilation, the OA Flow Setpoint Selectorscheduling input could be set to provide ventilation for the maximumoccupancy at all times. It is recommended however, that the OA FlowSetpoint Selector be time-scheduled to reflect the expected occupancyprofile. This minimizes the energy cost in systems with variableoccupancy while conforming to the ASHRAE Standard 62-1999 VRP.The Setpoint Selector scheduling input is operator determined to meetthe minimum OA flow requirement (for example; for an office spacewith a 100 person occupancy, the setpoint would be100 x 20 cfm/person = 2000 cfm). For a variable occupancy space, theOA Flow Setpoint Selector scheduling input can be changed accordingto a time programmed occupancy profile (Figure 3).

OutdoorAir Flow

Time of Day

Ventilation forMaximum ScheduledOccupancy

Ventilation for ScheduledOccupancy

minimum

12

Unoccupied Time

Minimum Outdoor AirFlow Required to MaintainSpace Static PressureControl (Varies with Exhaust Fan Activity)

Note: See Figure 6 in the section for an explanation of how this figure’s numbered points fit into the MVCA strategy.

Integration with Conventional AHU Controls

Figure 3: Time Scheduled Minimum Outdoor Air Flow for Ventilationand Space Static Pressure Control


CO2-Based High Limit Ventilation Control

For this function, DX-9100 logic uses differential (indoor - outdoor)CO2 concentrations.

In this feature, a CO2-based high limit ventilation control functionmonitors the CO2 concentration in the RA duct or in a critical zone(a critical zone is one most likely to experience occasional variableoccupancy at a higher than expected level) and resets the setpoint ofthe OA Flow Controller should the CO2 concentration approach apredetermined high level. The key words here are high limit. TheOA flow setpoint can only be adjusted by this function above the baselevel determined by expected occupancy or space pressurizationrequirements.

When used alone (without OA flow measurement and closed loopventilation control) the common HVAC industry terminology forCO2-based ventilation control is demand controlled ventilation.It should be emphasized that demand-controlled ventilation alonedoes not meet the requirements of the ASHRAE Standard 62-1999Ventilation Rate Procedure. A high level of CO2 is an indicator ofinsufficient ventilation, however a CO2 level below the maximumallowable limit of the CO2 high limit controller does not ensure thatventilation is sufficient to meet the standard. The CO2 concentrationin a RA duct or in a critical zone reaches a steady level only some timeafter the higher-than-expected occupancy begins. During lowoccupancy periods, the CO2-based demand controlled ventilationwould be reduced and the concentration of the air contaminants fromsources other than people may reach unacceptably high levels. Thus,demand controlled ventilation based only on CO2 measurement canresult in ventilation lower than required.


The CO2-based high limit ventilation control, however, is animportant supplement to ventilation control based on expectedoccupancy and makes the ventilation control adaptive to non-typicaloccupancy conditions. In the MVCA strategy, CO2-based high limitventilation control uses CO2 measurement in the RA duct or,preferably, in selected critical zones of the occupied space.(This optional feature does not use CO2 as a tracer gas but as anoccupancy indication.) It provides a low setpoint limit control ofventilation air flow rate such that the CO2 concentration does notexceed a preset limit. The CO2 high limit control function thuscomplements the scheduled OA flow function. The operator canspecify a relatively aggressive occupancy profile based setpointschedule, for the Outdoor Air Flow Setpoint Selector scheduling input,which minimizes the energy cost. The actual OA flow rate iscontrolled by the higher of the two ventilation needs, one reflecting theexpected occupancy (scheduled ventilation) and the other reflecting theactual occupancy (demand ventilation). See Figure 4.

Ventilation forMaximum ScheduledOccupancy

Ventilation for ScheduledOccupancy

CO -based High Limit Ventilation ControlStrategy Increases Ventilation Rate in Response toHigher-than-Scheduled Occupancy(e.g., an Overpopulated Conference Room)

2

2

3

1

highlimit

OutdoorAir Flow

Time of Day

Unoccupied Time

Minimum Outdoor AirFlow Required to MaintainSpace Static PressureControl (Varies with Exhaust Fan Activity)



Figure 4: Minimum Outdoor Air Flow for Ventilation and Space StaticPressure Control with CO2-Based High Limit Ventilation Control

This feature requires a RA duct CO2 sensor, or preferably awall-mounted CO2 sensor placed in a representative critical zoneof the occupied space. (With the MVC-2000 Multipoint CO2 Samplermethod for OA flow measurement, the RA duct CO2 sensing point isalready provided.)


Return Air CO2 Alarm Capability

For this function, DX-9100 logic uses differential (indoor - outdoor)CO2 concentrations.

The optional RA CO2 alarm utilizes the CO2 measurement in theRA duct or in a critical zone to provide operator alarms indicating apossible mechanical system failure, or sensor failure. The alarmfunctions are illustrated in Figure 5. A high alarm triggers an operatormessage display in case the CO2 concentration exceeds the ASHRAEStandard 62-1999 limit of 1000 parts per million (ppm) with a650 ppm differential. A high limit warning triggers an operatormessage display if the CO2 concentration exceeds the upper CO2

concentration limit established for the CO2 high limit control function,typically 800 ppm (450 ppm differential). The Return Air CO2 AlarmProcessor also sends a diagnostic message if the CO2 concentrationfalls below the expected low level, signifying a sensor fault or a needfor system calibration.

OSHA Permissible Exposure Limit

ASHRAE Limit 1999

Typical Office

Outdoor Air

5000

1000

500

420

350

800

Alarm

Warning

SensorAlarm

300 ppm

800 ppm

1000 ppm

capability

CO Concentration(Parts Per Million)

2

Figure 5: Typical CO2 Concentrations andReturn Air CO2 Alarm Capability



A typical implementation of the MVCA strategy for closed-loopcontrol of OA flow is illustrated in Figure 6. The numbered signallines are intended for coordination of Figure 6 with Figure 3, Figure 4,and Figure 7.

SequencingDevice

HC

CC

OA Flow SetpointSupply vs. Return AirflowDifferential Setpoint

CO Outdoor Air2

CO Supply Air2

CO Return Air2

2

1

Supply AirTemperatureSetpoint

SequencingDevice

SequencingDevice

4

OutdoorAirflow

Calculator

SpacePressurization

Controller(Fan Volume

Matching Method)

Discharge AirTemperature

Controller

Flow MeasuringStation

ExhaustAir

OutdoorAir

ReturnAir

SupplyAir

Economizer

EconomizerSwitchoverTemperatureSetpoint

Maximum and MinimumCO Limitfor High Limit Control

2

OA FlowSetpoint

HighSelector

OA FlowContoller

Time ScheduleOA Flow Setpoint

3

ControllerManager

Supply AirStatic PressureSetpoint

Supply AirStatic Pressure

Controller

Supply AirStaticPressureSensor

C0High LimitController

2

implement

MixedAir

Figure 6: Outdoor Air Flow Controls, Typical Implementation


The Outdoor Air Flow Calculator implements the CO2 ConcentrationBalance calculation and provides a calculated OA flow value as acontrolled variable input for the Outdoor Air Flow Controller. Thesetpoint of the Outdoor Air Flow Controller is determined by theOutdoor Air Flow Setpoint Selector as the highest of three signals:the CO2 high limit controller function output, the time scheduledOA setpoint, and the fan volume matching differential setpoint.

The Outdoor Air Flow Controller is implemented as a reverse actingDirect Digital Controller (DDC) that uses a Proportional-Integral (PI)control algorithm.

The Outdoor Air Flow Controller output determines the position of thecoupled mixed air dampers. The RA damper operates in a directionopposite to the outdoor and exhaust air dampers. As the controlleroutput increases, the outdoor and exhaust air dampers open andsimultaneously the RA damper closes. This arrangement provides afull range of control for the Outdoor Air Flow Controller. The OutdoorAir Flow Controller can control the OA flow up to 100% of SA flow.

The Outdoor Air Flow Controller output represents the lowestallowable control signal for the OA damper (a minimum position ofthe coupled mixed air dampers) required to provide the OA flow forventilation. The Outdoor Air Flow Controller is a low limit control thatoverrides the mixed air damper’s control signal provided byconventional discharge air temperature/economizer controls. Figure 6schematically shows these controls. A discharge air temperaturecontroller sequentially modulates the heating valve, mixed airdampers, and the cooling valve. The damper control signal is furtherprocessed by an economizer that positions the dampers to a presetminimum position when the OA temperature is above the economizerswitch over temperature. The Controller Manager is used to select(with logic to ensure stability) the higher of the Outdoor Air FlowController and the discharge air temperature controller/economizerdamper control signals and uses it to control the mixed air dampers.


Figure 7 illustrates the relationship of the OA flow control and thedischarge air temperature/economizer controls. In the free cooling zoneand below the economizer switch over temperature, the conventionaleconomizer controls will control the mixed air dampers to provideOA flow higher than that required for ventilation. The Outdoor AirFlow Controller functions, along with the Outdoor Air Flow SetpointSelector and its three input signals, to provide a low limit of thesetpoint for the OA flow. The Outdoor Air Flow Controller can takeover the control of mixed air dampers at cold OA temperatures, and atwarm OA temperatures above the economizer switch over temperature.

OutdoorAir Flow

Outdoor Air Temperature

0 %

100 %

4

EconomizerSwitchover

Temperature

economizer

System operates inFree Cooling mode.

System operates withmaximum outdoor airand mechanical refrigeration.

System operates withminimum outdoor airand mechanical refrigeration.

Minimum Outdoor Airfor Ventilation and SpaceStatic Pressure Control

Minimum Outdoor Airfor Ventilation and SpaceStatic Pressure Control



Figure 7: Outdoor Air Flow with Discharge Air Temperature Controland Economizer


As previously discussed and as illustrated in Figure 4, CO2-based highlimit ventilation control can be utilized as a supplement of the baselineventilation control for scheduled occupancy. The CO2-based high limitventilation control function is schematically shown as a span block inFigure 6. As its controlled variable input, the high limit function uses aCO2 concentration value measured by a CO2 sensor placed in theRA duct, or preferably, in a critical zone of the occupied space(for example, a conference room, class room, cafeteria, etc.).(Again, with the MVC-2000-1 Multipoint CO2 Sampler method forOA flow measurement, the RA duct CO2 sensing point is alreadyprovided.) The upper limit of the span block reflects the maximumallowable CO2 concentration and would be typically set at 800 ppm(450 ppm differential), well below the maximum occupied space limitof 1000 ppm specified by the ASHRAE Standard 62-1999. The spanblock output represents one of three low setpoint limits for theOutdoor Air Flow Controller, determining the minimum opening ofthe OA damper.Note: DX-9100 logic uses differential (indoor - outdoor) CO2

concentrations.Another low setpoint limit for the OA flow is represented by theoutdoor makeup air flow required for maintaining space pressurization.Controls for VAV systems must ensure a minimum OA flow for spacepressurization. Building pressure quickly go negative if makeup air isnot provided to replace exhausted air. Figure 6 schematically showsthe typical fan volume matching controller, in dual fan systems, thatattempts to maintain a positive space static pressure by maintaininga fixed difference between the supply and RA flows.In conventional systems the positioning of the OA damper(by discharge air temperature control and economizer controls) limitsthe potential range of outdoor makeup air flow for pressurization.If the OA damper is not sufficiently open, OA availability for buildingpressurization will be limited and the volume matching flow controlstrategy will be starved. The volume matching controls reduces thereturn fan volume to the minimum possible and yet will not be able tomaintain the required differential between the supply and RA flow andwill, therefore, lose control over building pressurization.The OA Flow Controller provides a means to improve spacepressurization control as load varies in a VAV system. The setpointof the volume matching controller represents the minimum requiredoutdoor makeup air flow rate. In this MVCA strategy, the volumematching controller setpoint is used as an input to the OA FlowSetpoint Selector. This arrangement ensures that the OA flow ascontrolled by the OA Flow Controller is never lower than the requiredmakeup air flow for building pressurization.


The control arrangement shown in Figure 6 implements the highestlow setpoint limit for the OA flow illustrated in Figure 4. Therequirements of scheduled OA flow control, CO2 high limit controland space static pressure control together (acting through the SetpointSelector and OA Flow Controller), establish a minimum value for theOA flow control signal provided by the conventional discharge airtemperature/economizer controls. It ensures the required ventilation airflow and also improves the reliability of the space static pressurecontrol. The minimum makeup OA flow required by the fan volumematching controller is maintained under all system load conditions.

System Applicability

The MVCA strategy employing the CO2 Concentration Balancemethod is applicable to any air handling unit controlled by MetasysApplication Specific Controllers (ASCs). This strategy depends oncommunication over a network to a workstation for documentation ofventilation performance. It does not require a flow measuring station inthe OA intake. This strategy provides a comprehensive set of tools thatcan be used to implement the ASHRAE Standard 62-1999 VentilationRate Procedure on a variety of systems. Figure 10 provides anoverview of the required and optional strategies and of alternativeimplementations. The choice of individual strategies depends on thetype of air handling unit, occupancy profile, existing controls,mechanical equipment configuration, and also on the available budget.

For constant volume systems, the ventilation air flow rate for expectedoccupancy can be established by the TAB (Test Adjust Balance)process. If any of the following functions are desired, the MVCAstrategy can be implemented in constant volume systems similarly toVAV systems:

• variable OA flow rate for scheduled occupancy

• documentation of the OA flow rate

• overcome significant wind effects that are causing problems withbuilding ventilation

For variable air volume systems, the ventilation air flow rate must bemeasured and controlled. Direct OA flow measurement can be used ifthe configuration of OA duct and dampers allows installation of a flowmeasuring station with sufficient lengths of straight duct run and withhigh enough air velocity to provide accurate measurement. In addition,direct measurement requires consideration of the OA design conditionsand the temperature rating of the flow measuring station. IndirectOA flow measurement requires only that the system be equipped witha SA flow measuring station.


The OA flow rate can be controlled at a fixed level corresponding tothe highest expected occupancy. In systems serving spaces withvariable occupancy, controlling the OA flow rate according to a timeschedule based on the expected occupancy profile can reduce energyusage. The OA flow control can be optionally complemented by CO2

high limit control and by RA CO2 Alarms.

The strategy shown in Figure 1 can be implemented on existingsystems by adding sensors and controls but requires no changes inmechanical equipment (assuming the system has capacity to handle theincreased OA flow for ventilation). Thus, this strategy is specificallysuitable for retrofitting existing systems. The method does not requireany ductwork modifications and is easy to install. Because theCO2 sensing is not affected by the mechanical equipment configuration(preheat coils, heating or cooling coils, humidifiers, etc.), theCO2 Concentration Balance method can be utilized on any type of airhandling unit.

The strategies shown in Figure 6 are central system-level OA flowcontrol strategies. They ensure the required ventilation, but assumeproper air distribution in the individual zones of the occupied space.Strategies for zone-level OA flow control in multiple zone systems arenot considered at this time.

In system design, sufficient heating, cooling, humidification, anddehumidification capacities must be provided to reflect the higher OAflow required for ventilation. In cold climates, the system design mustincorporate provisions for freeze protection, preferably a preheat coilin face/bypass configuration. Existing freeze protection thermostatsmay require relocation or recalibration to avoid nuisance tripping at thehigher ventilation air flow rates.

When adding ventilation controls per the ASHRAE Standard 62-1999to an existing air handling unit, it may be necessary to install a largercooling coil and/or a preheat coil in order to accommodate the possiblyincreased OA flow rate and still maintain control of mixed air andSA temperature.


MALL (Mixed Air Low Limit) Considerations

MALL controls are operating controls (not safety devices) employedprimarily to limit mixed air temperature to an operator adjusted lowlimit during economizer operation in cold environments. This preventsthe economizer from delivering air that is too cold for the occupantswho may experience this air before it is well mixed in the conditionedspace. Perhaps more significantly, it helps prevent nuisance tripping offreeze prevention thermostats due to momentary low temperaturescaused by stratification and/or cycling controls.

MALL control operation does not consider ASHRAEStandard 62-1999 VRP minimums and goes below that level if themixed air temperature demands it. Operation below VRP minimums,due to MALL, is most likely to occur during the coldest months of theheating season.

The MVCA strategy is intended to eliminate MALL in order toprovide the required ventilation under all conditions, even in coldclimates. The Controller Manager automatically overrides any MALLresiding in an application specific discharge temperature/economizercontroller. A pneumatic MALL is recommended to be disabled whenMVCA is installed and commissioned.

This may require a preheat coil in the OA intake (face and bypassconfiguration recommended) to allow for the required OA ventilationflow while avoiding the nuisance of false alarms provided by freezeprevention thermostats.

The above is the standard application (which satisfies the ventilationrequirements); however, if a customer requires MALL, it can beprovided relatively easily if the damper actuation is pneumatic.A pneumatic MALL (T-3610) can be provided, which overrides theventilation controls because it acts directly on the actuator signal.The customer needs to be made aware that this compromises the VRPventilation rates during cold weather. The MALL setting should be nohigher than that required to prevent nuisance tripping of the freeze upthermostats.

If the damper actuation is electric and MALL must be provided, usethe MVC-2000-1 and program the external dischargetemperature/ economizer controller to include MALL overriding theventilation controls. This is less expensive than adding a preheat coilbut includes the penalty of not meeting ventilation standards duringpart of the season.


Common Ductwork (Common Plenum) Considerations

Configuration of the supply, return and exhaust ductwork (plenums)must be considered for successful MVCA application. Any of theseplenums could be common to other air handling units. Typicalsituations include:

• Common Supply Plenum: Add MVCA to all the air handlerssupplying air to the common plenum at the same time. AddingMVCA to only one of several air handlers on a common plenumamounts to trying to ventilate all the conditioned space with only aportion of the system. To meet the ASHRAE Standard 62-1999VRP requirements, apply one MVCA to each of the air handlers onthe common plenum. For accuracy of the OA flow calculation, theSA CO2 sensing points must be before the supply duct joins thecommon plenum.

• Common Return Plenum: MVCA can be successfully applied toindividual air handlers on a common return plenum if the supplyplenum is not common. For accuracy of the OA flow calculation,the RA CO2 sensing point must be in the return stream to theindividual air handler. If the optional CO2-based high limitventilation control function is used, a separate space mountedCO2 sensor is required.


CommonSupply

CommonReturn

x

xx

xx

x

x CO Sensorin Critical Zonesfor Carbon Dioxide-basedHigh Limit Ventilation Control(Optional)

2

AHU 1 AHU 2 AHU 3

Zone 1

Zone 2

Zone 3

Zone 4

Zone 5

Zone 6

x Carbon DioxideSensing Locations

common

SA

RA

EAEA EAOAOA

RA

SA SA

RA

OA

EA - Exhaust AirRA - Return AirOA - Outside AirSA - Supply Air

RARARA

Figure 8: Common Supply and Return Plenum


Implementation of Outdoor Air Flow Control (MVCA) Strategy

Tracer Gas Concentration Balance Method and MVC-2000-1Multipoint CO2 Sampler

The multiplexed method of CO2 measurement (MVC-2000-1) that isused to provide accurate CO2 concentration values for the OutdoorAir Flow Calculator has additional capabilities in that it cancompensate for exhaust air bypass and mixing plenum air leaks. It isthe only method that can distinguish between outdoor and re-entrainedreturn or exhaust air.

The method is derived from equations describing the mixing of theoutdoor and RA streams in a common air handling unit (as illustratedin Figure 1 or Figure 6). Each of these air streams contains someconcentration of the tracer gas, CO2.

The outdoor air flow rate can then be determined as:

SAOA2,RA2,

SA2,RA2,OA cfm

COCO

COCOcfm ⋅

−−

=

using the SA volumetric flow rate in cfm (or m3/sec) and the CO2

concentrations in ppm (parts per million).

The expression OARA

SARA

COCO

COCO

,2,2

,2,2

−−

can be viewed as a flow coefficient

that determines the OA fraction in the SA. The typical RA CO2

concentration in an occupied building is in the range of500 to 1000 ppm while the OA CO2 concentration is in the range of350 to 450 ppm. The mixing of the outdoor and RA streams alwayscauses the SA CO2 concentration to be higher than that of the OA andlower than that of the RA. When the outdoor and exhaust air dampersare fully closed and all the RA is being recirculated, the SA CO2

concentration is equal to that of the RA and the flow coefficient has avalue of zero, correctly indicating that no OA is being introduced intothe space. When the outdoor and exhaust air dampers are fully open,the SA CO2 concentration is equal to that of the OA and the flowcoefficient has a value of one, indicating that the air handling unit isusing 100% OA.


A single CO2 sensor with a sampling air pump and appropriatesoftware is used to measure and store, in sequence, CO2 concentrationsof the three air streams. Two solenoid air valves are used to connectthe appropriate sampling line to the air sampling pump and to thesensor. Adequate time is provided for purging each sampling line andfor the time response of the CO2 sensor. The complete multiplexingcycle takes several minutes. This causes a dead time in the control loopresponse that has to be reflected in selecting the initial tuningparameters for the controller. Please refer to the Metasys softwareapplication note descriptions for the typical initial tuning parameters.

With the use of a single CO2 sensor, the relative differences betweenCO2 concentrations can be measured with an error of less than 5 ppm.The effect of sensing errors such as drift, temperature effect, and shortterm output variations is identical for all three CO2 measurements.Because the flow coefficient requires only calculation of the ratio ofthe CO2 differentials, the identical errors in the individualmeasurements will cancel out. Only infrequent field calibration of theCO2 sensor is required because only the differentials are used, ratherthan absolute values.

The RA CO2 concentration, one of the three CO2 concentrations readand stored during the multiplexing cycle, can be utilized in some casesfor purposes other than indirect OA flow calculation. For example,it can be used for CO2 high limit control and for Return Air CO2

Alarms. For these applications, when an absolute CO2 measurement isneeded, accurate CO2 sensor calibration is required.


concentrations.


CO2 Sensing Point Location

Selection of the CO2 sensing locations is relatively simple. SeeFigure 1. The sampling tube [typically a 6.35 mm (1/4 in.) diameterplastic tube] is inserted into the duct in any convenient and easilyaccessible section of the ductwork. Note that, contrary to temperaturesensing, the CO2 concentration in mixed air is identical to theCO2 concentration in the SA. Therefore, there is never any need tosense CO2 in the mixed air plenum where an averaging sensing probewould be required. Because the CO2 concentration of an air stream isnot affected by heating coils, cooling coils or humidifiers, the sensingpoint for the supply can be located downstream of the supply fan toensure that the outdoor and RA streams are well mixed and haveminimum stratification. The RA sensing point can be located in theRA duct, upstream or downstream of the return fan, using a tube of upto 30.48 m (100 ft) in length.

The SA sensing point is subject to the fastest changes inCO2 concentration, as the linked dampers change position. Whenpresented with choices regarding equipment location, mount theMVC-2000-1 in a location that will minimize the length of the SAsensing tube, using a tube of up to 9.14 m (30 ft) in length.

The OA sensing point should be located in free air outside the buildingor, alternatively, in the OA intake. The preferred method is placing theOA CO2 sensing point in free air outside of the building (this wouldrequire a weather protected enclosure to avoid entrainment of rain orsnow by the sampling tube). If the OA CO2 sample is obtained from alocation that is isolated from the building exhausts, the CO2

Concentration Balance method automatically compensates for air thatshort-cycles from the exhaust louvers to the OA intake. Either locationcompensates for air that short cycles from the fan room into the mixingplenum.

Just by placing the OA CO2 sensing point in a location that isisolated from the building exhausts, this method allows calculationof the true fresh air portion of the OA flow intake from thethree CO2 measurements and the SA flow. The outdoor CO2 sensingpoint, if placed in the OA intake duct for convenience reasons, shouldbe placed far enough on the upstream side of the OA damper so that itsreading is not affected by a possible backwash of the mixed air atlarger OA damper openings. A good practical test is to check theOA CO2 sensing point reading while positioning the OA damper fromits fully closed to its fully open position and verify that the sensorreading does not change (this assumes no re-entrainment of exhaust airby the OA intake.)


Limitations of the CO2 Tracer Gas Method

The CO2 Concentration Balance method of OA flow measurement canbe used as long as there is at least a 30 ppm gain in the RA CO2

concentration over the OA CO2 concentration (CO2,RA – CO2,OA).The typical CO2 gain in an occupied building ventilated in compliancewith ASHRAE Standard 62-1999 is much higher, approximately150 to 450 ppm. A CO2 gain as low as 30 ppm or less can occur onlywhen the OA flow is significantly higher than that required forventilation, for example when a large amount of OA is used for freecooling. It can also occur during very low occupancy when theminimum OA flow required for maintaining space static pressure isagain significantly higher than the ventilation air flow required for thegiven occupancy level. The CO2 differential will be too low for anaccurate calculation only during the times when the control of OA flowis taken over by discharge air temperature controls, or by space staticpressure controls. At these times, the OA flow is controlled at a levelhigher than that required for ventilation. A backup controller functionis provided to ensure continuous stable operation for spacepressurization purposes.

CO2 Sensor Auto Zero Function

The Auto Zero software appropriately commands the two solenoid airvalves to read the OA CO2 concentration for one hour daily, between2:00 a.m. and 3:00 a.m. An algorithm resident in the CO2 sensorrequires that the sensor be exposed to a CO2 concentration of 500 ppmor less for 30 minutes per day.

Outdoor Air Flow Controller Backup

The indirect OA flow measurement methods can only functionproperly when there are sufficiently large differentials between theCO2 concentrations used for the calculation. During time periods oflow differential, the output of the OA Flow Calculator is notdependable and is forced to a value of zero. When this occurs, theOA Flow Controller output is temporarily replaced by a backup valuethat determines the OA damper position. In the DX-9100implementation of this application, the backup value is simply anoperator defined minimum damper position.


Return Air CO2 Alarm Capability


concentrations for this CO2 Alarm Capability. (The differentialtechnique is preferred for locations with widely varyingoutdoor CO2 levels.)

This Alarm function is used to monitor the CO2 concentration in theRA duct or in a critical zone of the occupied space. The use of aMVC-2000-1 Multipoint CO2 Sampler, a RA duct CO2 sensor or, awall mounted CO2 sensor placed in the critical zone is required. Thealarm functions are illustrated in Figure 5.

When the RA CO2 Analog Data (AD) constant (DX-9100) is in highalarm, it triggers an operator message display in case the CO2

concentration exceeds the ASHRAE standard limit of 1000 ppm.

When the RA CO2 AD constant (DX-9100) is in high warning, ittriggers an operator message display in case the CO2 concentrationexceeds the upper CO2 concentration limit established for the CO2

high limit controller (typically 800 ppm).

The Return Air CO2 Alarm Processor also displays a diagnosticmessage if the CO2 concentration falls below the expected low levelof OA CO2 concentration (300 ppm), signifying a sensor fault or aneed for system calibration.

Controller Manager

This logic selects between the Outdoor Air Flow Controller and theconventional discharge air temperature controller/economizer forcontrol of the position of the mixed air dampers. Time delay forcontrol loop stability is applied. The selection criterion is the OA flowrate, the controller that provides the higher OA flow rate is selected.


Implementation Requirements

To successfully apply this control strategy, the following conditionsand equipment are needed (also see Figure 8):

• A Variable Air Volume (VAV) air handling unit (or constant airvolume air handling unit) controlled by an ACS (Access ControlSystem) and using a workstation for trending and documentation.

• A minimum of one SA flow station (both supply and returnstations preferred).

• The MVC-2000-1 Multipoint CO2 Sampler and controller areessential for measurement and control of OA flow. Refer to theMetasys Ventilation Controller Application for the DX-9100Technical Bulletin (LIT-653425) and order through customerservice in Milwaukee.

• The Metasys DX-9100 MVCA program module files (*.dxs) andthe GX-9100 programming tool.

MVCA Software

This section discusses Metasys DX-9100 programmable modules thatimplement the MVCA Strategy. Each module is described briefly. Formore information, see the Accessing MVCA Modules Via GX-9100Software section in this document.

You can access every module of the MVCA for the GX-9100 from thestart screen of the application. Figure 9 is an example of a modulescreen. See Table 2 for an overview of the attributes within eachmodule.


Figure 9: Sample Screen from GX-9100 Software

The recommended minimum installation for monitoring only includesthe following modules:

• CO2 Multiplexer

• Outdoor Air Flow Calculator


The recommended installation for ventilation control includes theabove plus the following modules:

• Flow Zero Function

• Sequencer for the CO2 Multiplexer

• Denominator Check

• Outdoor Air Flow Controller

• Outdoor Air Flow Setpoint Selector

• Supply Air Flow Calculator

• Square Root of Velocity Pressure Calculator

• CO2 High Limit/MALL Selector

• Concentration Viewer

• CO2 Concentration Filter

CO2 Multiplexer

The CO2 Multiplexer provides sequence control for the MVC-2000-1Multipoint CO2 Sampler and implements the multiplexed CO2 sensingmethod as described in the Tracer Gas Concentration Balance Methodand MVC-2000 Multipoint CO2 Sampler section. The CO2 Multiplexercontrols the switching sequence of two solenoid air valves used in theMVC-2000-1, and reads and stores the CO2 sensor output analogvalues representing CO2 concentrations in the outdoor, supply, andreturn air.

Outdoor Air Flow Calculator

This calculator function implements the CO2 Concentration BalanceMethod of indirect OA flow measurement. It is used in conjunctionwith the CO2 Multiplexer and requires the use of the MVC-2000-1.The sampler provides the input analog values representing CO2

concentrations. The calculator receives four input analogvalues-CO2 concentrations of outdoor, supply and RA, and the SAflow.

The calculator implements the CO2 Concentration Balance Methodformula given in the Tracer Gas Concentration Balance Method andMVC-2000-1 Multipoint CO2 Sampler section. In order to ensure apositive value of the flow coefficient and ensure that its value alwaysis within the limits, even during transient conditions, the range islimited. The flow coefficient values are limited to a 0 to 1 range.

The Outdoor Air Flow Calculator provides the numerator of the OAflow equation and displays CO2 measurements from the variousairstreams.


Flow Zero Function

Research shows that when the difference between the CO2

concentrations in the RA and OA become very small (less than30 ppm), the calculated OA flow will be inaccurate. If this is the case,the calculated OA flow is zeroed. If the concentration difference ismore than 30 ppm, the calculated OA flow is multiplied by one.

Sequencer for the CO2 Multiplexer

This module sequences through the three airstreams as triggered by theexpiration of the timer in the CO2 Multiplexer

Denominator Check

The Denominator Check tests the denominator of the calculated OAflow fraction. The RA CO2 concentration minus the OA CO2

concentration should be less than 30 ppm. The Denominator Checkalso compares the measured SA flow against a minimum supply airflow rate. A measured supply air flow should be greater than3000 cfm.

Outdoor Air Flow Controller

The OA Flow Controller provides closed-loop control of the OA flowrate. It is configured as a PI controller and its interaction with the restof the air handling unit controls is shown in Figure 6. As its input, itaccepts the OA flow analog value calculated by the OA FlowCalculator. The setpoint is determined by the OA Flow ControllerSetpoint Selector and can be fixed or time-scheduled to reflect theexpected occupancy profile, based on the OA flow requirementsspecified in ASHRAE Standard 62-1999 VRP. The output of the OAFlow Controller is one of the control signals used, after furtherprocessing, to control the mixed air dampers. The output connection tothe other mixed air damper controls is shown in Figure 6. The OAFlow Controller is configured as a Reverse Acting controller. Anincrease in the controller’s input (OA flow value) causes a decrease inits output (OA damper position). The Reverse Action is obtained byspecifying the proportional band as a negative number for DX-9100implementations.

The proportional band can be initially specified as a number higherthan the supply fan capacity (140% of supply fan cfm is suggested).For example, for an air handling unit with 14,000 cfm supply fancapacity, the initial proportional band for the Outdoor Air FlowController can be specified as 20,000 cfm.


The initial estimate of the integral time can be based on the expectedtime constant of the controlled process and on the dead time causedby the CO2 Multiplexer that provides the controllers input. As a rule ofthumb, it should be 200 seconds or more.

Outdoor Air Flow Setpoint Selector


concentrations. This is the preferred technique for locationswith widely varying outdoor CO2 levels.

This portion of the logic is used to select the highest setpoint for theOA Flow Controller, as well as providing closed-loop high limitcontrol of the CO2 concentration of the occupied space. In this CO2

high limit control function, it adjusts ventilation air flow rate such thatthe CO2 concentration does not exceed a preset limit.

The DX-9100 programmable module contains a standard span blockobject, serving as a high limit proportional only (P) controller. Itaccepts, as its input an analog value representing the CO2

concentration in the RA duct, or in a selected critical zone. Themaximum limit is typically set to 800 ppm, well below the ASHRAEStandard 62-1999 specified maximum limit of 1000 ppm.

The output of CO2 high limit control span block is one of the controlsignals used as a setpoint signal for the OA Flow Controller, whichcontrols the mixed air dampers. The CO2 high limit control span blockrelationship to other setpoint signals and the Setpoint Selector isshown in Figure 6. An increase in the input (CO2 concentration) causesan increase in its output (OA damper position setpoint signal).

The proportional band for the CO2 high limit control span block is setat 50 ppm, which means the lower limit of the span block is 50 ppmbelow its maximum limit. This narrow proportional band is necessaryin order to implement the limit control. Any larger proportional bandmay cause the CO2 high limit control function to interfere with theoperation of the other mixed air damper controls.

Supply Air Flow Calculator

This module uses the velocity pressure in the SA ductwork, the flowconstant, and the ventilation guidelines found in ASHRAEStandard 62-1999 to calculate SA flow. The Supply Air FlowCalculator receives the supply air flow from the Square Root ofVelocity Pressure Calculator. Then the Calculator multiplies thenumber of occupants in the space by the ventilation rate of 0.02 kcfmper person. This module also takes the high select of the calculatedventilation requirements and the low select MALL or the minimumdamper position.


Square Root of Velocity Pressure Calculator

This module calculates the square root of the velocity pressuremeasured in the SA ductwork. The Square Root of Velocity PressureCalculator receives the velocity pressure from the pressure transduceron the supply air flow measuring station. That number is thenmultiplied by 4.005 (kcfm) times the SA duct area (in square feet).

CO2 High Limit/MALL Selector

This module sets the high limit for CO2 concentration differencesbetween the RA and the OA. If this concentration difference exceedsthe high limit, the system switches to maximum supply flow. The highlimit protects against high levels of CO2 that can indicate high levelsof dangerous other contaminants.

The MALL protects against low temperatures in the mixed air used forventilation. This function prevents the temperature from fallingbecause of the economizer damper allowing too much cold OA intothe system. As the temperature rises, the damper open percentagesdrop.

Concentration Viewer

This function allows for viewing of the CO2 concentrations of allthree airstreams through the DX-9100 screen. No changes may bemade via this module.

CO2 Concentration Filter

The CO2 filter prevents particulate and moisture contamination of thepump and CO2-sensing chamber. However, filtering the CO2 signal isnot necessary to system operation. A filter weight of 0.0 is the mostfiltering and a weight of 1.0 is the least filtering. Different filterweights may be used for each airstream; outdoor CO2 concentrationschange the least while supply air concentrations change the most.


Attributes within the GX-9100 Modules

See Table 2 for a list of Module Attributes.

Table 2: Module Attributes

Attribute Name Description DefaultMultiplexer Timer (Mux Timr)

TIMER #1 TYPE Timer type: On Delay 4

Input Connection #1 60s xpir

Reset Connection #1 Unused

Time Period #1 Time each airstream flows through the solenoid valves 60

Time Units#1 Time units: seconds 0

TIMER #2 TYPE Timer type: pulse 1

Input Connection #2 60s xpir


Time Period #2 Time each airstream is sampled and held 2

Time Units #2 Time units: seconds 0

TIMER #3 TYPE Timer type: disabled 0

Input Connection #3 Unused


Time Period #3 10.


TIMER #4 TYPE Timer type: disabled 0

Input Connection #4 Unused


Time Period #4 10.


Outdoor Air Flow Calculator (FlowCalc)

CH#1 EQUATION TYPE Equation: K1*I1 + K2*I2 1

Input I1 Return Air CO2 concentration (ppm) CO2[PPM]

Constant K1 Constant 1.

Input I2 No input Zero



Input I1 Outdoor Air CO2 concentration (ppm) CO2[PPM]





Input I1 Supply Air CO2 concentration (ppm) CO2[PPM]




Continued on next page . . .


Attribute Name (Cont.) Description DefaultOutdoor Air Flow Calculator (FlowCalc)

CH#4 EQUATION TYPE Equation: K1*I1 - K2*I2 (Denominator of OA flow equation) 2

Input I1 Return Air CO2 concentration (ppm) RACO2 FV


Input I2 Outdoor Air CO2 concentration (ppm) OACO2 FV


Outdoor Air Flow Calculator (FlowCalc) Data 2

CH#5 EQUATION TYPE Equation: K1*I1 - K2*I2 (Numerator of OA flow fraction) 2

Input I1 Return Air CO2 concentration (ppm) RACO2 FV


Input I2 Supply Air CO2 concentration (ppm) SACO2 FV


CH#6 EQUATION TYPE Equation: K1*I1 / K2*I2 (Fraction of OA in the SA) 4

Input I1 Return Air CO2 minus Supply Air CO2 concentration RA-SA FV


Input I2 Return Air CO2 minus Outdoor Air CO2 concentration RA-OA FV


CH#7 EQUATION TYPE Equation: K1*I1 * K2*I2 3

Input I1 Ratio produced from Channel 6 (RA-SA/RA-OA) ActFloCo


Input I2 Measured Supply Air Flow SuplyFlo


CH#8 EQUATION TYPE Channel disabled 0

Input I1 Unused


Input I2 Unused


Outdoor Air Flow Zero (FlowZero)

Input #1-Connection FlowCoef

Input #2-Connection FlowCoef

Input #3-Connection Unused


Input #5-Connection RA-OA>30


High Limit-Connection 1.

Low Limit- Connection 0.

Input #1-K[n] 0.

Input #2-K[n] 1.

Input #3-K[n] 1.

Input #4-K[n] 1.

Input #1-C[n] 0.



Attribute Name (Cont.) Description Default

Input #2-C[n] 0.

Input #3-C[n] 0.

Input #4-C[n] 0.

Sequencer for CO2 Multiplexer (Mux Seq)

Sequence Module Mode Module mode: Sequential 2

Chain Next PM (0=N) 0

Define Sets 1

Stg #1 first of 1

Stg #2 first of 1

Stg #3 first of 0

Stg #4 first of 0

Stg #5 first of 0

Stg #6 first of 0

Stg #7 first of 0

Stg #8 first of 0

Increase/Ana. Conn Seq Step

Decrease Conn Unused

Fast Step Dwn Conn MuxReset

Disable Stage #1 Unused








Denominator Check (Denm Chk)

CHANNEL TYPE #1 Channel: High Limit Comparator 1

Analog Input #1 Return Air CO2 minus Outdoor Air CO2 concentration RA-OA FV

Set Point #1 Unused

Set Point Value #1 Maximum ppm CO2 from Analog Input 1 below whichChannel 1 is True

30.

Differential #1 2.

CHANNEL TYPE #2 Channel: Low Limit Comparator 2

Analog Input #2 Measured Supply Air flow SuplyFlo

Set Point #2 Unused

Set Point Value #2 Minimum SA flow rate (kcfm) above which Channel 2 is true 3.

Differential #2 0.5

CHANNEL TYPE #3 Channel: Disabled 0

Analog Input #3 Unused

Set Point #3 Unused

Set Point Value #3 0.

Differential #3 2.




CHANNEL TYPE #4 Channel: Disabled 0.

Analog Input #4 Unused

Set Point #4 Unused

Set Point Value #4 0.

Differential #4 2.

Outdoor Air Flow Control (OAFlowPI)

Ena Shutoff: 0=N 0

Shutoff Out Level 0.

Ena Startup:0=N 0

Startup Out Level 100.

Ena Symm Mode: 0=N 0

ExtForce Out Level Set to minimum OA damper position 10.

Ena PID to P: 0=N 0

Remote Mode: 0=N 1

Ena OFF Trans: 0=N 0

Process Variable Calculated Outdoor Air flow Calc OAF

Remote Setpoint Outdoor Air flow setpoint OAFSP Hi

Reference Variable Unused

Proportional Band Unused

Off Mode Control Unused

Standby Control Unused

Reverse Action Unused

External Forcing SuplySta

Output Bias Unused

Minimum WSP Unused

Maximum WSP Unused

Outdoor Air Flow Control (OAFlowPI) Data 2

Local Set Pt. (LSP) 0.

Proport. Band (PB) Set to 100% to 140% maximum SA flow (kcfm) -100

Reset Action (TI) 1.

Rate Action (TD) 0.

Standby Bias (BSB) 0.

Off Mode Bias (BOF) 0.

Symmetry Band (SBC) 5.

Err Deadband (EDD) Defaults at 1% of the proportional band 1.

Output Bias (OB) 0.

Out High Limit (HIL) 100.

Out Low Limit (LOL) 0.

Dev H.H.Limit (DHH) Set to greater than maximum SA flow 10.

Dev High Limit (DH) Set to greater than maximum SA flow 5.

Dev Low Limit (DL) Set to less than zero SA flow 5.

Dev L.L.Limit (DLL) Set to less than zero SA flow 10.




Minimum WSP (MNWS) Minimum Working Setpoint equals minimum SA flow (kcfm) 0.

Maximum WSP (MXWS) Maximum Working Setpoint equals maximum SA flow (kcfm) 40.

Outdoor Air Flow Setpoint Select (Stpt Sel)

Input #1-Connection CO2 Demand Controlled Ventilation (DCV) setpoint (kcfm) CO2 DCV

Input #2-Connection Minimum ventilation for acceptable IAQ, default is20 cfm/1000 per person times the number of occupants (kcfm)

20*Occup

Input #3-Connection Building Pressurization Setpoint (kcfm) BldPrsSp






High Limit-Connection Set to maximum SA flow 40.

Low Limit-Connection 0.

Input #0-K[n] 0.

Input #1-K[n] 1.

Input #2-K[n] 1.

Input #3-K[n] 1.

Input #4-K[n] 1.

Input #5-K[n] 1.

Input #6-K[n] 1.

Input #7-K[n] 1.

Input #8-K[n] 1.

Supply Flow Calculator (SA Flow)


Input I1 VelPr^.5


Input I2 Unused

Constant K2 Flow constant (4005/1000 for DX-9100 controller) (cfm) 4.005


Input I1 Flow constant in kcfm times the supply duct area 4005*A


Input I2 Unused

Constant K2 Supply duct area (ft2) 15.5

CH#3 EQUATION TYPE Equation: K1*I1 * K2*I2 (multiply occupancy and0.02 kcfm/person ventilation rate)

3

Input I1 Number of occupants Occupncy


Input I2 Unused

Constant K2 Ventilation rate (kcfm) 2.e-002

CH#4 EQUATION TYPE Equation: Maximum (K1*I1, K2*I2) (High select of the VRP) 6

Input I1 VRP DPR





Input I2 EconDpr%


Supply Flow Calculator (SA Flow) Data 2

CH#5 EQUATION TYPE Equation: Maximum (K1*I1, K2*I2) 6

Input I1 Answer from Channel 4 VRPorEcn


Input I2 Minimum Damper Position MinDprPs


CH#6 EQUATION TYPE Equation: MIN(K1*I1, K2*I2) 5

Input I1 Answer from Channel 5 #4orACO4


Input I2 Mixed Air Low Limit MALL


CH#7 EQUATION TYPE Equation: Channel Disabled 0

Input I1 Unused


Input I2 Unused



Input I1 Unused


Input I2 Unused


Square Root of SA Velocity Pressure (Vel Pres)






Input #6-Connection SupFloVP



High Limit-Connection 100.

Low Limit-Connection 0.

Eqn (1 or 2)-Connection 2

Input #0-K[n] 0.

Input #1-K[n] 0.

Input #2-K[n] 0.

Input #3-K[n] 0.

Input #4-K[n] 0.

Input #5-K[n] 0.




Input #6-K[n] 1.

Input #7-K[n] 0.

Input #8-K[n] 1.

Input #9-K[n] 0.

CO2 High Limit/MALL

Segment CH#1 #0-X Return Air CO2 minus Outside Air CO2 (ppm) 0.




Segment CH#1 Ana. Input-X Return Air CO2 minus Outside Air CO2 concentration RA-OA FV

Segment CH#2 #0-X Mixed Air Temperature (°F) 0.

Segment CH#2 #1-X Mixed Air Temperature (°F) 32.

Segment CH#2 #2-X Mixed Air Temperature (°F) working with the MPB(MALL Proportional Band)

32+MPB

Segment CH#2 #3-X Mixed Air Temperature (°F) 32+MPB

Segment CH#2 Ana. Input-X Mixed Air Temperature (°F) MixdAirT

Segment CH#1 #0-Y 0.

Segment CH#1 #1-Y 0.

Segment CH#1 #2-Y Maximum Supply Air Flow (kcfm) 40.

Segment CH#1 #3-Y Maximum Supply Air Flow (kcfm) 40.

Segment CH#2 #0-Y Economizer Damper Command: 0% 0.




View CO2 at Front Panel (viewing only)

Ena Shutoff: 0=N 0

Shutoff Out Level 0.

Ena Startup: 0=N 0

Startup Out Level 100.

ExtForce Out Level 0.

Ena PID to P: 0=N 0

Remote Mode: 0=N 0

Ena OFF Trans: 0=N 0

Process Variable Outdoor Air CO2 concentration OACO2 PV

Remote Setpoint #1 Supply Air CO2 concentration SACO2 PV

Ref. Variable #1 Unused

Proportional Band Unused

Off Mode Control Unused

Standby Control Unused

Reverse Action Unused

External Forcing Unused

Remote Setpoint #2 Return Air CO2 concentration RACO2 PV

Ref. Variable #2 Unused




Output Bias #1 Unused

Output Bias #2 Unused

Minimum WSP Unused

Maximum WSP Unused


CH#1 EQUATION TYPE Equation: K1*I1 + K2*I2 (filtering of Return Air CO2) 1

Input I1 Return Air CO2 present value RACO2 PV

Constant K1 Filter weight for the present value (K1 plus K2 must be 0.) 1.

Input I2 Return Air CO2 filtered value RACO2 FV

Constant K2 Filter weight for the filtered value (K1 plus K2 must be 0.) 0.

CH#2 EQUATION TYPE Equation: K1*I1 + K2*I2 (filtering of Outdoor Air CO2) 1

Input I1 Outdoor Air CO2 present value OACO2 PV


Input I2 Outdoor Air CO2 filtered value OACO2 FV


CH#3 EQUATION TYPE Equation: K1*I1 + K2*I2 (Filtering of Supply Air CO2) 1

Input I1 Supply Air CO2 present value SAVO2 PV


Input I2 Supply Air CO2 filtered value SACO2 FV




Input I1 Unused


Input I2 Unused



Procedure OverviewTo ensure the MVCA system will function correctly, evaluate thestatus of the entire HVAC system. Once the MVCA is running, checkthe dampers and system calculations.

Table 3: MVCA System Checks

To Do This Follow These Steps:

Evaluate the HVAC SystemIntegrity

Perform a walk-through inspection. Look forand correct unstable control loops. Replacebroken or disconnected linkages, correct ductseparation or leaks, and consider the state ofany separate exhaust systems. Confirm thatactuators, linkages, and controls are capableof achieving specific flow rates. Confirm OA,RA, and exhaust air dampers are controlled bythe same signal and can close tightly. Ensurethat variable frequency drives, inlet guidevanes, and other equipment used to modulatethe supply and return fan capacities are ableto achieve the specified flow modulation.Remove any mechanical or software stopsthat may limit OA, RA, and exhaust damperfunction.

Check the MVCA SystemStability

With the OA damper fully closed, RA damperfully open, and exhaust damper fully closed,verify at various SA flows that all RA isrecirculated and RA and RA flows are equal.Verify that OA temperature and CO2 sensingpoints are not affected by changes in the OAdamper position. If this occurs, relocatesensing points further upstream in the OAintake. Verify that when the OA damper is fullyopen, the mixed air temperature is equal tothe OA temperature, and the SA CO2 is equalto the OA CO2. If they are different, investigatethe sensor placement and calibration. Verifythat when the OA damper is fully closed, themixed air temperature is equal to the RAtemperature, and the SA CO2 is equal to theRA CO2. If different, consider sensorplacement and calibration. When SA stationand CO2 measurement are verified and theOA damper is fully open, verify that the RAdamper is fully closed and the OA flow isequal to the SA flow. If the OA flow is lowerthat the SA flow, trace for possible leaks withpowder and seal the leaks.

Access MVCA Modules ViaGX-9100 Software

Once the GX-9100 software is running, opendxmvca.dxs. When the module schematicappears, right click on the module you wish toedit. Select Data. Edit the attributes. Click OKwhen done. For modules with two screens,select Data 2 to see the next screen.


Detailed Procedures

Evaluating the HVAC System Integrity

Note: Be aware that, for the product team developing and testing thisstrategy, 70% of test site effort was spent to get the air handlingunit to operate properly before the application could proceed.If a retrofit application is involved, the equipment and controlsmust be evaluated to bring the system up to the intendedoperating level before applying this control strategy.

To evaluate the system, perform a walk-through inspection:

1. Look for and correct unstable control loops by checking transducerand pilot positioner calibration as well as controller tuning.

2. Replace broken or disconnected linkages.

3. Correct duct separations and major leaks.

4. Consider the controls and present operational state of any separatebuilding exhaust systems.

Note: Instability of any of the following existing control loops woulddegrade the operation of the OA Flow Controller: discharge airtemperature/economizer control loop, supply fan static pressurecontrol loop (which may in turn be affected by individual VAVbox control loop instability), and space pressurization (volumematching) control loop.

5. Confirm that the mixed air damper actuators, linkages, and controlsare operable and capable of achieving the specified flow rates.

6. Confirm that the OA, RA, and exhaust air dampers are controlledby the same signal and can close off without excessive leakage.

7. Ensure that the variable frequency drives, inlet guide vanes, orother equipment employed to modulate the capacity of the supplyand return fans is operating and capable of achieving the specifiedflow modulation for the job.

8. Remove any mechanical or software stops that may be limitingthe operation of the OA, RA, and exhaust air dampers.

Note: The minimum OA damper position for the existing controlsystem is redundant when this strategy is applied and should beeliminated. Separate minimum position OA dampers can bepermanently closed off or allowed to modulate in parallel withthe main OA dampers.


Checking the MVCA System Stability

Note: These checks, at fully closed and fully open OA damperpositions can, in addition to verification of the flow sensingaccuracy, also be used for rough verification of accuracy oftemperature and CO2 sensing and for verification of properplacement of OA temperature and CO2 sensing points.

Perform verification of new and existing equipment and controls withthe following functional checks at the extreme positions of the coupledOA, RA, and exhaust air dampers:

1. Verify when the OA damper is fully closed (0% position), thereturn (recirculating) air damper is fully open and the exhaust airdamper is also fully closed. Under this condition, verify that all RAis recirculated (with all separate building exhausts shutdown) andthe SA and RA flows are equal. Verify this at various supply flows.

Note: This method can be used as a quick check that verifies the SAflow station accuracy against the RA flow station.

2. Verify that the OA temperature and CO2 sensing points are notaffected by changes in the OA damper position. If this occurs,relocate the sensing points further upstream in the OA intake ductbecause the sensing points are being affected by the RA backwashfrom the mixed air plenum.

3. Verify that when the OA damper is fully open, the mixed airtemperature is equal to the OA temperature, and the SA CO2 isequal to the OA CO2. If large differences exist, investigate theplacement of sensing points and calibration of the sensors.

4. Verify that when the OA damper is fully closed, the mixed airtemperature is equal to the RA temperature, and the SA CO2 isequal to the RA CO2. Reconsider the placement of the sensingpoints if large differences exist.

5. Perform another functional check once the SA flow station andCO2 measurement are verified, with the OA damper fully open(100% position). Verify that, in this condition, the RA damper isfully closed and the OA flow (calculated from CO2 concentrations)is equal to the SA flow (measured by the flow station).

Note: Perform this check at various supply flows and identify anydifference between the two air flow measurements.


6. Determine if the OA flow is lower than the SA flow as it could becaused by leaks of equipment room air or RA into the negativelypressurized mixed air plenum. Trace the leaks with a powder gunand locate. Sealing the leaks enhances system performance, energyefficiency, and ventilation.

Accessing MVCA Modules Via GX-9100 Software

To access modules with GX-9100 software:

1. Open dxmvca.dxs. The module schematic appears (Figure 10).

Figure 10: MVCA in GX-9100 Software

2. Right click on the module you wish to edit.

3. Select Data from the options list.

4. Edit the attributes.

5. Click OK when done.

Note: For modules with two screens, select Data 2 to see the nextscreen.


Notes


Notes

Controls Group www.johnsoncontrols.com507 E. Michigan Street FAN 653P.O. Box 423 M-ApplicationsMilwaukee, WI 53201 Printed in U.S.A.

metasys ventilation controller application (mvca

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