Troubleshooting Variable Air Volume Modular Assembly (VMA ...

Technical BulletinIssue Date 11/01/01

© 2001 Johnson Controls, Inc. www.johnsoncontrols.comCode No. LIT-6363135 Software Release 5.0

TECHNICAL BULLETIN

Troubleshooting Variable Air Volume Modular Assembly(VMA) 1400 Series Controllers

Troubleshooting VMA1400 Series Controllers...................................3

Introduction......................................................................................................... 3

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

Troubleshooting VAV System Operation .........................................................................5

Hardware Concerns.........................................................................................................6

VMA Control Performance Measures ..............................................................................9

Occupant Hot/Cold Complaints .....................................................................................12

VMA LED Indicator Status.............................................................................................14

Tools for Detecting Communication Problems...............................................................14

N2 Bus Communication Problems.................................................................................14

Zone Bus Problems.......................................................................................................15

Lack of Heat During Commissioning..............................................................................16

Detailed Procedures......................................................................................... 17

Detecting Airflow and Temperature Problems................................................................17

Troubleshooting Airflow Leakage at a Fully Closed Damper..........................................20

Troubleshooting Low Air Velocity...................................................................................20

Correcting Distorted Flow Patterns Due to Duct Design ................................................20

Correcting Unstable Sensor (AI) Readings ....................................................................21

Checking Flow Problems by Verifying Velocity Pressure Sensor Operation ..................22

Checking Airflow Pickups for Debris or Water................................................................22

Checking for Incorrectly Wired Sensors and Misapplied Room Assignments ................23

Troubleshooting Balancer’s Flow Reading.....................................................................24

Detecting Communication Problems Using the VMA LED .............................................24

Identifying N2 Bus Configuration Problems ...................................................................25

Testing for N2 Bus Opens, Shorts, and Crossed Wires .................................................26

Verifying Proper Transformer Installation.......................................................................27

Troubleshooting VMA1400 Series Controllers Technical Bulletin2

Checking for Proper Device Isolation and Ground Loops ..............................................27

Troubleshooting Heating Problems................................................................................28

Troubleshooting VMA1400 Series Controllers Technical Bulletin 3

Troubleshooting VMA1400 SeriesControllers

Introduction Although the Variable Air Volume Modular Assembly (VMA) hasbeen designed to provide years of reliable service, problems candevelop in the related building and Heating, Ventilating, and AirConditioning (HVAC) systems. Problems involving the VMA usuallysurface in the form of occupant comfort issues or communicationfailures. The causes of these problems range from device failures toHVAC system maintenance and design issues, installation errors, orchanges in use of a zone. Troubleshooting must consider all of thesevariables.

Note: This document focuses on the VMA1410, 1420, and1430 controllers. The VMA1400 Series also includes theVMA1440, which is used exclusively as part of theMetasys Zoning Package. See the Metasys Zoning PackageProduct Bulletin (LIT-639050) and the Metasys ZoningPackage Overview Technical Bulletin (LIT-639100) forinformation on this specialized product.

This document describes how to:

• detect airflow and temperature problems

• troubleshoot airflow leakage at a fully closed damper

• troubleshoot low air velocity

• correct distorted flow patterns due to duct design

• correct unstable sensor (Analog Input [AI]) readings

• check flow problems by verifying velocity pressure sensoroperation

• check airflow pickups for debris or water


• check for incorrectly wired sensors and misapplied roomassignments

• troubleshoot Balancer’s flow reading

• detect communication problems using the VMA Light-EmittingDiode (LED)

• identify N2 Bus configuration problems

• test for N2 Bus opens, shorts, and crossed wires

• verify proper transformer installation

• check for proper device isolation and ground loops

• troubleshoot heating problems

Note: Where this document refers to HVAC PRO software,substitute EURO PRO in Europe.


Key Concepts

Troubleshooting Variable Air Volume (VAV) System Operation

Controller Configuration Problems

Mechanical, flow, and wiring problems occur far more often thanconfiguration problems. This is especially true sinceProportional-Integral-Derivative (PID) loop tuning is automatic withthe VMA.

As a rule of thumb, ensure all the equipment and wiring in the systemchecks out before investigating errant configuration parameters inHVAC PRO software.

Refer to the HVAC PRO User’s Guide for more information whenreviewing VMA configuration parameters.

HVAC PRO Software

HVAC PRO software provides various diagnostics to help youpinpoint temperature and airflow control problems. Controllerinformation, VAV Box Flow Test, Collect VAV Diagnostics, and theVMA Balancer Tool each have a role in diagnosis and correction ofVariable Air Volume (VAV) system problems, whether mechanical,electrical, or configuration related.

Collected and calculated data about VMA inputs, outputs, and controlloop performance can be displayed through the Parameters list box inthe Commissioning mode. Refer to the HVAC PRO User’s Guide formore information when using HVAC PRO tools.

VAV Box Flow Test

This test is provided in HVAC PRO software to collect flowcharacteristics for the VMA. It can help determine if the VMA ismaintaining minimum flow for ventilation and for staged electricreheat. It can also check for maximum flow and diagnose a starvedbox, reversed polarity, or a loose set screw on an actuator.


Hardware Concerns

Sensor Errors

Zone temperature and pressure sensor errors can cause controlproblems. The Analog Input (AI) range may be improperly selected inthe HVAC PRO configuration file. The sensor may need an offset forcable length. This offset, whose default value is -0.8°C (-1.5°F), is anattribute of the Zone Temperature AI. In addition, the sensor may beimproperly installed or an element may be damaged. Sensor errors canoccur when the temperature sensor is affected by the sun, other zones,or supply air from the diffuser.

In the worst case, the Differential Pressure (DP) sensor may drift up to±0.1793 Pascal per °C (±0.0004 inch W.C. per °F). This is usuallyinsignificant. However, it may produce noticeable flow measurementerror during periods of large ambient temperature change. Examples ofsuch periods are transitions between unoccupied and occupied, orduring project startup when electrical power may be off and buildingtemperature is not controlled. The controller autocalibrates the DPsensor every two weeks by default to offset the long term affects oftemperature and humidity changes. If greater flow measurementaccuracy is required at low flow rates where box inlet velocity is under1 m/s (200 fpm [feet per minute]), the Autocalibration Period can bereduced to recalibrate every three hours.

IMPORTANT: During Autocalibration, airflow to the zone drops tozero for the following durations (worst case):

VMA1410/1420 = 90 seconds (30 seconds to driveopen + 30 seconds to drive close + 30 seconds sensorsettle time)

VMA1430 = the sum of the damper actuator stroketime and 30 seconds (amount of time to drive close+ 30 seconds sensor settle time)

Damper/Actuator Operation

Damaged damper seals, bent damper blades, poorly designed dampers,or a mis-aligned actuators can cause air leakage during the fully closedposition. This causes a small offset that the Balancer can usuallycompensate for. However, if tight shutoff is required for theapplication, replace the defective VAV box.


Ductwork Design

Air velocity is non-uniform if turns or transitions in hard duct or sagsin flexible duct are within close proximity to the flow pickups. In thiscase, the pickup ports may not represent the true average air velocity.In addition, different flow rates may distinctly change the velocityprofile.

A minimum of three duct diameters of straight, unrestricted ductupstream from the airflow pickups is recommended. The flow profileproblem can be corrected by installing straightening vanes in theoffending duct section or changing the duct configuration to providegreater separation between the transition and the pickup.

Flow rate measurement problems can also be caused by a duct lengththat causes significant pressure drop or by sags present in flexible duct.This can also be avoided by installing hard duct three diameters inlength, starting at the VAV box inlet.

Flow Pickup Performance

Flow pickup performance suffers when the device is not installedcorrectly. It can also collect debris and must be checked for pluggedports and leaks between the high and low-pressure sides.

Air Flow

Total Pressure

Static Pressure

Static Pressure

Pickup HighPressure Manifold Pickup Low

Pressure Manifold

Area of Increased Velocityand Decreased Pressure

Duct Wall

Airflow

Figure 1: Interaction of the Pickup and Air Stream

Referencing Figure 1 above, the upstream ports are exposed to totalpressure. In order to sense true static pressure, the pickup must haveopenings that are perpendicular to the direction of flow. Thelow-pressure ports open downstream, and the passing air exerts a pullon these openings, resulting in a pressure less than static. This resultsin airflow pickup gain, resulting in a differential pressure of1.5- to 3-times the velocity pressure.


Following are typical flow pickup designs. Usually the cross and ringtypes perform better than straight tubes because the sensing ports arebetter distributed across the duct area.

Sensing Ports

Cross Tubes Squared Rings Straight TubesSensing

Figure 2: Common Flow Pickups

VAV Box Size

The VAV box size and flow pickup gain must be entered accuratelyfor the controller to calculate proper airflow. VAV boxes may beoversized for quieter operation or to reserve cooling capacity. Inletsize and pickup gain for the VAV box are entered inHVAC PRO software during configuration.

The size and capacity of the VAV box should match the zone loads.If the installed unit is too small, insufficient cooling results. Inaddition, high flow rates may cause the unit to emit an audible noise. Ifthe installed unit is too large, then proper control of airflow is difficult.


VMA Control Performance Measures

Temperature Control Measures

Table 1 lists key parameters indicating temperature control problems.

Table 1: Temperature Control Key Parameters

Parameter DescriptionMovAvg ZT Err Average of the zone temperature control loop error (setpoint minus temperature)

over the preceding eight hours of control in normal control modes. This calculationis stopped in Shutdown, Warmup, Low Limit, Water System Flush, and when theZone Temperature sensor is unreliable. If the VMA is interlocked with supply air(fan) availability, and the zone is well designed, this measure should always bewithin ±0.5°C (±1°F). Since controller error is defined as setpoint minus processvariable feedback, negative numbers indicate a warm zone and positive valuesindicate a cold zone.

MovAvg ABS ZT Err Average of the absolute or unsigned zone temperature control loop error (absolutevalue of setpoint minus temperature) over the preceding eight hours of control innormal control modes. This calculation is stopped in Shutdown, Warmup, LowLimit, Water System Flush, and when the Zone Temperature sensor is unreliable. Itis just like the MovAvg ZT Err, except that the error is always considered positive. Ifthe VMA is interlocked with supply air (fan) availability, and the zone is welldesigned, this measure should always be less than 0.5°C (1°F). Larger values mayindicate a cooling or heating problem, a cycling control, an extended cooldown, orwarmup in progress.

Inadequate Cooling When True, this indicates the zone cooling demand cannot be satisfied.Specifically, it means the following:

• the controller is calling for cooling

• the controller is in neither Unoccupied nor Shutdown mode

• the Zone Temperature is reliable

• the box is not starved

• the cooling PID has been saturated high for 15 minutes

Inadequate Heating When True, this indicates the zone heating demand cannot be satisfied.Specifically, it means the following:

• the controller is not in Heating Lockout

• the controller is calling for heating

• the controller is in neither Unoccupied nor Shutdown mode

• the Zone Temperature is reliable

• the heating PID to be sequenced has been saturated high for ten minutes.

This may indicate heating media is not available and Heating Lockout is False.

Zone TemperatureStatus

If the sensed value of the zone temperature sensor is outside of the normaloperating range -45 to 121°C (-50 to 250°F) for several readings, the PresentValue of Temperature Loop is Unreliable. This condition is normally caused by anopen or short in the sensor.


Airflow Control Measures

Table 2 lists key parameters that can indicate airflow control problems.

Table 2: Airflow Control Key Parameters

Parameter DescriptionMovAvg Flow Err Average of the flow control loop error (setpoint minus calculated flow) over the

preceding 20 minutes of control in normal control modes. This calculation isstopped in Shutdown, Warmup, Low Limit, Water System Flush, and when theDelta P sensor is unreliable. Since error is (SP – flow), negative numbers indicatea zone with too much flow.

MovAvg ABS Flow Err Average of the absolute or unsigned flow control loop error (difference betweensetpoint and calculated) over the preceding 20 minutes of control in normal controlmodes. This calculation is stopped in Shutdown, Warmup, Low Limit, WaterSystem Flush, and when the Delta P sensor is unreliable. It is just like the MovAvgFlow Err, except that the error is always considered positive. If the VMA isinterlocked with supply air (fan) availability, and the supply system is well designed,this measure should always be less than the cfm calculated in MovAvg Flow Errabove. Larger values may indicate a lack of supply air, duct blockage ordisconnection, or a damper or actuation problem.

Starved Box When True, this indicates the airflow setpoint cannot be satisfied. Specifically, itmeans the controller has been calling for a 100% open damper position for at leastten minutes, and is in neither Unoccupied nor Shutdown, and the Delta P isreliable.

Delta P Status If the sensed value of the velocity pressure (Delta P) sensor is outside of thenormal operating range for several readings, the Present Value of Flow Loop isUnreliable. This condition is normally caused by an open or short in the sensor ortoo high a DP in duct.

Balancing Errors

Pressure independent VAV control jobs frequently require accuracywithin 5-10% of actual flow and indicated flow. The balancingcontractor must adjust and certify the flow rates specified by theconsulting engineer. Sometimes the Balancer’s readings disagree withflow indicated by the controller.

When airflow readings disagree, a problem may exist or some airdelivery system fact may not be known or understood. There aremargins for error in the measurement equipment used by thecontroller, as well as that used by the Balancer. Therefore, it isimportant that both controls contractors and Balancers understand eachother’s equipment, techniques, and expectations.


Typical factors that contribute to Balancer flow reading errors include:

• not calibrating the flow hood as specified. Flow hood accuracy isspecified by the manufacturer and may range from ±5% to ±3% offull scale reading on the better instruments.

• calibrating the flow hood on a different type of diffuser.Calibrating the hood with one type of diffuser and then takingmeasurements on a different type of diffuser results in less accuratevalues.

• using multiple diffusers supplied by a single VAV box. When theflow hood is placed over one diffuser, the hood may present arestriction, causing less flow from the measured diffuser and moreflow from the others. In this case, the Balancer’s sum of thereadings taken at all diffusers served by the box is less than theactual flow.

• performing airflow measurements using a hood rather than avelocity probe type of instrument with a slotted diffuser. A slotteddiffuser consists of one to three slots, each about one inch wideand four or more feet long. Tests show that hood readings of someslotted diffusers may be as much as 40% erroneous. The diffusermanufacturer’s literature specifies how to measure airflow andwhat instrument to use.

• incorrectly mounting the balancing damper. If mounted directly onthe diffuser, turbulent flow patterns entering the hood may occur,resulting in erroneous hood indication.

• mismatching the flow hood with the diffuser size such that thediffuser is not completely covered

The Balancer may take two additional measurements to help find thecause of flow reading discrepancies:

• verifying the controller differential pressure reading with ahigh-accuracy differential pressure meter

• performing a duct traverse as detailed in 2001 ASHRAE HandbookFundamentals, I-P Edition, 14.16-17.


Occupant Hot/Cold Complaints

Zone temperature control problems, which are usually reported asoccupant hot/cold complaints, can have causes ranging from thebuilding or mechanical system to the control components. Thefollowing guide can help locate the cause of zone temperature controlproblems with Pressure Independent VAV Terminals.

Table 3: Occupant Hot/Cold Complaints

Symptom Cause Action Occupant HotComplaints

Zone setpoint too high Lower the common setpoint.Calibrate remote setpoint potentiometer.

Controller not in Occupied mode Check to see if mode is commanded by networkfeatures.

Occupied Cooling Bias too large Set Occupied Cooling Bias to 1 degree or less forgreatest comfort.

Occupant HotComplaints orMovAvg ZT Errless than 1°C (-2°F)

Insufficient airflow Check Starved Box.Check terminal box inlet obstructions.

Airflow not in control.(Indicated by MovAvg Flow Errgreater than sq ft Box Area * 40.)

Tighten damper shaft coupling.Increase minimum flow setpoint.Reconnect duct.Check for differential pressure sensing problem(refer to Variable Air Volume Modular Assembly[VMA] 1400 Series Overview and EngineeringGuidelines Technical Bulletin [LIT-6363120]).

Reheat and/or supplemental heatstuck on, valve stuck open orwrong stroke time/range

Verify heating device operation.

VAV box inlet air too warm With the box at maximum cooling flow, verifydiffuser air temperature is 10 to 15°C (50 to 60°F).Warmer air temperature may indicate supply airtemperature problem, stuck reheat valve, or boxseries fan adjusted to flow higher than maximumcooling.

Zoning problem - VAV boxserves multiple rooms/areas

See Zones in the Definition of Terms section ofthe Variable Air Volume Modular Assembly (VMA)1400 Series Overview and EngineeringGuidelines Technical Bulletin (LIT-6363120).

Zone sensor placement resultingin non-representativetemperature sensed

See Room Sensor Placement in the Variable AirVolume Modular Assembly (VMA) 1400 SeriesOverview and Engineering Guidelines TechnicalBulletin (LIT-6363120).

Zone sensor wiring crossed Rewire VMA to appropriate sensor.

VAV outlet duct crossed Change duct routing or cross sensor wiring.

Cooling load exceeds design dueto change in use, or addedequipment or people

Investigate possibility of increasing the MaxCooling Flow setpoint. (How much greater ispresent load than design? Is sufficient supply airavailable? Is box large enough?)

Continued on next page . . .


Symptom (Cont.) Cause Action Occupant ColdComplaints

Zone setpoint too low Raise the Common Setpoint.Calibrate remote setpoint potentiometer.

Controller not in Occupied mode Check to see if mode is commanded by networkfeatures.

Occupied Heating Bias too large Set Occupied Heating Bias to 1 degree or less forgreatest comfort.

Occupant ColdComplaints orMovAvg ZT ErrGreater than 1°C(2°F)

Airflow not in control(Indicated by MovAvg Flow Errgreater than 40 times the BoxArea [measured in sq ft].)

Tighten damper, shaft coupling (refer to theMounting the VMA1410/1420 section in Mountingand Wiring the Variable Air Volume ModularAssembly [VMA] 1400 Series Controllers[LIT-6363125]). Decrease minimum flow setpoint.Check for differential pressure sensing problem(refer to Variable Air Volume Modular Assembly[VMA] 1400 Series Overview and EngineeringGuidelines Technical Bulletin [LIT-6363120]).

Too much airflow Often due to minimum airflow when zone is notoccupied. An occupancy sensor may help.

VAV box inlet air too cold Supply air temperature too low. 10 to 15°C(50 to 60°F) is typical. When most boxes areoperating near their minimum cooling flow, supplyair temperature may be reset higher via networkfeatures.

Reheat and/or supplementalheating media not available

Often caused by disabling heating based onarbitrary means like calendar dates. Use networkfeatures to enable heating based on demand.

Reheat heating media availableand QA path increases flowsetpoint on full heating.

Reheat media temperature toolow.

If reheat water temperature is too low, or airflow istoo great, zone does not warm. Verify watertemperature and airflow against designspecifications.

Reheat media not available andQA path increases flow setpointon full heating.

Issue a Heating Lockout via network features toprevent increased cold airflow when heating is notavailable.

Cold spots or drafts due to lowvelocity air dropping onoccupants

Raise the supply air temperature, replace thediffuser with one designed for low velocity, installa series fan, or control a parallel fan from flowrather than temperature.

Reheat and/or supplemental heatstuck off or valve stuck closed orwrong stroke time or range

Verify heating device operation.

Zoning problem - VAV boxserves multiple rooms/areas

See the Zones topic in the Definition of Termssection in the Variable Air Volume ModularAssembly (VMA) 1400 Series Overview andEngineering Guidelines Technical Bulletin(LIT-6363120).

Zone sensor placement resultingin non-representativetemperature sensed

See Room Sensor Placement in the Variable AirVolume Modular Assembly (VMA) 1400 SeriesOverview and Engineering Guidelines TechnicalBulletin (LIT-6363120).

Zone sensor wiring crossed Rewire VMA to appropriate sensor.

VAV outlet duct crossed Change duct routing or cross sensor wiring

Heating load exceeds design dueto change in use or lessequipment and lights or fewerpeople

Investigate possibility of decreasing the Occ InCooling Flow setpoint (how much ventilation isneeded for the occupants?)


VMA LED Indicator Status

The VMA LED provides general diagnostics for VMA power supplyand communication status. Viewing the LED signal and understandingwhat each type of signal indicates quickly isolates some problems. SeeDetecting Communication Problems Using the VMA LED in theDetailed Procedures section.

Tools for Detecting Communication Problems

Many communication problems can be detected with a DMM (digitalvoltmeter). Ordinarily, this is the only equipment needed totroubleshoot communication problems, such as shorts, opens, andcrossed wires. Refer to the N2 Communications Bus Technical Bulletin(LIT-636018).

N2 Bus Communication Problems

N2 Bus Communications problems between the VMA and thesupervisory system are related to improper N2 Bus installation,configuration errors, or both.

Typical communications problems include:

• broken or frayed wires

• improper transformer installation (improper wiring length orgauge, improper termination, or wrong transformer type)

• incorrect End-of-Line (EOL) settings

Note: Incorrect EOL settings may cause signal reflections thatinterfere with communication. The VMA is aself-terminating device, which means it has EOLterminations built in. However, the Metasys NetworkControl Module (NCM), as well as other devices, provide anEOL switch or jumper that the installer needs to set.

• internally shorted N2 device

• too many N2 devices

• N2 Bus too long

• “T” or “Y” connections on N2 Bus

• defective surge protector, defective repeater, or communicationsterminal board failure at the NCM


• N2 address set to 0, 254, or 255 on the controller when softwareaddressing is not used

Note: Software addressing is enabled when hardware switches areset to 0 or 255 and when the VMA has B12 firmware or laterand HVAC PRO Release 7.02 or later. Address 254 isreserved for VMA broadcast messages and should not beused. For more information on N2 addressing, refer to the N2Address Switches topic in the Mounting and Wiring VariableAir Volume Modular Assembly (VMA) 1400 SeriesControllers Technical Bulletin (LIT-6363125).

• more than one controller set to the same N2 address

• ground loops

Note: The N2 Bus shield, if used, must be earth-grounded at onlyone location, preferably at the Network Control Unit (NCU).The N2+, N2-, and REF lines can never be earth grounded.Tie the shield to the SHLD terminal (soft ground) on the N2connector of the VMA.

Refer to the N2 Communications Bus Technical Bulletin (LIT-636018)for more information.

Zone Bus ProblemsThe Zone Bus uses +15 Volts Direct Current (VDC) from the VMA topower the CablePRO or CVTPRO interface devices (24 VoltsAlternating Current [VAC] is not required). A communication errormay occur while using the HVAC PRO commissioning tool with theVMA over the Zone Bus. The cause of the error is often a loose orimproper connection between the interface device, laptop PersonalComputer (PC), and the controller. A defective COM port on thelaptop could also be at fault. At other times, a defective controller cancause an error.

Note: It takes 20 seconds for a VMA to reset and resumecommunication after being downloaded or after power up.

An effective troubleshooting technique is to use a CBLCON andobserve its LEDs. The red LED indicates power from the VMA andthe green LED indicates that the Zone Bus is present. With no laptopconnected to the Zone Bus, the green LED blinks once every10 seconds. With a laptop connected, the LED blinks several times persecond. If this is not the case, try exchanging a component that may bedefective with an identical working component.

A noisy wire adjacent to the Zone Bus can also cause communicationerrors. Noise can be periodically induced into the Zone Bus, therebycausing sporadic communication failures between the laptop and theVMA. Most often, noisy lines cause intermittent disruption, not totalloss of communication.


For more information on HVAC PRO software, refer to theHVAC PRO User’s Guide. For more information on CablePRO andCVTPRO interface devices, see the Auxiliary Gear Technical Bulletin(LIT-6363080).

Lack of Heat During Commissioning

A common method to enable heating during commissioning is tooverride the zone temperature to less than the actual heating setpoint.This method may not produce the expected results. The expectationwith this tactic is that the heating will come on immediately. However,the VMA uses saturation timers to switch between modes, and thesetimers require a minimum of ten minutes before switching modes.Refer to the Application Logic section of the Variable Air VolumeModular Assembly (VMA) 1400 Series Application Note(LIT-6375125) for a more detailed explanation of the theory behind theVMA State Machine.

See the Detailed Procedures section of this document for appropriatetroubleshooting tactics to address heating problems.


Detailed Procedures

Detecting Airflow and Temperature Problems

To detect airflow and temperature problems:

1. Connect a PC running HVAC PRO software to the VMA for theVAV box you are testing.

2. On the Options menu, click Commissioning View.

3. Click Parameters to view the Parameters list box. Scroll andobserve parameters from Table 5 for problems.

4. On the Action menu, click VAV Box Flow Test to detect theproblems listed in Table 5. Use the Flow Test Parameters in Table4. Refer to the HVAC PRO User’s Guide for more information onhow to set up this diagnostic.

Table 4: Flow Test Parameters

Parameter ValueStep Amount (%) 50%

Settle Time (0-60 sec) 15 sec

5. On the Action menu, click Collect Diagnostics. Print out datagraphing. Make any needed software adjustments.


Table 5: Parameters Used to Detect Airflow and Temperature Problems

If the Parameter Status Is ThenZone Temperature PresentValue = Unreliable

Test for an open or a short on the zone temperature sensor. See Correcting Unstable Sensor (AI) Readings in this document.

Zone Temperature PresentValue = 37.8°C (100°F)

Check sensor for an open condition after confirming that 37.8°C (100°F)isn’t the actual temperature and that Reliability = Reliable.

Supply Delta P Present Value =Unreliable

See Correcting Unstable Sensor (AI) Readings, Checking Flow Problemsby Verifying Velocity Pressure Sensor Operation, or Checking AirflowPickups for Debris or Water in this document.

Damper Command Reliability =Stalled During Positioning

Either the damper’s movement is obstructed or the actuator has failed.Visually inspect the damper.

If Starved Box = True • Perform Box Flow Test.

• Check the damper linkage.

• Check the static pressure near the box. Refer to Troubleshooting LowAir Velocity section in this document.

• Check that the flow setpoints are realistic in comparison to theinstalled box dimensions.

If Inadequate Cooling = True • Check the Starved Box parameter to determine if this is a flowproblem. If so, see the Starved Box parameter in this table for moreinformation.

• Increase Flow Setpoint Cooling Max Flow.

• Check for adequate supply air temperature from fan system.

If Inadequate Heating = True Check for a stuck valve actuator, incorrect spring range on pneumaticvalves, or other equipment related problems in the heating system ordevice.

MovAvg ABS Flow Err =(is much larger than thedeadband)

See Correcting Distorted Flow Patterns Due to Duct Design in thisdocument to troubleshoot this problem.


Table 6: VAV Box Flow Test Results

Symptom Cause ActionFlow high at Position 0and low at Position 100

Actuator DirectionIncorrect

Change the Direction To Close attribute of the DamperActuator Output or Stepper Motor Output (SMO).

Flow too high atPosition 0(Flow > Area * 80)

Damper Leakage Refer to Troubleshooting Airflow Leakage at a FullyClosed Damper section of this document.

No change in flow fromPosition 0 to 100

Loose ActuatorLinkage

Refer to the Checking Flow Problems by VerifyingVelocity Pressure Sensor Operation section of thisdocument, or type in -100% to cause the damperactuator to close.

Unexpected stroke time(See Table 7.)

Damper andActuator stopsnot aligned

1. Reset stop positions. Refer to the Mounting andWiring Variable Air Volume Modular Assembly(VMA) 1400 Series Controllers Technical Bulletin(LIT-6363125).

2. Reset the controller using HVAC PRO software. Onthe Action menu, click Reset Controller.

Loose Actuatorscrew

1. Tighten screw. Refer to the Mounting and WiringVariable Air Volume Modular Assembly (VMA) 1400Series Controllers Technical Bulletin (LIT-6363125).


Bound Damper 1. Reset stop positions. Refer to Mounting and WiringVariable Air Volume Modular Assembly (VMA) 1400Series Controllers Technical Bulletin (LIT-6363125).

2. Check for bent damper shaft.

3. Check for duct damage.


Blockage in Duct 1. Open duct and clear blockage.


Actuator travelspast damper fullopen

1. Reset stop positions. Refer to Mounting and WiringVariable Air Volume Modular Assembly (VMA) 1400Series Controllers Technical Bulletin (LIT-6363125).


Flow increases fromPosition 0 to 50and decreases fromPosition 50 to 100

45°°°° box withdamper stops setat 90°°°°

Reset stop positions. Refer to Mounting and WiringVariable Air Volume Modular Assembly (VMA) 1400Series Controllers Technical Bulletin (LIT-6363125).

Table 7: Actuator Rotation Stroke Times for the VMA1410/1420 (Damper fullclosed to full flow open)

Damper Rotation (Degrees) Nominal Stroke Time (Seconds)90 30

60 20

45 15

30 10

Note: These times may vary when an external actuator is used (VMA1430).


Troubleshooting Airflow Leakage at a Fully Closed Damper

To troubleshoot airflow leakage at a fully closed damper:

1. Check damper stop positions. Refer to the Variable Air VolumeModular Assembly (VMA) 1400 Series Overview and EngineeringGuidelines Technical Bulletin (LIT-6363120). On a 90° box,install at the closed position.

2. Check that the actuator collar (or other linkage) is tightly lockedto the damper shaft. Refer to the Variable Air Volume ModularAssembly (VMA) 1400 Series Overview and EngineeringGuidelines Technical Bulletin (LIT-6363120). Visually inspect fora damaged damper seal, bent damper blades, or a poorly designeddamper. Replace if necessary.

Troubleshooting Low Air Velocity

To troubleshoot low air velocity:

1. Check airflow velocity in the Parameters list box inHVAC PRO software. Typical minimum is 2 m/s (400 fpm).

2. Check whether the flow is due to an oversized box. If so, checkwith the Balancer to determine if adjusting the box constant orother system changes are reasonable.

Correcting Distorted Flow Patterns Due to Duct Design

To correct distorted flow patterns due to duct design:

1. Observe whether flexible duct is installed and connected to theVAV box inlet.

2. Whether hard or flexible duct is installed, check if there is a ducttransition less than three diameters in length upstream of theairflow pickups.

3. Check if inlet duct length is excessive. This also may result inpressure drops that restrict airflow. These design conditions cancause non-uniform air velocity across the flow pickup location.

4. To eliminate turbulent flow problems, install a section of straighthard duct between the VAV box inlet and the flexible duct (orduct transition). The straight hard duct must be at least three timesthe length of the duct diameter.


Correcting Unstable Sensor (AI) Readings

To correct unstable sensor (AI) readings:

1. Within the HVAC PRO configuration file, verify that the AI range(temperature, cfm, gpm, etc.) is defined properly in the hardwaretables.

2. Check sensor calibration. Verify that the input reading is the sameas the standard. For temperature readings, a glass thermometerwith 0.05°C (0.1°F) accuracy is necessary. Adjust the AI offset inthe configuration file. For velocity pressure readings, ensure thatAutocalibration was run from HVAC PRO software, then verifyflow readings within 5% of balancing hood reading.

3. Use the HVAC PRO commissioning tool to verify that theAI point is reading steady (not unstable). If the velocity pressureis unstable, this could be due to turbulence in the duct.

4. Ensure that there are 3-5 duct diameters of straight ductworkupstream of the airflow pickups.

5. If AI point is still unstable, check the sensor for the followingproblems:

• Is the wrong type sensor installed or is it installed improperly?

• Is the averaging element securely fastened?

• Are the wires secure?

6. If AI point is still unstable, use the anti-spike filter in the AnalogInput Modify option of HVAC PRO software to average out thepeaks and valleys and eliminate instability. Select the AI screenthen double click on the AI point. Set anti-spike to True and setthe spike fraction between 0.0 (none) to 1.0 (any amplitude spike).The default attributes for zone temperature have anti-spike set toTrue and spike function set to 0.3. This should correct mostproblems. If not, increase the filter order from none to first orsecond order. The filter weight can be set from 0.0 (low) to1.0 (high).

Note: Filter values greater than 0.5 are not recommended and, iftoo large, could make the loop respond too slowly. Filteringmust not be done on the velocity pressure sensor, because theP-adaptive algorithm uses air turbulence values toautomatically tune.


Checking Flow Problems by Verifying Velocity Pressure SensorOperation

Note: To ensure accurate readings of the velocity pressure sensor,allow at least 30 minutes stabilization time before installing aproduct that just came from a storage environment whichdiffers from the installed temperature.

To check flow problems by verifying velocity pressure sensoroperation:

1. Recalibrate differential pressure by labeling and disconnecting thehigh and low side tubing from the box pickup pressure taps(preferred), or by commanding Autocalibration fromHVAC PRO software, which closes the VAV box to obtainnear-zero flow.

2. Autocalibrate the velocity pressure sensor while inCommissioning mode in HVAC PRO software by settingAutocalibration Request parameter to True.

3. Reconnect the high and low side tubing, ensuring the connectionsare correct.

Checking Airflow Pickups for Debris or Water

!WARNING: Never blow into the VMA tubing. The velocity

pressure transducer is very sensitive and will bedestroyed by as little as 70 mbar (1 psi) ofpressure.

To check airflow pickups for debris or water:

1. Examine the pickup pressure taps inside the VAV box.

2. Check for plugged pickup ports and internal leaks between thehigh-and low-pressure sides of the sensor.

3. Disconnect the high and low tubes between the VMA and boxpickup pressure taps to observe if there is water or condensationdisrupting the pressure reading. If necessary, clear water from thetubes and reconnect.

4. Ensure that the VMA is installed vertically on the duct for bestperformance. If mounted horizontally, Autocalibration adjusts forposition effect.


Checking for Incorrectly Wired Sensors and Misapplied RoomAssignments

To check for incorrectly wired sensors and misapplied roomassignments:

1. When experiencing problems with poor control, check that theroom sensor is connected to the correct VMA. Sometimes sensorsare miswired to an adjoining room’s VAV box (see Figure 3).

2. Check that the VMA is controlling the right room. Crossedductwork leads to mistakes similar to the one shown in Figure 4.

VMAVMA

Miswired

RoomSensor

RoomSensor

Figure 3: Incorrectly Wired Sensors

VMAVMA

RoomSensor

Crossed

RoomSensor

Figure 4: Crossed Ductwork


Troubleshooting Balancer’s Flow Reading

To troubleshoot Balancer’s flow reading:

Determine if there is a discrepancy of more than 5 to 10% between theactual flow and Balancer’s indicated flow. If so, speak with theBalancer about recalculating the Box Constant for the VAV BoxCalculation in HVAC PRO software. The default is 2.25.

Detecting Communication Problems Using the VMA LED

To detect communication problems using the VMA LED, use Table 8to help to determine the source of the communications problem.

Table 8: Checking Performance with the VMA LED

If the LED Response Is ThenNot lit Power is not applied to the VMA or +15 VDC is shorted.

Irregular flashing four timesevery second

Communications between the VMA and the supervisory system is normal.

One flash every second The VMA is offline. Verify N2 address switches, and continuetroubleshooting possible causes as discussed in the following sections ofthis document:

• Identifying N2 Bus Configuration Problems

• Testing for N2 Bus Opens, Shorts, and Crossed Wires

• Verifying Proper Transformer Installation

• Checking for Proper Device Isolation and Ground Loops

On solid If on steady for longer than 20 seconds, replace the VMA.

Rapid flashing 20 times persecond

The VMA memory is corrupted. Download the firmware code into thecontroller.


Identifying N2 Bus Configuration Problems

To identify N2 Bus configuration problems, use Table 9 totroubleshoot common offline configuration problems:

Table 9: Configuration Errors that Cause N2 Bus Communications Problems

Error/TroubleCondition

Possible Causes Solution

VMA cyclesonline and offline.

Two or more controllers have thesame address.

Change each duplicate address to a uniquenumber.

A NCM process is using anunconfigured point (i.e., not listed inHVAC PRO.prn file).

Delete the use of the unconfigured point in theNCM process.

There are ‘Y’ or ‘T’ taps. Refer to the N2 Communications Bus TechnicalBulletin (LIT-636018).

The repeater lost power or is wiredwrong.

Refer to the N2 Communications Bus TechnicalBulletin (LIT-636018).

The EOL is not set at the NCM orN30.


VMA does notcome online.

Two or more controllers have thesame address.

Change each duplicate address to a uniquenumber.

The address of the VMA waschanged without its power beingcycled afterward.

Cycle power on the VMA.

N2 Bus contains too many devices. A maximum of 100 N2 devices are allowed perNCM.

VMA does not have power. Power the VMA.Remove the short from the +15 VDC to COM.

N2 cable runs are broken. Locate and correct the wiring.

Entire N2 Bus isoffline.

EOL jumpers on NCM/N30 orW3 jumper on MM-CVT101 are notinstalled.

Install EOL jumpers or W3 jumper properly.

MM-CVT101 is not plugged into PCor 120 VAC source.

Plug MM-CVT101 into PC or plug it into a 120 VACsource.

There are ‘Y’ or ‘T’ taps or therepeater lost power or is wiredwrong.


N2 Bus wires open near the NCM. Connect the wires.


Testing for N2 Bus Opens, Shorts, and Crossed Wires

!CAUTION: To avoid electrical shocks when troubleshooting,

measure the voltage of each N2 wire to earthground with a 100 K ohm resistor at the DMM.If line-voltage is measured, have a qualifiedelectrician locate the fault.

Test for N2 Bus opens, shorts, and crossed wires by either selecting allof the N2 devices and performing a CommDisable at the OperatorWorkstation (OWS), or by connecting an MM-CVT101 in place of theNCM and verifying the NCM/N30 EOL separately. Use a DMM(digital voltmeter) to detect polarity, shorts, crossed wires, and grounds.

To test for N2 Bus opens, shorts, and crossed wires:

1. Connect the DMM across the N2+ and REF screws on theN2 terminal block.

2. Write down the DC voltage reading. Repeat for N2- and REF,then for N2+ and N2-.

3. Compare the voltages on the DMM to the following values:

• N2+ to REF = +2.45 to 2.98 VDC

• N2– to REF = +2.06 to 2.54 VDC

• N2+ to N2– = +0.36 to 0.92 VDC

If the DMM readings are not approximately within the rangeslisted above, the wire is grounded, shorted, or crossed withanother wire.

4. Perform the following checks:

• Check that the polarity of the wiring is consistent from deviceto device all along the N2 Bus.

• Check for breaks in the 24 American Wire Gauge (AWG)(0.6 mm) wire.

• Check if the VMA is internally shorted by unplugging theN2 connector and measuring the voltages from Step 3 on thepins of the VMA. If these do not match Step 3 with power onthe VMA, replace it.

• N2 Bus is improperly terminated. There must be one switchedend-of-line device on each N2 Bus, typically the NCM orN30.

• N2 Bus wiring has “T” or “Y” connections installed.


5. Correct and measure again. If the readings are approximatelywithin the ranges listed above, the Bus is properly wired.Otherwise, proceed to Checking for Proper Device Isolation andGround Loops.

Verifying Proper Transformer Installation

To verify proper transformer installation:

1. Connect the DMM across the 24 VAC and the 24 VAC commonterminal of the VMA. Read the voltage with all typical loadsenergized. A reading of 24-26 VAC is required when the linevoltage is nominal. If the DMM reads a value less than 22 VAC,split the load in half and add a second transformer.

2. Check if the primary voltage matches the transformer’s voltagerating.

3. Check the 24 VAC cable length versus wire gauge.

4. Recalculate the power requirements for the VMAs and loads onthat transformer.

If the procedures above did not correct the communications problem,refer to the N2 Bus Communications Technical Bulletin (LIT-636018)for more information.

Checking for Proper Device Isolation and Ground Loops

To check for proper device isolation and ground loops:

1. Connect the leads of the DMM in parallel with the 100 K ohmresistor from the DC power supply (+15 VDC) output terminal ofthe VMA to earth ground.

2. Read the voltage. If the voltage is 5 VDC/VAC or greater, thecircuit is improperly isolated or a valve actuator is connected,which ties COM to an earth grounded Terminal 1. A single earthground is allowed.

3. Remove all the field wires and N2 Bus wires from the VMA, butleave the transformer wires attached.

4. With the DMM still connected, reconnect each set of field wiresone at a time until the voltage is 5 VDC/VAC or greater. At thispoint, you have discovered one cause of the ground loop.

5. Correct the problem. Continue to reconnect each set of field wiresuntil all ground loops are found and corrected. All grounds arecorrected when the voltage is less than 5 VDC/VAC on the DMM.


Troubleshooting Heating Problems

To troubleshoot heating problems:

1. In HVAC PRO software, switch to Commissioning view.

2. Check the VAV Box Mode Present Value parameter. Use Table10 below to determine the next step based on the value of thisparameter.

Table 10: VAV Box Mode Present Value

If the VAV Box ModePresent Value Is

Then

Cooling Check the Cooling PID Present Value. Go to Table 11.

Satisfied Check the Heating Available parameter (in theCommand Mode parameter group) and the ActualHeating Setpoint parameter (in the TemperatureSetpoint parameter group). Go to Table 13.

Heating Check the Box Heating PID Present Value. Go toTable 14.

Note: The logic in this procedure also applies forsupplemental heating.

Table 11: Cooling PID Present Value

If the Cooling PIDPresent Value Is

Then

= 0% Check the Cooling PID Saturation Status. Go to Table12.

> 0% VAV Box Mode does not switch to Satisfied until:

• PID deintegrates to 0%

• Saturation Status = Low(Saturation Status switches from Normal to Lowafter Cooling PID Present Value is equal to 0% forten minutes.)

Note: To test the heating mode and save time during commissioning, override theVAV Box Mode Present Value to Satisfied with the Zone Temperature lessthan the Actual Heating Setpoint. This bypasses the Cooling modesaturation timers.


Table 12: Cooling PID Saturation Status

If the Cooling PIDSaturation Status Is

Then

Normal Wait ten minutes for Low Saturation time to expire.

Note: Saturation times can be viewed in theDiagnostic View. On the Options menu, clickView to change to Diagnostic view.

Low Recheck VAV Box Mode Present Value. It now equalsSatisfied.

Note: To test the heating mode and save time during commissioning, override theVAV Box Mode Present Value to Satisfied with the Zone Temperature lessthan the Actual Heating Setpoint. This bypasses the Cooling modesaturation timers.

Table 13: Heating Available and Actual Heating Setpoint

If ThenHeating Available =False

BD169 is being commanded from MetasysSupervisory System.

Actual HeatingSetpoint > ZoneTemperature

and

VAV Box Mode PresentValue = Satisfied

Configuration was generated usingHVAC PRO software Release 7.00 and saved usingRelease 7.01.

Workaround: Upgrade the controller inHVAC PRO software Release 7.01. On the Uploadmenu, click Upgrade Controllers. See theTroubleshooting section of the Variable Air VolumeModular Assembly (VMA) 1400 Series ApplicationNote (LIT-6375125) for more information.

Table 14: Box Heating PID Present Value

If Box Heating PIDPresent Value is

Then

=0% Check Box Heating PID Saturation Status. Go to Table15.

>0%

and

Box Heating = eitherBinary Output(normally open valve)or Binary Output(normally closed valve)

Check the Box Heating Make Limit parameter (in theBox Heating Output parameter group). The BoxHeating PID Present Value must exceed the BoxHeating Make Limit before the Box Heating CommandOutput will become active.

>0%

and

Box Heating = Electric1-stage

Check the Box Heating Make Limit parameter (in theBox Heating Output parameter group). The BoxHeating PID Present Value must exceed the BoxHeating Make Limit before the Box Heating OutputPresent Value and EHS Box Heating CommandOutput will change from Off to Stage 1.


Table 15: Box Heating PID Saturation Status

If Box Heating PIDSaturation Status is

Then

Low Wait 60 seconds and recheck Box Heating PIDSaturation Status. The status should change tonormal. The period for the PID is 60 seconds, and itmay take the full time to switch to Normal and begin toramp up the PID.

Normal Wait for 60 seconds and recheck Box Heating PIDPresent Value. The period for the PID is 60 seconds,and it may take the full time to begin to ramp up thePID.

Controls Group507 E. Michigan StreetP.O. Box 423 www.johnsoncontrols.comMilwaukee, WI 53201 Printed in U.S.A.

Troubleshooting Variable Air Volume Modular Assembly (VMA ...

Documents

Transcript of Troubleshooting Variable Air Volume Modular Assembly (VMA ...