© 2001 Johnson Controls, Inc. www.johnsoncontrols.com
Code 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
Checking for Proper Device Isolation and Ground Loops ...27 Troubleshooting Heating Problems...28
Troubleshooting VMA1400 Series
Controllers
Introduction
Although the Variable Air Volume Modular Assembly (VMA) has been designed to provide years of reliable service, problems can develop in the related building and Heating, Ventilating, and Air Conditioning (HVAC) systems. Problems involving the VMA usually surface in the form of occupant comfort issues or communication failures. The causes of these problems range from device failures to HVAC system maintenance and design issues, installation errors, or changes in use of a zone. Troubleshooting must consider all of these variables.
Note: This document focuses on the VMA1410, 1420, and 1430 controllers. The VMA1400 Series also includes the VMA1440, which is used exclusively as part of the
Metasys Zoning Package. See the Metasys Zoning Package Product Bulletin (LIT-639050) and the Metasys Zoning Package Overview Technical Bulletin (LIT-639100) for information 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 sensor
operation
• check for incorrectly wired sensors and misapplied room assignments
• troubleshoot Balancer’s flow reading
• detect communication problems using the VMA Light-Emitting Diode (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 than configuration problems. This is especially true since
Proportional-Integral-Derivative (PID) loop tuning is automatic with the VMA.
As a rule of thumb, ensure all the equipment and wiring in the system checks out before investigating errant configuration parameters in HVAC PRO software.
Refer to the HVAC PRO User’s Guide for more information when reviewing VMA configuration parameters.
HVAC PRO Software
HVAC PRO software provides various diagnostics to help you pinpoint temperature and airflow control problems. Controller
information, VAV Box Flow Test, Collect VAV Diagnostics, and the VMA Balancer Tool each have a role in diagnosis and correction of Variable Air Volume (VAV) system problems, whether mechanical, electrical, or configuration related.
Collected and calculated data about VMA inputs, outputs, and control loop performance can be displayed through the Parameters list box in the Commissioning mode. Refer to the HVAC PRO User’s Guide for more information when using HVAC PRO tools.
VAV Box Flow Test
This test is provided in HVAC PRO software to collect flow characteristics for the VMA. It can help determine if the VMA is maintaining minimum flow for ventilation and for staged electric reheat. It can also check for maximum flow and diagnose a starved box, reversed polarity, or a loose set screw on an actuator.
Hardware Concerns
Sensor Errors
Zone temperature and pressure sensor errors can cause control
problems. The Analog Input (AI) range may be improperly selected in the HVAC PRO configuration file. The sensor may need an offset for cable length. This offset, whose default value is -0.8°C (-1.5°F), is an attribute of the Zone Temperature AI. In addition, the sensor may be improperly installed or an element may be damaged. Sensor errors can occur 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 usually insignificant. However, it may produce noticeable flow measurement error during periods of large ambient temperature change. Examples of such periods are transitions between unoccupied and occupied, or during project startup when electrical power may be off and building temperature is not controlled. The controller autocalibrates the DP sensor every two weeks by default to offset the long term affects of temperature and humidity changes. If greater flow measurement accuracy is required at low flow rates where box inlet velocity is under 1 m/s (200 fpm [feet per minute]), the Autocalibration Period can be reduced to recalibrate every three hours.
IMPORTANT: During Autocalibration, airflow to the zone drops to zero for the following durations (worst case): VMA1410/1420 = 90 seconds (30 seconds to drive open + 30 seconds to drive close + 30 seconds sensor settle time)
VMA1430 = the sum of the damper actuator stroke time 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 closed position. This causes a small offset that the Balancer can usually compensate for. However, if tight shutoff is required for the application, replace the defective VAV box.
Ductwork Design
Air velocity is non-uniform if turns or transitions in hard duct or sags in flexible duct are within close proximity to the flow pickups. In this case, the pickup ports may not represent the true average air velocity. In addition, different flow rates may distinctly change the velocity profile.
A minimum of three duct diameters of straight, unrestricted duct upstream from the airflow pickups is recommended. The flow profile problem can be corrected by installing straightening vanes in the offending duct section or changing the duct configuration to provide greater separation between the transition and the pickup.
Flow rate measurement problems can also be caused by a duct length that causes significant pressure drop or by sags present in flexible duct. This can also be avoided by installing hard duct three diameters in length, starting at the VAV box inlet.
Flow Pickup Performance
Flow pickup performance suffers when the device is not installed correctly. It can also collect debris and must be checked for plugged ports and leaks between the high and low-pressure sides.
Air Flow
Total Pressure
Static Pressure Static Pressure
Pickup High
Pressure Manifold Pickup Low Pressure Manifold
Area of Increased Velocity and Decreased Pressure
Duct Wall
Airflow
Figure 1: Interaction of the Pickup and Air Stream
Referencing Figure 1 above, the upstream ports are exposed to total pressure. In order to sense true static pressure, the pickup must have openings that are perpendicular to the direction of flow. The
low-pressure ports open downstream, and the passing air exerts a pull on these openings, resulting in a pressure less than static. This results in airflow pickup gain, resulting in a differential pressure of
Following are typical flow pickup designs. Usually the cross and ring types perform better than straight tubes because the sensing ports are better distributed across the duct area.
Sensing Ports
Cross Tubes Squared Rings Straight Tubes
Sensing
Figure 2: Common Flow Pickups
VAV Box Size
The VAV box size and flow pickup gain must be entered accurately for the controller to calculate proper airflow. VAV boxes may be oversized for quieter operation or to reserve cooling capacity. Inlet size and pickup gain for the VAV box are entered in
HVAC 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. In
addition, high flow rates may cause the unit to emit an audible noise. If the 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 Description
MovAvg 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 calculation is stopped in Shutdown, Warmup, Low Limit, Water System Flush, and when the Zone Temperature sensor is unreliable. If the VMA is interlocked with supply air (fan) availability, and the zone is well designed, this measure should always be within ±0.5°C (±1°F). Since controller error is defined as setpoint minus process variable feedback, negative numbers indicate a warm zone and positive values indicate a cold zone.
MovAvg ABS ZT Err Average of the absolute or unsigned zone temperature control loop error (absolute value of setpoint minus temperature) over the preceding eight hours of control in normal control modes. This calculation is stopped in Shutdown, Warmup, Low Limit, Water System Flush, and when the Zone Temperature sensor is unreliable. It is just like the MovAvg ZT Err, except that the error is always considered positive. If the VMA is interlocked with supply air (fan) availability, and the zone is well designed, this measure should always be less than 0.5°C (1°F). Larger values may indicate a cooling or heating problem, a cycling control, an extended cooldown, or warmup 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 Temperature Status
If the sensed value of the zone temperature sensor is outside of the normal operating range -45 to 121°C (-50 to 250°F) for several readings, the Present Value of Temperature Loop is Unreliable. This condition is normally caused by an open 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 Description
MovAvg 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 is stopped in Shutdown, Warmup, Low Limit, Water System Flush, and when the Delta P sensor is unreliable. Since error is (SP – flow), negative numbers indicate a zone with too much flow.
MovAvg ABS Flow Err Average of the absolute or unsigned flow control loop error (difference between setpoint and calculated) over the preceding 20 minutes of control in normal control modes. This calculation is stopped in Shutdown, Warmup, Low Limit, Water System Flush, and when the Delta P sensor is unreliable. It is just like the MovAvg
Flow Err, except that the error is always considered positive. If the VMA is
interlocked 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 Err above. Larger values may indicate a lack of supply air, duct blockage or
disconnection, or a damper or actuation problem.
Starved Box When True, this indicates the airflow setpoint cannot be satisfied. Specifically, it means the controller has been calling for a 100% open damper position for at least ten minutes, and is in neither Unoccupied nor Shutdown, and the Delta P is reliable.
Delta P Status If the sensed value of the velocity pressure (Delta P) sensor is outside of the normal operating range for several readings, the Present Value of Flow Loop is Unreliable. This condition is normally caused by an open or short in the sensor or too high a DP in duct.
Balancing Errors
Pressure independent VAV control jobs frequently require accuracy within 5-10% of actual flow and indicated flow. The balancing contractor must adjust and certify the flow rates specified by the consulting engineer. Sometimes the Balancer’s readings disagree with flow indicated by the controller.
When airflow readings disagree, a problem may exist or some air delivery system fact may not be known or understood. There are margins for error in the measurement equipment used by the controller, as well as that used by the Balancer. Therefore, it is
important that both controls contractors and Balancers understand each other’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 is
specified by the manufacturer and may range from ±5% to ±3% of full 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 taking measurements on a different type of diffuser results in less accurate values.
• using multiple diffusers supplied by a single VAV box. When the flow hood is placed over one diffuser, the hood may present a restriction, causing less flow from the measured diffuser and more flow from the others. In this case, the Balancer’s sum of the readings taken at all diffusers served by the box is less than the actual flow.
• performing airflow measurements using a hood rather than a velocity probe type of instrument with a slotted diffuser. A slotted diffuser consists of one to three slots, each about one inch wide and four or more feet long. Tests show that hood readings of some slotted diffusers may be as much as 40% erroneous. The diffuser manufacturer’s literature specifies how to measure airflow and what instrument to use.
• incorrectly mounting the balancing damper. If mounted directly on the diffuser, turbulent flow patterns entering the hood may occur, resulting in erroneous hood indication.
• mismatching the flow hood with the diffuser size such that the diffuser is not completely covered
The Balancer may take two additional measurements to help find the cause of flow reading discrepancies:
• verifying the controller differential pressure reading with a high-accuracy differential pressure meter
• performing a duct traverse as detailed in 2001 ASHRAE Handbook Fundamentals, I-P Edition, 14.16-17.
Occupant Hot/Cold Complaints
Zone temperature control problems, which are usually reported as occupant hot/cold complaints, can have causes ranging from the building or mechanical system to the control components. The
following guide can help locate the cause of zone temperature control problems with Pressure Independent VAV Terminals.
Table 3: Occupant Hot/Cold Complaints
Symptom Cause Action
Occupant Hot
Complaints 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 network
features.
Occupied Cooling Bias too large Set Occupied Cooling Bias to 1 degree or less for greatest comfort.
Occupant Hot Complaints or MovAvg ZT Err less than 1°C (-2°F)
Insufficient airflow Check Starved Box.
Check terminal box inlet obstructions.
Airflow not in control.
(Indicated by MovAvg Flow Err greater 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 Engineering Guidelines Technical Bulletin [LIT-6363120]).
Reheat and/or supplemental heat stuck on, valve stuck open or wrong stroke time/range
Verify heating device operation.
VAV box inlet air too warm With the box at maximum cooling flow, verify diffuser air temperature is 10 to 15°C (50 to 60°F). Warmer air temperature may indicate supply air temperature problem, stuck reheat valve, or box series fan adjusted to flow higher than maximum cooling.
Zoning problem - VAV box
serves multiple rooms/areas See Zones in the Definition of Terms section ofthe Variable Air Volume Modular Assembly (VMA)
1400 Series Overview and Engineering Guidelines Technical Bulletin (LIT-6363120).
Zone sensor placement resulting in non-representative
temperature sensed
See Room Sensor Placement in the Variable Air
Volume Modular Assembly (VMA) 1400 Series Overview and Engineering Guidelines Technical Bulletin (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 due
to change in use, or added equipment or people
Investigate possibility of increasing the Max Cooling Flow setpoint. (How much greater is present load than design? Is sufficient supply air available? Is box large enough?)
Symptom (Cont.) Cause Action
Occupant Cold
Complaints 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 network
features.
Occupied Heating Bias too large Set Occupied Heating Bias to 1 degree or less for greatest comfort. Occupant Cold Complaints or MovAvg ZT Err Greater than 1°C (2°F)
Airflow not in control
(Indicated by MovAvg Flow Err greater than 40 times the Box Area [measured in sq ft].)
Tighten damper, shaft coupling (refer to the
Mounting the VMA1410/1420 section in Mounting and Wiring the Variable Air Volume Modular Assembly [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 Engineering Guidelines Technical Bulletin [LIT-6363120]).
Too much airflow Often due to minimum airflow when zone is not occupied. 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 are operating near their minimum cooling flow, supply air temperature may be reset higher via network features.
Reheat and/or supplemental
heating media not available Often caused by disabling heating based onarbitrary means like calendar dates. Use network features to enable heating based on demand. Reheat heating media available
and QA path increases flow setpoint on full heating. Reheat media temperature too low.
If reheat water temperature is too low, or airflow is too great, zone does not warm. Verify water temperature and airflow against design specifications.
Reheat media not available and QA path increases flow setpoint on full heating.
Issue a Heating Lockout via network features to prevent increased cold airflow when heating is not available.
Cold spots or drafts due to low velocity air dropping on occupants
Raise the supply air temperature, replace the diffuser with one designed for low velocity, install a series fan, or control a parallel fan from flow rather than temperature.
Reheat and/or supplemental heat stuck off or valve stuck closed or wrong stroke time or range
Verify heating device operation.
Zoning problem - VAV box
serves multiple rooms/areas See the Zones topic in the Definition of Termssection in the Variable Air Volume Modular
Assembly (VMA) 1400 Series Overview and Engineering Guidelines Technical Bulletin (LIT-6363120).
Zone sensor placement resulting in non-representative
temperature sensed
See Room Sensor Placement in the Variable Air
Volume Modular Assembly (VMA) 1400 Series Overview and Engineering Guidelines Technical Bulletin (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 due
to change in use or less equipment and lights or fewer people
Investigate possibility of decreasing the Occ In Cooling Flow setpoint (how much ventilation is needed for the occupants?)
VMA LED Indicator Status
The VMA LED provides general diagnostics for VMA power supply and communication status. Viewing the LED signal and understanding what each type of signal indicates quickly isolates some problems. See Detecting Communication Problems Using the VMA LED in the Detailed Procedures section.
Tools for Detecting Communication Problems
Many communication problems can be detected with a DMM (digital voltmeter). Ordinarily, this is the only equipment needed to
troubleshoot communication problems, such as shorts, opens, and crossed wires. Refer to the N2 Communications Bus Technical Bulletin (LIT-636018).
N2 Bus Communication Problems
N2 Bus Communications problems between the VMA and the supervisory 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 or gauge, improper termination, or wrong transformer type) • incorrect End-of-Line (EOL) settings
Note: Incorrect EOL settings may cause signal reflections that interfere with communication. The VMA is a
self-terminating device, which means it has EOL terminations built in. However, the Metasys Network
Control Module (NCM), as well as other devices, provide an EOL 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 communications terminal board failure at the NCM
• N2 address set to 0, 254, or 255 on the controller when software addressing is not used
Note: Software addressing is enabled when hardware switches are set to 0 or 255 and when the VMA has B12 firmware or later and HVAC PRO Release 7.02 or later. Address 254 is reserved for VMA broadcast messages and should not be used. For more information on N2 addressing, refer to the N2 Address Switches topic in the Mounting and Wiring Variable Air Volume Modular Assembly (VMA) 1400 Series
Controllers 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 only one 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 N2 connector of the VMA.
Refer to the N2 Communications Bus Technical Bulletin (LIT-636018) for more information.
Zone Bus Problems
The Zone Bus uses +15 Volts Direct Current (VDC) from the VMA to power the CablePRO or CVTPRO interface devices (24 Volts
Alternating Current [VAC] is not required). A communication error may occur while using the HVAC PRO commissioning tool with the VMA over the Zone Bus. The cause of the error is often a loose or improper connection between the interface device, laptop Personal Computer (PC), and the controller. A defective COM port on the laptop could also be at fault. At other times, a defective controller can cause an error.
Note: It takes 20 seconds for a VMA to reset and resume
communication after being downloaded or after power up. An effective troubleshooting technique is to use a CBLCON and observe its LEDs. The red LED indicates power from the VMA and the green LED indicates that the Zone Bus is present. With no laptop connected to the Zone Bus, the green LED blinks once every
10 seconds. With a laptop connected, the LED blinks several times per second. If this is not the case, try exchanging a component that may be defective with an identical working component.
A noisy wire adjacent to the Zone Bus can also cause communication errors. Noise can be periodically induced into the Zone Bus, thereby causing sporadic communication failures between the laptop and the VMA. Most often, noisy lines cause intermittent disruption, not total loss of communication.
For more information on HVAC PRO software, refer to the
HVAC PRO User’s Guide. For more information on CablePRO and CVTPRO interface devices, see the Auxiliary Gear Technical Bulletin (LIT-6363080).
Lack of Heat During Commissioning
A common method to enable heating during commissioning is to override the zone temperature to less than the actual heating setpoint. This method may not produce the expected results. The expectation with this tactic is that the heating will come on immediately. However, the VMA uses saturation timers to switch between modes, and these timers require a minimum of ten minutes before switching modes. Refer to the Application Logic section of the Variable Air Volume Modular Assembly (VMA) 1400 Series Application Note
(LIT-6375125) for a more detailed explanation of the theory behind the VMA State Machine.
See the Detailed Procedures section of this document for appropriate troubleshooting 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 the VAV box you are testing.
2. On the Options menu, click Commissioning View.
3. Click Parameters to view the Parameters list box. Scroll and observe parameters from Table 5 for problems.
4. On the Action menu, click VAV Box Flow Test to detect the problems listed in Table 5. Use the Flow Test Parameters in Table 4. Refer to the HVAC PRO User’s Guide for more information on how to set up this diagnostic.
Table 4: Flow Test Parameters
Parameter Value
Step Amount (%) 50%
Settle Time (0-60 sec) 15 sec
5. On the Action menu, click Collect Diagnostics. Print out data graphing. Make any needed software adjustments.
Table 5: Parameters Used to Detect Airflow and Temperature Problems
If the Parameter Status Is Then
Zone Temperature Present
Value = Unreliable Test for an open or a short on the zone temperature sensor.See Correcting Unstable Sensor (AI) Readings in this document.
Zone Temperature Present
Value = 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 Airflow Pickups 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 Low
Air Velocity section in this document.
• Check that the flow setpoints are realistic in comparison to the installed box dimensions.
If Inadequate Cooling = True • Check the Starved Box parameter to determine if this is a flow
problem. If so, see the Starved Box parameter in this table for more information.
• 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 pneumatic valves, or other equipment related problems in the heating system or device.
MovAvg ABS Flow Err = (is much larger than the deadband)
See Correcting Distorted Flow Patterns Due to Duct Design in this document to troubleshoot this problem.
Table 6: VAV Box Flow Test Results
Symptom Cause Action
Flow high at Position 0 and low at Position 100
Actuator Direction
Incorrect Change the Direction To Close attribute of the DamperActuator Output or Stepper Motor Output (SMO).
Flow too high at Position 0 (Flow > Area * 80)
Damper Leakage Refer to Troubleshooting Airflow Leakage at a Fully
Closed Damper section of this document.
No change in flow from
Position 0 to 100 Loose ActuatorLinkage Refer to the Checking Flow Problems by VerifyingVelocity Pressure Sensor Operation section of this
document, or type in -100% to cause the damper actuator to close.
Unexpected stroke time (See Table 7.)
Damper and Actuator stops not aligned
1. Reset stop positions. Refer to the Mounting and
Wiring Variable Air Volume Modular Assembly (VMA) 1400 Series Controllers Technical Bulletin (LIT-6363125).
2. Reset the controller using HVAC PRO software. On the Action menu, click Reset Controller.
Loose Actuator
screw 1. Tighten screw. Refer to the Mounting and WiringVariable Air Volume Modular Assembly (VMA) 1400 Series Controllers Technical Bulletin (LIT-6363125).
2. Reset the controller using HVAC PRO software. On the Action menu, click Reset Controller.
Bound Damper 1. Reset stop positions. Refer to Mounting and Wiring
Variable Air Volume Modular Assembly (VMA) 1400 Series Controllers Technical Bulletin (LIT-6363125).
2. Check for bent damper shaft. 3. Check for duct damage.
4. Reset the controller using HVAC PRO software. On the Action menu, click Reset Controller.
Blockage in Duct 1. Open duct and clear blockage.
2. Reset the controller using HVAC PRO software. On the Action menu, click Reset Controller.
Actuator travels past damper full open
1. Reset stop positions. Refer to Mounting and Wiring
Variable Air Volume Modular Assembly (VMA) 1400 Series Controllers Technical Bulletin (LIT-6363125).
2. Reset the controller using HVAC PRO software. On the Action menu, click Reset Controller.
Flow increases from Position 0 to 50 and decreases from Position 50 to 100
45°°°° box with
damper stops set at 90°°°°
Reset stop positions. Refer to Mounting and Wiring
Variable Air Volume Modular Assembly (VMA) 1400 Series Controllers Technical Bulletin (LIT-6363125).
Table 7: Actuator Rotation Stroke Times for the VMA1410/1420 (Damper full closed to full flow open)
Damper Rotation (Degrees) Nominal Stroke Time (Seconds)
90 30
60 20
45 15
30 10
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 Volume Modular Assembly (VMA) 1400 Series Overview and Engineering Guidelines Technical Bulletin (LIT-6363120). On a 90° box, install at the closed position.
2. Check that the actuator collar (or other linkage) is tightly locked to the damper shaft. Refer to the Variable Air Volume Modular Assembly (VMA) 1400 Series Overview and Engineering
Guidelines Technical Bulletin (LIT-6363120). Visually inspect for a damaged damper seal, bent damper blades, or a poorly designed damper. Replace if necessary.
Troubleshooting Low Air Velocity
To troubleshoot low air velocity:
1. Check airflow velocity in the Parameters list box in
HVAC PRO software. Typical minimum is 2 m/s (400 fpm). 2. Check whether the flow is due to an oversized box. If so, check
with the Balancer to determine if adjusting the box constant or other 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 the VAV box inlet.
2. Whether hard or flexible duct is installed, check if there is a duct transition less than three diameters in length upstream of the airflow pickups.
3. Check if inlet duct length is excessive. This also may result in pressure drops that restrict airflow. These design conditions can cause non-uniform air velocity across the flow pickup location. 4. To eliminate turbulent flow problems, install a section of straight
hard duct between the VAV box inlet and the flexible duct (or duct transition). The straight hard duct must be at least three times the 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 hardware tables.
2. Check sensor calibration. Verify that the input reading is the same as the standard. For temperature readings, a glass thermometer with 0.05°C (0.1°F) accuracy is necessary. Adjust the AI offset in the configuration file. For velocity pressure readings, ensure that Autocalibration was run from HVAC PRO software, then verify flow readings within 5% of balancing hood reading.
3. Use the HVAC PRO commissioning tool to verify that the AI point is reading steady (not unstable). If the velocity pressure is unstable, this could be due to turbulence in the duct.
4. Ensure that there are 3-5 duct diameters of straight ductwork upstream of the airflow pickups.
5. If AI point is still unstable, check the sensor for the following problems:
• 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 Analog Input Modify option of HVAC PRO software to average out the peaks and valleys and eliminate instability. Select the AI screen then double click on the AI point. Set anti-spike to True and set the spike fraction between 0.0 (none) to 1.0 (any amplitude spike). The default attributes for zone temperature have anti-spike set to True and spike function set to 0.3. This should correct most problems. If not, increase the filter order from none to first or second order. The filter weight can be set from 0.0 (low) to 1.0 (high).
Note: Filter values greater than 0.5 are not recommended and, if too large, could make the loop respond too slowly. Filtering must not be done on the velocity pressure sensor, because the P-adaptive algorithm uses air turbulence values to
Checking Flow Problems by Verifying Velocity Pressure Sensor
Operation
Note: To ensure accurate readings of the velocity pressure sensor, allow at least 30 minutes stabilization time before installing a product that just came from a storage environment which differs from the installed temperature.
To check flow problems by verifying velocity pressure sensor operation:
1. Recalibrate differential pressure by labeling and disconnecting the high and low side tubing from the box pickup pressure taps (preferred), or by commanding Autocalibration from HVAC PRO software, which closes the VAV box to obtain near-zero flow.
2. Autocalibrate the velocity pressure sensor while in Commissioning mode in HVAC PRO software by setting Autocalibration Request parameter to True.
3. Reconnect the high and low side tubing, ensuring the connections are correct.
Checking Airflow Pickups for Debris or Water
!
WARNING: Never blow into the VMA tubing. The velocity pressure transducer is very sensitive and will be destroyed by as little as 70 mbar (1 psi) of pressure.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 the high-and low-pressure sides of the sensor.
3. Disconnect the high and low tubes between the VMA and box pickup pressure taps to observe if there is water or condensation disrupting the pressure reading. If necessary, clear water from the tubes and reconnect.
4. Ensure that the VMA is installed vertically on the duct for best performance. If mounted horizontally, Autocalibration adjusts for position effect.
Checking for Incorrectly Wired Sensors and Misapplied Room
Assignments
To check for incorrectly wired sensors and misapplied room assignments:
1. When experiencing problems with poor control, check that the room sensor is connected to the correct VMA. Sometimes sensors are miswired to an adjoining room’s VAV box (see Figure 3). 2. Check that the VMA is controlling the right room. Crossed
ductwork leads to mistakes similar to the one shown in Figure 4.
VMA VMA Miswired Room Sensor Room Sensor
Figure 3: Incorrectly Wired Sensors
VMA VMA Room Sensor Crossed Room Sensor
Troubleshooting Balancer’s Flow Reading
To troubleshoot Balancer’s flow reading:Determine if there is a discrepancy of more than 5 to 10% between the actual flow and Balancer’s indicated flow. If so, speak with the
Balancer about recalculating the Box Constant for the VAV Box Calculation 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 8 to help to determine the source of the communications problem.
Table 8: Checking Performance with the VMA LED
If the LED Response Is Then
Not lit Power is not applied to the VMA or +15 VDC is shorted.
Irregular flashing four times
every second Communications between the VMA and the supervisory system is normal. One flash every second The VMA is offline. Verify N2 address switches, and continue
troubleshooting possible causes as discussed in the following sections of this 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 per second
The VMA memory is corrupted. Download the firmware code into the controller.
Identifying N2 Bus Configuration Problems
To identify N2 Bus configuration problems, use Table 9 to troubleshoot common offline configuration problems:
Table 9: Configuration Errors that Cause N2 Bus Communications Problems
Error/Trouble
Condition Possible Causes Solution
VMA cycles
online and offline. Two or more controllers have thesame address.
Change each duplicate address to a unique number.
A NCM process is using an unconfigured point (i.e., not listed in HVAC PRO.prn file).
Delete the use of the unconfigured point in the NCM process.
There are ‘Y’ or ‘T’ taps. Refer to the N2 Communications Bus Technical
Bulletin (LIT-636018).
The repeater lost power or is wired wrong.
Refer to the N2 Communications Bus Technical
Bulletin (LIT-636018).
The EOL is not set at the NCM or
N30. Refer to the N2 Communications Bus TechnicalBulletin (LIT-636018).
VMA does not
come online. Two or more controllers have thesame address.
Change each duplicate address to a unique number.
The address of the VMA was changed without its power being cycled afterward.
Cycle power on the VMA.
N2 Bus contains too many devices. A maximum of 100 N2 devices are allowed per NCM.
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 is
offline. EOL jumpers on NCM/N30 orW3 jumper on MM-CVT101 are not installed.
Install EOL jumpers or W3 jumper properly.
MM-CVT101 is not plugged into PC
or 120 VAC source. Plug MM-CVT101 into PC or plug it into a 120 VACsource. There are ‘Y’ or ‘T’ taps or the
repeater lost power or is wired wrong.
Refer to the N2 Communications Bus Technical
Bulletin (LIT-636018).
Testing for N2 Bus Opens, Shorts, and Crossed Wires
!
CAUTION: To avoid electrical shocks when troubleshooting, measure the voltage of each N2 wire to earth ground with a 100 K ohm resistor at the DMM. If line-voltage is measured, have a qualified electrician locate the fault.Test for N2 Bus opens, shorts, and crossed wires by either selecting all of the N2 devices and performing a CommDisable at the Operator Workstation (OWS), or by connecting an MM-CVT101 in place of the NCM 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 the N2 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 ranges listed above, the wire is grounded, shorted, or crossed with another wire.
4. Perform the following checks:
• Check that the polarity of the wiring is consistent from device to 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 the N2 connector and measuring the voltages from Step 3 on the pins of the VMA. If these do not match Step 3 with power on the VMA, replace it.
• N2 Bus is improperly terminated. There must be one switched end-of-line device on each N2 Bus, typically the NCM or N30.
5. Correct and measure again. If the readings are approximately within the ranges listed above, the Bus is properly wired.
Otherwise, proceed to Checking for Proper Device Isolation and Ground Loops.
Verifying Proper Transformer Installation
To verify proper transformer installation:1. Connect the DMM across the 24 VAC and the 24 VAC common terminal of the VMA. Read the voltage with all typical loads energized. A reading of 24-26 VAC is required when the line voltage 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 voltage rating.
3. Check the 24 VAC cable length versus wire gauge.
4. Recalculate the power requirements for the VMAs and loads on that 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 ohm resistor from the DC power supply (+15 VDC) output terminal of the VMA to earth ground.
2. Read the voltage. If the voltage is 5 VDC/VAC or greater, the circuit is improperly isolated or a valve actuator is connected, which ties COM to an earth grounded Terminal 1. A single earth ground is allowed.
3. Remove all the field wires and N2 Bus wires from the VMA, but leave the transformer wires attached.
4. With the DMM still connected, reconnect each set of field wires one at a time until the voltage is 5 VDC/VAC or greater. At this point, you have discovered one cause of the ground loop.
5. Correct the problem. Continue to reconnect each set of field wires until all ground loops are found and corrected. All grounds are corrected 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 Table
10 below to determine the next step based on the value of this parameter.
Table 10: VAV Box Mode Present Value
If the VAV Box Mode
Present Value Is Then
Cooling Check the Cooling PID Present Value. Go to Table 11.
Satisfied Check the Heating Available parameter (in the Command Mode parameter group) and the Actual Heating Setpoint parameter (in the Temperature Setpoint parameter group). Go to Table 13.
Heating Check the Box Heating PID Present Value. Go to Table 14.
Note: The logic in this procedure also applies for supplemental heating.
Table 11: Cooling PID Present Value
If the Cooling PID
Present Value Is Then
= 0% Check the Cooling PID Saturation Status. Go to Table 12.
> 0% VAV Box Mode does not switch to Satisfied until: • PID deintegrates to 0%
• Saturation Status = Low
(Saturation Status switches from Normal to Low after Cooling PID Present Value is equal to 0% for ten minutes.)
Note: To test the heating mode and save time during commissioning, override the VAV Box Mode Present Value to Satisfied with the Zone Temperature less than the Actual Heating Setpoint. This bypasses the Cooling mode saturation timers.
Table 12: Cooling PID Saturation Status
If the Cooling PID
Saturation Status Is Then
Normal Wait ten minutes for Low Saturation time to expire. Note: Saturation times can be viewed in the
Diagnostic View. On the Options menu, click View to change to Diagnostic view.
Low Recheck VAV Box Mode Present Value. It now equals Satisfied.
Note: To test the heating mode and save time during commissioning, override the VAV Box Mode Present Value to Satisfied with the Zone Temperature less than the Actual Heating Setpoint. This bypasses the Cooling mode saturation timers.
Table 13: Heating Available and Actual Heating Setpoint
If Then
Heating Available = False
BD169 is being commanded from Metasys Supervisory System.
Actual Heating Setpoint > Zone Temperature and
VAV Box Mode Present Value = Satisfied
Configuration was generated using
HVAC PRO software Release 7.00 and saved using Release 7.01.
Workaround: Upgrade the controller in
HVAC PRO software Release 7.01. On the Upload menu, click Upgrade Controllers. See the
Troubleshooting section of the Variable Air Volume Modular Assembly (VMA) 1400 Series Application Note (LIT-6375125) for more information.
Table 14: Box Heating PID Present Value
If Box Heating PID
Present Value is Then
=0% Check Box Heating PID Saturation Status. Go to Table 15.
>0% and
Box Heating = either Binary Output (normally open valve) or Binary Output (normally closed valve)
Check the Box Heating Make Limit parameter (in the Box Heating Output parameter group). The Box Heating PID Present Value must exceed the Box Heating Make Limit before the Box Heating Command Output will become active.
>0% and
Box Heating = Electric 1-stage
Check the Box Heating Make Limit parameter (in the Box Heating Output parameter group). The Box Heating PID Present Value must exceed the Box Heating Make Limit before the Box Heating Output Present Value and EHS Box Heating Command Output will change from Off to Stage 1.
Table 15: Box Heating PID Saturation Status
If Box Heating PID
Saturation Status is Then
Low Wait 60 seconds and recheck Box Heating PID Saturation Status. The status should change to normal. The period for the PID is 60 seconds, and it may take the full time to switch to Normal and begin to ramp up the PID.
Normal Wait for 60 seconds and recheck Box Heating PID Present Value. The period for the PID is 60 seconds, and it may take the full time to begin to ramp up the PID.
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