Johnson Controls VMA1200 Series Operating instructions

LONWORKS Systems Manual 1162
VMA1200 Section
Issue Date 0401
© 2001 Johnson Controls, Inc. www.johnsoncontrols.com
Code No. LIT-1162380
TECHNICAL BULLETIN
Troubleshooting VMA1200 Series Controllers
Troubleshooting VMA1200 Series Controllers ...................................3
Introduction........................................................................................................*3
Key Concepts...................................................................................................... 4
Troubleshooting Tools......................................................................................................*4
Hardware Concerns .........................................................................................................*5
VMA Control Performance Measures (Diagnostics) ........................................................*7
Procedure Overview......................................................................................... 11
Detailed Procedures......................................................................................... 12
Investigating Occupant Hot/Cold Complaints.................................................................*12
Troubleshooting Airflow and Temperature Problems.....................................................*14
Detecting Problems Using VMA LEDs ...........................................................................*19
Verifying Proper Transformer Installation.......................................................................*20
Checking for Proper Device Isolation and Ground Loops ............................................... 20
* Indicates those sections where changes have occurred since the last printing.

VMA12002

Troubleshooting VMA1200 Series Controllers 3
Troubleshooting VMA1200 Series
Controllers
Introduction
Although the Variable Air Volume Modular Assembly (VMA) has
been designed to provide reliable service, problems can develop in the
related building and HVAC (Heating, Ventilating, and Air
Conditioning) systems that affect VMA performance. Problems
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, to
installation errors, or to changes in use of a zone. Consider all of these
variables when troubleshooting.
This document discusses the indicators, tools, and procedures available
to troubleshoot systems employing the VMA. This document describes
how to:
●investigate occupant hot/cold complaints
●troubleshoot airflow and temperature problems
●detect problems using VMA Light-Emitting Diodes (LEDs)
●verify proper transformer installation
●check for proper device isolation and ground loops

VMA12004
Key Concepts
Troubleshooting Tools
Building Automation System (BAS)
The MetasysBAS gives access to VAV (Variable Air Volume)
system operation. Using the information available though Focus
windows (either Direct Map or Control System [CS] objects), data from
a malfunctioning system can be monitored to help problem analysis and
resolution.
LONWORKSNetwork Management Tools
LONWORKS network management tools provide additional
diagnostics. These third-party tools also allow you to make parameter
changes to correct temperature and airflow control problems.
Refer to the user’s manual of the specific network management tool
for more information.
Comm Pro Commissioning Tool
The Comm Pro commissioning tool is software that allows users to
view and modify device parameters to correct temperature and airflow
control problems. It also allows users to download new firmware to the
controller. Comm Pro software is part of M-Tool and can be launched
from M-Pro. For more information, refer to Comm Pro User’s Guide
(LIT-1162550).
VMA1200 Balancing Tool (VBT) Software
The VBT software helps you balance VAV systems with VMA1200
Series controllers. The software runs on a Palmcompatible handheld
device (Version 2.0 or higher operating system). The VBT commands
the VMA to desired flow setpoints, temporarily overrides some
outputs, and corrects configuration properties (parameters) stored in
the VMA. For more information, refer to Using the VMA1200
Balancing Tool (VBT) Software Technical Bulletin (LIT-1162400).

Troubleshooting VMA1200 Series Controllers 5
Hardware Concerns
Temperature Problems
Zone temperature sensor errors cause control problems. The AI range
may be improperly selected in the M-Pro software used to configure
the VMA1200. The temperature sensor may need an offset for cable
length. In addition, the temperature sensor may be improperly installed
or the element may be damaged. The temperature sensor may be
placed where it is affected by the sun, other zones, or supply air from
the diffuser. Refer to Variable Air Volume Modular Assembly (VMA)
1200 Series Overview and Engineering Guidelines Technical
Bulletin (LIT-1162350) for other sensor location issues.
Airflow Problems
The VMA auto calibrates the pressure sensor within the first hour after
power up and then periodically every two weeks (default is every
336 hours), to correct offset tolerance and effects of mounting
position. If greater flow measurement accuracy is required at low flow
rates where box inlet velocity is under 1.25 m/s (250 fpm), the Auto
Calibration Period can be decreased. Remember that during Auto
Calibration, the damper is commanded to fully close and the
airflow to the zone drops to zero for up to 3 minutes.
Refer to the VMA Airflow Rate section of the Variable Air Volume
Modular Assembly (VMA) 1200 Series Overview and Engineering
Guidelines Technical Bulletin (LIT-1162350) for more information.
Damaged damper seal, bent damper blades, poorly designed dampers,
or a misaligned actuator at end of travel causes air leakage during the
fully closed position. This causes offset errors during the pressure
sensor calibration.
Control problems can result from a slipping damper shaft or an
obstruction that causes the actuator to stall. If the damper overrotates,
control problems occur. You must provide fixed end stops at each end;
one where the damper is fully closed, and one at the fully open
position. For 90°boxes, the stops are built into the VMA. For 45°and
60°boxes, the VMA must be installed with the damper either fully
open or fully closed to provide its end stop.

VMA12006
Ductwork Design Issues
Air velocity is non-uniform if turns or transitions in rigid 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.
A minimum of three duct diameters of straight, unrestricted duct,
upstream from the airflow pickups is recommended. Correct flow
reading problems by installing straightening vanes in a duct section or
changing the duct configuration to provide greater separation between
the transition and the pickup.
Long duct length or sags in flexible duct cause significant pressure
drop. Avoid this by installing rigid duct three times the diameter in
length, starting at the VAV box inlet.
Flow Pickup Performance
The flow pickup must be installed properly. Poor installation causes
debris collection, plugged ports, or leaks between the high and low
pressure manifold.
Air Flow
Tota l Pre ss ur e
Static Pressure
Static Pressure
Pickup
High Pressure
Manifold
Pickup
Low Pressure
Manifold
Area of Increased
Velocity and
Decreased Pressure
Duct Wall
Pickup
Figure 1: Interaction of the Pickup and Air Stream
As shown in Figure 1, the pickups must have openings that are
perpendicular to the direction of flow. The low pressure ports face
downstream, and the passing air exerts a pull on these openings,
causing a pressure less than static. This results in an airflow pickup
gain, and a differential pressure of 1.5 to 3 times the velocity pressure.
Figure 2 shows 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.

Troubleshooting VMA1200 Series Controllers 7
Sensing Ports
Cross Tubes Squared Rings Straight Tubes
Sensing
Figure 2: Common Flow Pickups
VAV Box Size
The size and capacity of the VAV box should match the zone loads.
If the installed box is too small, insufficient cooling results and audible
noise caused by high flow rates is emitted. If the installed unit is
too large, proper control of airflow is difficult.
The VAV box area and flow pickup gains must be entered into the
VMA1200 for the controller to accurately calculate airflow. Enter
these values during either configuration or commissioning.
VMA Control Performance Measures (Diagnostics)
Temperature Control Measures
Key network variables that indicate temperature control problems are:
nvoEwmaSpace (network variable output Exponentially Weighted
Moving Average Space). This is an 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 all modes except heat and cool. If the VMA is interlocked
with supply air 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.
nvoEwmaAbsSpace. This is an average of the absolute value of space
temperature error (absolute value of setpoint minus temperature) over
the preceding eight hours of control. This calculation is stopped in all
modes except heat and cool. It is just like the nvoEwmaSpace, 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

VMA12008
indicate a cooling or heating problem, a cycling control, or an
extended cooldown/warmup in progress.
Airflow Control Measures
Key network variables that indicate airflow control problems are:
nvoEwmaFlow. This is an 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 during
morning warmup or when the Heat/Cool mode is off.
Increasing positive values indicate a starved box. Negative values
indicate a stuck damper or stalled actuator.
nvoEwmaAbsFlow. This is an average of the absolute or unsigned
flow control loop error (difference between setpoint and measured)
over the preceding 20 minutes of control in normal control modes.
This calculation is stopped during morning warmup or when the Heat
Cool mode is off.
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. An open or short in the
sensor normally causes this condition. This status is available in the
Alarm field of the nvoUnitStatus variable.
nvoMvgAveDutyCyc. This is the duty cycle of the damper. It
measures the performance of the damper output. A large value
indicates that the flow controller is having problems controlling.
nvoMvgAveRev. This is the number of reversals of the damper
output. It measures the performance of the damper output. A large
value indicates that the flow controller is having problems controlling.

Troubleshooting VMA1200 Series Controllers 9
Balancing Errors
Pressure independent VAV control jobs frequently require a flow
accuracy of 5-10%. The balancing contractor must adjust and certify
the flow rates specified by the consulting engineer.
When actual vs. measured airflow readings disagree, a problem exists
in the measurement equipment used by the controller, as well as that
used by the Balancer. Therefore, it is important that both the controls
contractor and Balancer understand the equipment, techniques, and
expectations of each other.
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 is anywhere from ±5% of full
scale to ±3% of reading on better instruments. A dated calibration
certificate shows the last calibration.
•Calibrating the flow hood on a different type of diffuser other than
that used on the job.
•Using multiple diffusers supplied by a single VAV box. When the
flow hood is placed over one diffuser, the hood presents 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 total flow.
•Performing airflow measurements using a hood rather than a
velocity probe type of instrument with a slotted diffuser. Tests
show that hood readings of some slotted diffusers are 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 1993 ASHRAE Handbook
Fundamentals, I-P Edition, 13.14-15.

VMA120010
VMA LED Indicator Status
The VMA LED provides general diagnostics for VMA power supply
and operating status. Viewing the LED signal and understanding what
each type of signal indicates quickly isolates some problems. See the
Detecting Problems Using VMA LEDs section for details.
LONWORKS Network Bus Communication Problems
LONWORKS Network Bus communications problems between the
VMA and the supervisory system are related to improper installation,
configuration errors, or both.
Typical communications problems include:
•broken or frayed wire
•internally shorted device
•too many devices (64 maximum per segment allowed)
•LONWORKS Network Bus too long
•ground loops--The LONWORKS Network Bus shield, if used, must
be earth grounded at only one location (usually the NCM). The
LON+ and LON- lines are never earth grounded. Tie the shield to
the SHLD terminal on the LONWORKS Network connector of the
VMA.
•missing, improperly located, or wrong value of end of line
terminators
Zone Bus Problems
If a communication error occurs while using the commissioning tool
with the VMA over the Zone Bus, the cause is often a loose or
improper connection between the CablePRO (AS-CBLPRO) or
CVTPRO (AS-CVTPROx00-0), the Palm compatible handheld device,
and the controller. Low batteries on the handheld device could also be
at fault.
The Zone Bus LEDs on the CablePRO or CVTPRO blink only when
sending or receiving data from the handheld device. The handheld
device only sends or receives data when it synchronizes with the VMA
or upon a command.
An effective troubleshooting technique is to observe the LEDs on
either a CVTPRO converter or a CBLCON cable connector
(AS-CBLCON). The red LED indicates power from the VMA and the
green LED indicates that the Zone Bus is present. With no handheld
device connected to the Zone Bus, the green LED blinks once every
second. When a handheld device is communicating, the LED blinks
several times per second.