Johnson Controls VMA1200 Series Operating instructions

LONWORKS Systems Manual 1162

VMA1200 Section

Issue Date 0401

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

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.

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