Functional Testing GuidanceTerminal Units

This functional testingguidance is designed to aid in developing test procedures for a specific project by describing the steps involved in testing a particular system. The guidance should be adapted as necessary to address the configuration and performance requirements of the system being tested. Additionally, codes may require specific testing procedures that may not be addressed in this document. All tests based on this guidance should be reviewed carefully to ensure that they are complete and appropriate.

Test Procedure:Variable Volume Terminal Units

Overview

The test guidance provided below will focus on variable air volume terminal units with hot water reheat. Specific test procedures applicable to electric reheat will be provided when applicable. Many of the test procedures outlined for variable air volume terminal units will be applicable to other system types as well, including cooling-only units, constant volume units, and fan-powered units.

This document will provide guidance on verifying proper operation of the features and elements of terminal units (TUs). This includes the spot checking of programming, construction checklists and diagnostic functions as well as the testing of CO2controls, occupancy sensors, temperature, flow, reheat, and fans. The areas that this test guidance covers include, but are not limited to:

  • Air flow multi-meter for verification of air flow station calibration or use during test if no air flow stations.
  • Flow (cfm) control (damper, flow sensor and control loop)
  • Unoccupied, warm up, night low limit, night high limit
  • Diagnostic and self-tuning functions (auto-zero, flow error, duty cycle, etc.)
  • Interactions with air handler (duct static pressure setpoint and fan speed control)
  • Fan control (both series and parallel fan-powered units)

System Description

Air terminal units are devices that regulate the flow of air at the zone level from a central air handling unit. TUs come in a variety of configurations like constant volume, variable volume, fan-powered, and reheat.

Test Conditions

The primary air handler must be capable of providing air though the TU that is being tested. But the air handler does not need to be in automatic control nor provide conditioned air. A technician should be available to command and adjust the control points in the building automation system in order to simulate various test conditions. Specific test condition requirements are provided with each respective test method.

Example Test

The following test form was created using this guidance document. It is available at

  • Terminal Unit Reheat Functional Test. ID# 417

Test Equipment

Required test equipment will always include a digital thermometer. If temperature calibrations will be checked, the contractor’s original calibrating instrument should be used. If using another instrument, it should be checked against the original instrument and adjusted prior to the calibration check. Additionalequipment may include a hydronic pressure meter for valve or coil measurements.

Test Procedure Outline

  1. Preparation

1.1Create a test form

1.2Develop a sampling strategy

1.3Determine acceptance criteria

1.4Take necessary precautions

1.5Completeprefunctional checklist

1.6Specify test participants and roles/responsibilities

  1. Component Testing

2.1Static inspections

2.2Reheat coil water flow

2.3Strainer cleanliness

2.4Sensor calibration

2.53-way valve check

2.6Actuator calibration checks

2.7Control programming

  1. System Testing: Sequence of Operations

3.1Normal cooling to heating sequencing

3.2Heating coil valve control loop stability

3.3Space temperature stability

3.4Discharge air control

3.5CO2 Control/Demand-controlled ventilation

3.6Occupancy sensor control

3.7Heating coil valve leakage

3.8Unoccupied and override control

3.9Night low limit and morning warm-up

3.10Night high limit and morning cool-down

3.11Special fan powered TU guidelines

3.12Special guidelines for constant volume TUs: Hospital operating rooms

3.13Interactions of TUs with air handler fan speed control

  1. Trend Analysis

4.1Heating coil control loop

4.2Space temperature control

4.3Excessive reheat of overcooled zones

  1. Automated Testing

5.1Semi-automated testing

5.2Automated testing

1.Preparation

1.1Create a test form. Testing will be easier, more conclusive, and more efficient if the test procedure is thought through and documented before conducting the test. Developing a test form will assist in data collection and subsequent evaluation and may allow less experienced staff to execute the tests. Each test procedure should reference the specific sequence it is verifying.

1.2Develop a sampling strategy. For most commercial applicationsit is impractical to verify complete system performance on all installed terminal units. Certain features may be checked on a larger sample of terminal units,but in general only a small fraction of the terminal units warrant full functional performance testing. Typically the sampling strategy is applied separately to each type of TU (constant volume, variable volume reheat, variable volume cooling only, etc.). If a set fraction of TUs fail, another set fraction of TUs istested. To avoidtesting all the features of the TU when only one element or feature hasfailed, the TU test canbe divided up into sections and failures can be tracked by section. Additional testing is only necessary for the failed sections. For laboratories, hospitals,or other critical applications,sample sizes up to 100% may be required.When automated or semi-automated testing can be used, sample sizes may be increased accordingly. The example sampling plan below illustrates these concepts.

Example Sampling Plan

The specifications call for a random sample of 5% of all TUs of similar type to be tested(minimum of 3). Total number to be tested of similar type = 8. The specifications also require that if 10% of the sampled TUs fail (any No Pass items), then another 5% of the total population must be tested. This applies to the featuresof the test, i.e., if a feature fails, only that feature of additional TUs needs to be tested. Record test results in the table below:

Sub-Section / % Failed of 1st Sample / % Failed of 2nd Sample
I. Static Inspections
II. Sensor calibration
IV. Actuator calibrations
V. Programming
VI. Functional tests

1.3Determine acceptance criteria. Acceptance criteria should be provided with the test procedures. Many elements are simple pass/fail, but others like space temperature control and reheat valve control loop stability are more complex. Part of the acceptance criteria is the balancing report, so it will be necessary to have a copy in hand. Example criteria are provided later in this guidance document.

1.4Take necessary precautions. Precautions should be taken to ensure the safety of equipment and comfort of occupants. Terminal unit testing can result in dramatic changes in space temperatures of enclosed offices and occupants should be advised before testing takes place.

1.5Complete prefunctional checklist. Verify that the system is ready for functional testing. Prefunctional checklist items include, but are not limited to, the following:

  • Central air handling unit and respective terminal units have been balanced
  • Air handler is fully operational, including all safeties and interlocks
  • ForTUs equipped with hot water reheat,check:
  • Hydronic piping is flushed and clean
  • Heating water valve is balanced
  • Pumps and heating plant are operating
  • All control sequences are programmed per design intent

1.6Specify test participants and roles/responsibilities. The testing guidance provided in this document can assist in verifying proper system performance in both new construction and existing building applications. At a minimum, the following people should participate in the testing process. Refer to the Functional Testing Basics section of the Functional Test Guide for a description of the general role and responsibility of the respective participant throughout the testing process. The roles and responsibilities should be customized based on actual project requirements.

New Construction Project / Existing Building Project
Commissioning Provider / Commissioning Provider
Mechanical Contractor / Building Operating Staff
Control Contractor / Controls Contractor
Test, Adjust, and Balance Contractor

2.Component Testing

This test describes procedures for a VAV TU with reheat. Specific guidelines for fan powered boxes and constant volume boxes are providedin section3.1.

2.1Static inspections (completed on all units in the sample unless otherwise noted).

Visually check the following items from the construction checklist completed by the contractor:

  • Unit is properly labeled and accessible.
  • Filter is clean (fan powered units).
  • Inlet conditions to the TU are:
  • Smooth, round, straight duct for 3 duct diameters for velocity pressure sensor, 2 duct diameters at minimum
  • Smooth, round, straight duct for 3 to 5 duct diameters for single point electronic sensors
  • If straight duct is not possible and an elbow is necessary, use hard duct elbows (with straighteners if possible) instead of installing an elbow in the flex duct.

2.2Reheat coil water flow(check only on halfof the units sampled).

The intent of this step is to spot check the TAB report and verify that the reheat coil design flow rate is met. This procedure is the same for systems with either automatic or manual balancing valves.

  • With the hot water plant operating under normal conditions, command the reheat coil control valve to the 100% open position and measure the differential pressure (dP) across either the balancing valve or the reheat coil itself. Most valve and coil manufacturers will have performance data that correlates dP with flow rate through the device.
  • Using the manufacturer’s dP vs. flow rate data, use the measured dP to find the flow rate through the device and compare this value with the design flow rate specified for the reheat coil.
  • If the measured flow rate is not within ±10% of design, consider it deficient. However, note that the performance of manual balancing valves is dependent on overall loop pressure. If nearly all of reheat valves are closed when the test is performed, there is a chance the pressure within the water loop may exceed normal operating conditions. If the measured flow rate is 10% greater than design, consider commanding a significant number of reheat valves open in order to simulate a normal hot water pressure and retest the coil.

2.3Strainer cleanliness(check only on the other half of the units sampled).

The intent of this step is to verify the water loop is clear of debris that may prevent design water flow through the coil. This check is only necessary on the terminal units not checked for design flow rate within the testing sample.

  • Valve off the heating coil, remove the strainer, and check for cleanliness.
  • To pass, basket strainers must have an unclogged area greater than or equal to 80% of the strainer area. In-line strainers with area equal to pipe cross section must be 90% clean.

2.4Sensor calibration (complete on all units in the test sample).

Normally there are only two sensors that control a terminal unit –space temperature and terminal unit air flow. However, some projects will also have a discharge air temperature sensor. The air flow is calibrated by the balancer and is typically most efficiently checked during a random check of all the balancing work. The space sensor that controls the terminal unit should have already been calibrated and is checked at this time. If the supply air is monitored only, the sensor may not need to be checked, but if it is used for control, its calibration should be checked at this time. Some contractors may insist that the sensors are factory calibrated and account for the wire length of the sensor. However, field experience has shown that calibrations are still needed on many sensors.

To avoid disputes, it is best to either use the same instrument that the contractor usedfor theoriginal calibration or to use a thermometer that has been adjusted against the contractor’sprior to testing.

  • Hold the calibrated test instrument within 6 inches of the site sensor.
  • Verify that the sensor reading atthe building automation systemis within the limitspecified when compared to the test instrument-measured value. Normally this limitshould be equal to the resolution of the calibrating instrument and BAS readout (typically ±0.1F).
  • If the sensor is outside the specified tolerance, the contractor should install an offset in the system so the reading matches the test instrument reading. For very large jobs, it may be more time-efficient to randomly check the calibration on all space sensors at one time rather than during anindividual terminal unit functional test.

It is important to use digital thermometers that are sufficiently accurate,or the calibration process can reduce the sensor accuracy. Withan RTD space sensor,ifthe overall BAS reading accuracy is desired to be ±1.0F, it will be necessary to use a calibrating instrument and transducer with a total accuracy of no less than ±0.9F. The typical digital thermometers used by many controls contractors do not meet this accuracy requirement. For example, a common brand’s best digital thermometer series is ±0.5F accurate, but when the thermocouple probe accuracy of ±2.0F is added (via square root of the sum of the squares), the total accuracy is ±2.06F, which cannot provide a check of a BAS reading within 1.0F accuracy. Thermometers that use thermistors rather than thermocouples are more accurate.

2.53-way valve check(complete on all units with 3-way valves in the sample).

Most terminal units will have 2-way valves but some have3-way heating valves, which should be checked for proper installation, set-up, and programming. When programmed or wired backwards, the valve will open when being commanded to close, causing the space to overheat. The balancer should have verified this, but the frequency of failures warrants another check.

  • If the zone is not already in cooling mode, lower the space temperature setpoint 10F below current zone temperature.
  • Verify proper 3-way valve set up is one of the following ways:
  • Check that the actual space temperature is within 2F of the setpoint.
  • Measure the supply air entering the space from the TU via the diffuser (it should be within 3F of the air handler primary air temperature).
  • Measure the return side of the TU coil (it should be 10F or more below the heating water temperature).

These readings should immediately provide indication of a malfunctioning valve.

2.6Actuatorcalibration checks(complete on all units in the test sample).

All terminal units will have an actuator on the primary cooling air flow damper. Dual duct VAV boxes will have actuators on both the primary cooling and heating air dampers, and units with a hot water reheat coil will have an actuator on the control valve. The air flow damper calibration will be checked later in the test during sequence of operation checks. The heating water valve should be calibrated (sometimes called “spanning”) atthe BAS by commanding the valve closed, full open and to an intermediate position while observing that the actuator shaft is representing the respective position. Small actuators may not provide valve position feedback to the BAS (only the commanded valve position is shown at the BAS), making visual verification necessary.

2.7Control programming(complete on all units in the test sample).

Checking the control programming can be an important area for locating inconsistencies and potential long-term performance problems. Often, the contractor will use a few terminal units todetermine the control parameters and then use these same parameters on all remaining units. Butthe response of terminal units varies depending on actual zone conditions and individual unit performance characteristics, and parameters must be customized for the specific application.

Ask the controls programmer to provide all programming and control parameters for the terminal unit being tested (parameters programmed in the TU controller or BAS). Comparethese parameters to the written sequences. All variances should be corrected. Note that some parameters and features available for a terminal unit may not be appropriate for every application. Regardless, all parameters should be documented to assist in system troubleshooting andenhancing system performance through control modifications. Below are some common parameters that should be verified and documented.

  • Auto TU diagnostics. In the control system diagnostics,
  • Check the controller and actuator accumulated run times, and the moving average flow error.
  • Check the moving average space temperature deviation from setpoint.
  • Check the ratio of actuator to controller runtime. Ideally it should be less than 3%, but less than 5% is acceptable.
  • Check the moving average flow error. It should be less than 10% of maximum cooling flow rate.
  • Check the moving average space temperature deviation. It should be less than 3F.
  • Address Check.TU address matches the TU location and ID on the plan drawings and control drawings.
  • Cooling minimum and maximum flow rate setpoints.Programmed maximum and minimumcooling flow ratesetpoints in the BAS match (within 10%) the latest plan drawings and balance report. Typically the minimum cooling air flow rate is based on the minimum design ventilation requirement for the zone being served. However, when zone occupancy loads are less than design, the minimum air flow rate may overcool the space resulting in excess reheat to maintain space temperature setpoint. A high minimum cooling air flow rate can also cause comfort complaints in a cooling-only VAV box application. It may be possible to reduce the minimum air flow rate setpoint if trending verifies that the reheat coil is enabled a significant number of hoursin what should be considered a “cooling” situation, or the space is continually below zone temperature setpoint in a cooling-only application.
  • Heating minimum and maximum flow rate setpoints.Programmed maximum and minimum heating flow rate setpoints in the BAS match (within 10%) the latest plan drawings and balance report. In most situations, the minimum heating air flow rate setpoint will be the same as the minimum cooling air flow setpoint. This may not be the case when electric reheat is used due to specific air flow requirements associated with electric reheat elements (refer to section3.1 for a detailed discussion). Occasionally the designer may stipulate in the control sequences that the amount of air delivered to the space during heating mode be increased from a minimum to maximum value. The misconception is that increasing the flow rate will deliver more heat to the zone. Unfortunately a higher flow rate means the temperature differential across the coil decreases and may cause comfort complaints because the hot air may feel “cold” to the occupants. It is more effective to reduce the amount of air flow during heating mode to ensure design temperature differential across the heating coil. If the minimum and maximum heating flow rate setpoints exceed the minimum cooling flow rate, consider raising thisas a control strategy issue with the designer.
  • K-factor (flow coefficient).K-factor programmed in the BAS is within 20% of the K-factor on the submitted control drawings, unless explained by the balancing contractor. If theactual K-factorvalue is less than 0.5 or greater than 4, further investigation is warranted.
  • Temperature adjustment range. Some zone temperature sensors provide occupants the ability adjust the temperature setpoint either up or down via a slider on the sensor. Document the range programmed in the BAS and compare with the design value (if applicable). Typically the range should be between 1F and 3F.
  • Occupied cooling and heating zone temperature setpoints. Note that some systems actually only have one central setpoint and then a bias above and below this central setpoint that defines the cooling and heating setpoint.
  • Unoccupied cooling and heating zone temperature setpoints.
  • Heating coil valve stroke.Measurestroke time for incremental valves.
  • Cooling space temperature setpoint proportional band. This controls the response speed of the primary air damper and affects over-and under-shoot.
  • Heating space temperature setpoint proportional band. This controls the response speed of the reheat valve (and potentially the primary air damper) and affects over-and under-shoot.
  • Primary air damper proportional band. This controls the response speed of the primary air damper and affects over-and under-shoot.
  • Duct area inlet. Compare to actual duct and plans. If they do not match,air flow will be inaccurate.
  • Damper stroke time. This value comes from controller specification/cut sheet.
  • Auto-zero function schedule.Set and enable.

3.System Testing: Sequence of Operations