Chapter 12: Terminal Equipment

12.1. Theory and Applications. 1

12.2. Commissioning the Terminal Equipment1

12.2.1. Functional Testing Field Tips. 2

Key Commissioning Test Requirements. 2

Key Preparations and Cautions. 2

Time Required to Test

12.2.2. Design Issues Overview. 4

12.3. Testing Guidance and Sample Test Forms

 

12.1. Theory and Applications

The terminal equipment associated with an HVAC system provides the interface between the HVAC process that conditions the air and the occupants and processes occurring in the space. For the HVAC system to be perceived as successful by the end users, the terminal equipment must reliably perform its intended function, otherwise the system will not fulfill its design intent, regardless of the level of performance at the central system.

The most common functions provided by terminal equipment are control of space temperature and indoor air quality. In addition to these functions, terminal humidity control and filtration systems are often employed in health care environments. These additional functions are also common in industrial processes like clean rooms and environmental chambers.

In many ways, the efficiency and proper adjustment of the processes used by the terminal equipment will ultimately set the overall efficiency of the air handling system. In many instances, the terminal equipment will directly or indirectly control the system flow rates and distribution temperatures. In addition, many of the central system design and performance parameters will be set by the requirements of the terminal equipment. For instance, the inlet static pressure requirement for the terminal equipment will be an important part of the design static pressure specified for the air handling unit. Some of the system and terminal equipment parameters are interactive. Minimum outdoor air settings at the central air handling unit are set based on the occupant level expected in the various zones. The minimum flow settings on the terminal units are set based on the central system’s minimum outdoor air setting. If the terminal units are poorly commissioned or improperly adjusted relative to the actual loads they are serving, significant amounts energy can be wasted.

12.2. Commissioning the Terminal Equipment

The following sections present benefits, practical tips, and design issues associated with commissioning an air handling system’s terminal equipment.

12.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements lists practical considerations for functional testing. Key Preparations and Cautions address potential problems that may occur during functional testing and ways to prevent them.

Key Commissioning Test Requirements

General

Terminal equipment can ultimately set the overall energy consumption of the systems they serve. Many terminal unit control problems can mask each other. For example, excessive air flow may cause excessive reheat. Thus proper commissioning of the terminal equipment is critical to ensuring the efficiency of the overall system. During design review, verify that VAV box minimum flow rates are acceptable to meet design loads.  Prior to functional testing, verify the minimum flow rate setting in the BAS control system is programmed per design.  During functional testing, use trends to help identify excess reheat.  One option to minimize reheat is to lower the minimum flow rate setpoints, especially if actual occupancy is less than design. Proper coordination and sequence of operations between zonal heat (i.e. finned tube, radiant panels, etc) and VAV box flow rate/reheat must also be verified, if applicable. 

Sampling

Due to the high number of terminal units frequently associated with air handling systems, most terminal system functional testing verifies the performance of the equipment through spot-checking randomly selected units. (The process assumes that someone other than the commissioning provider—usually the controls contractor and the testing and balancing contractor—has verified all functions on all units.)

For example, the project acceptance might initially be based on testing 20% of the terminal units, selected at random. If 90% of the units tested perform satisfactorily, the project will be accepted without further testing, pending correction of any deficiencies in the tested units and perhaps some focused testing and repair based on those deficiencies. If less than 90% of the terminal units fail to perform, then the number of units tested may be increased by another 10%, and so on.

It is important to adjust the test requirement percentages so they are realistic for the project. For example, specifying that 20% of the terminal units will be tested and the project will pass if 90% of those tested are satisfactory may be meaningless on a project with only 5 zones. Similarly, specifying testing of 100% of the terminal units and requiring a 98% success rate may be more rigor than is required for a spec office building with 300 zones.

One caveat to the random sampling of units is that all types of units common in the building should be tested. It is not unusual for an air handling system to have hundreds of terminal units or more. Often, the drawings and documentation for the terminal equipment use a standard detail that is applied to all similar units, with a note like “Typical of VAV units 1-23, 29, 40-55, 75-80, and 98." One mistake in the detail or its interpretation can result in field problems that are replicated hundreds of times. Therefore, it is important that each type of unit be tested.

For VAV boxes with electric resistance reheat, it may be necessary to check ALL units for proper minimum air flow rate to ensure the heating element will operate correctly.  Many VAV boxes with electric reheat may require a manual reset if the heating element shuts down due to inadequate flow across the coil.  This safety may not be caught if a sampling plan is utilized and minimum flow rate is not checked for each box.

Key Component Testing Requirements

The following are the commissioning provider’s checks on the selected sample of units:

1 Verify that the unit passes static inspection. Visually check the following items from the construction checklist completed by the contractor:

a the unit is properly labeled and accessible

b the filter on fan-powered units is clean

c the inlet conditions to the terminal unit will provide satisfactory conditions for the velocity pressure or airflow sensor (typically 3 to 5 diameters of straight duct.)

2 Verify that the reheat coil design flow rate is met.

3 Verify that the strainer is clean and that the water loop is clear of debris that may prevent design water flow through the coil.

4 Verify that the sensors (typically space temperature and terminal unit air flow) are calibrated and that the reading at the building automation system is within the limit specified when compared to the test instrument-measured value. Normally this limit should be equal to the resolution of the calibrating instrument and BAS readout (typically ±0.1degF). 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.

5 Verify that the heating coil valve closes fully, and does not leak water through the valve.

6 If present, verify proper 3-way valve setup, wiring, and programming. Most terminal units will have 2-way valves but some have 3-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.

7 If the terminal unit has a hot water valve that does not provide position feedback to the BAS, verify heating water valve calibration (sometimes called “spanning”) at the BAS by commanding the valve closed, full open and to an intermediate position while observing that the actuator shaft is representing the respective position.

8 Verify the function of the terminal unit control programming. Often, the contractor will use a few terminal units to determine the control parameters and then use these same parameters on all remaining units. But the response of terminal units varies depending on actual zone conditions and individual unit performance characteristics, and parameters must be customized for the specific application.

9 Use the automated terminal unit diagnostics to check unit performance as part of the control programming verification: In the control system diagnostics, check the controller and actuator accumulated run times, the moving average flow error, and the and moving average space temperature deviation from setpoint. The ratio of actuator to controller runtime should be ideally less than 3%, but less than 5% is acceptable. Moving average flow error should be less than 10% of maximum cooling flow rate. The moving average space temperature deviation should be less than 3degF.

10 Additional terminal unit control verification checks should include the following:

• Documentation.

• Cooling minimum and maximum flow rate setpoints.

• Heating minimum and maximum flow rate setpoints

• K-factor (flow coefficient).

• Zone temperature adjustment range

• Occupied cooling and heating zone temperature setpoints.

• Unoccupied cooling and heating zone temperature setpoints.

• Heating coil valve stroke rate for incremental valves.

• Cooling space temperature setpoint proportional band.

• Heating space temperature setpoint proportional band.

• Primary air damper proportional band.

• Damper stroke time. This value comes from controller specification/cut sheet.

• Duct area inlet.

• Auto-zero function schedule. Set and enable.

• Check three-way valves for proper installation, set-up, and programming (if present).

Key System Testing Requirements

System testing verifies that terminal units respond according to the sequence of operations. These tests are performed primarily by changing setpoints and observing response in the BAS.

1 Verify the ability of the terminal unit to respond to zone cooling and heating loads by simulating a call for maximum cooling and watching system response, then simulating a call for heating and watching system response. How these tests are performed depends on the types of units being tested. It may be necessary to check for outlet air stratification in heating mode.

2 Verify the heating coil valve control loop stability. Trend the HCV command and the space temperature at one to two minute intervals for 24 hours. The HCV command should not be hunting.

3 Verify space temperature stability. If all rooms can be trended, trend at 10 minute intervals for 3 days, or take a smaller sample for 7 days. Trend space temperature, HCV command, supply air temperature, and outside air temperature. Verify that the space is kept within ±1degF of setpoint at all times, except during start-up.

4 Verify the performance of demand controlled ventilation at the zone level, if present. For example, the VAV box minimum flow rate is reset based on CO2 concentration.

5 Verify occupancy sensor control, if present. For example, zone temperature setpoint and/or VAV box minimum flow rate is reset based on an occupancy signal.

6 Verify unoccupied and override control. First verify that the unoccupied schedule and override duration meets the actual schedule. Then engage the override button and observe the system reverts back to the “occupied” conditions. Wait for the length of the override duration and observe that the unit goes back into UNOCCUPIED mode.

7 Verify night low limit and night high limit control sequences. Note that these sequences are typically different for fan-powered terminal units than for non-fan-powered units.

8 Verify morning warm-up cycle and morning pre-cool cycle control sequences.

9 Verify the integration of the terminal unit control with the AHU fan speed control. Check that the duct static pressure sensor is 2/3 to 3/4 down the duct from the first to the last terminal box takeoff on the most hydraulically restrictive branch. Then, when the balancer determines the fixed duct static pressure setpoint, the most restrictive branch from the air handler down to the terminal unit should have all balancing dampers fully open.

Key Preparations and Cautions

Cautions

1 Testing the terminal equipment will affect the conditions in the zone under test and may generate unacceptable zone conditions, especially if the unit fails the test. The potential impact of testing on zone conditions should be considered when planning and scheduling testing and may preclude testing with the zones occupied.

2 Design and installation problems can be quickly replicated across hundreds of units due to standard detailing and installation practices. Similarly, a test procedure that has a bug can be replicated across the units. Therefore, it is advisable on projects with many zones to test run test procedures prior to replicating the necessary paperwork.

3 Due to high zone counts, problems uncovered during terminal unit testing can rapidly escalate into a significant time commitment to resolve.

4 Most terminal unit tests will manipulate the various settings in the units to force certain operating conditions and observe the results. It is critical that all parameters are returned to the correct settings and that the terminal units are verified as resuming normal function following the test. This is important with any functional test, but especially critical for terminal units since they have such a direct impact on zone comfort, indoor air quality, and on the efficiency of the air handling system serving them.

5 Testing a VAV box may require the use of ladders or perhaps mechanical lifts to reach the unit.  Proper safety precautions should be adhered to when working in an elevated position

Test Conditions

1 Generally, all other system functions need to be functional and stable to allow terminal equipment testing to proceed. The terminal equipment is dependent on the systems that provide their design inlet static pressure. Without control of the static pressure and flow delivered to the terminal equipment, it can be difficult to assess if an observed deficiency is the result of a terminal unit problem or a central system problem. So, 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.

2 Ideally, the conditions in the zone will reflect the occupied conditions, and perhaps even the design conditions. Sometimes, achieving design loads can be difficult, and the initial testing effort will need to be supplemented with ongoing trending during the first year of operation to verify that the design intent is met.

For very large jobs, it may be more time-efficient to check the calibration on all space sensors at one time (for the terminal units in the tested sample) rather than during an individual terminal unit functional test.

Instrumentation Required

Instrumentation requirements will vary from test to test but typically will include the following in addition to the standard tool kit listed in Functional Testing Basics:

1 Inclined manometers, Magnahelics, Shortridge Air Data Multimeters, and other instruments capable of measuring and documenting low air static and velocity pressures. This equipment can also be used to verify flow rates and calibration of the flow measuring elements on terminal equipment.

2 Flow hoods for use with the meters in item 1 to allow zone flow levels to be verified at the diffuser locations.

3 Particle counters for verifying cleanliness in applications where terminal filtration is employed.

Time Required to Test

There can be a wide range of variability in the time associated with terminal system testing, since the number of terminal units that exist and are tested varies based on the building and commissioning requirements. On a per terminal unit basis, the time required can vary from 15 to 30 minutes for simple systems with successful test results to hours or even days for complex systems and/or systems where the tests reveal major control problems. Qualification testing can be particularly time consuming in surgeries and clean rooms if a rigorous humidity, temperature, particle count, and sound test regime is specified. Generally all terminal units serving loads deemed critical should be functionally tested in addition to a random sampling of other units. On some projects, all zones may be tested if the processes they serve are subject to significant penalties in terms of loss of product, loss of productivity, energy inefficiency, or occupant safety if their terminal unit control functions are not properly implemented.

A commissioning provider's time to test can be adversely impacted if many of the terminal units fail the tests. When this occurs, the commissioning provider is forced to retest the failed units and may also be forced to test a higher percentage of the units on the project. On a project with a lot of problems and a large terminal unit count, the time required to complete terminal unit testing and troubleshooting can quickly spiral out of hand. These factors should be carefully considered when preparing the project specifications, budgets, and bids. Many commissioning providers include language in the specifications that obligates the contractor to pay the added costs incurred by repeated test failures and/or the need to test additional units due to poor performance of the initial test sample.

If initial tests on the first two or three terminal units reveal poor performance, it may be desirable to suspend testing of additional terminal units pending further work by the contractor.

 

12.2.2 Design Issues Overview

The Design Issues Overview presents issues that can be addressed during the design phase to improve system performance, safety, and energy efficiency. These design issues are essential for commissioning providers to understand, even if design phase commissioning is not a part of their scope, since these issues are often the root cause of problems identified during testing.

Many current technology DDC terminal unit control systems are capable of a wide variety of control strategies. These control strategies will produce a comfortable and safe environment when viewed from the occupants’ perspective, but they can have widely varying energy consumption associated with them. Matching the terminal control strategy to the requirements of the zone in a manner that provides a comfortable environment in the least energy intensive manner is critical to the overall efficiency of the building.

Many of the energy optimizing strategies employed on central systems will provide additional benefits if employed at the terminal unit level, by allowing the feature to be tailored to the requirements of the zone rather than the requirements of the overall system. Scheduling and demand controlled ventilation are good examples.

12.3. Testing Guidance and Sample Test Forms

Click the button below to access all publicly-available prefunctional checklists, functional test procedures, and test guidance documents referenced in the Testing Guidance and Sample Test Forms table of the Air Handler system module.

AHU Testing Guidance and Sample Test Forms