Functional Testing Guidance
Terminal Units
This functional testing guidance 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 CO2 controls, 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
www.ftguide.org/ftct/testdir.htm.

Terminal Unit Reheat Functional Test. ID# 417
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Functional Testing Guidance
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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. Additional equipment may include a hydronic pressure meter for valve or coil
measurements.
Test Procedure Outline
1.
2.
3.
Preparation
1.1
Create a test form
1.2
Develop a sampling strategy
1.3
Determine acceptance criteria
1.4
Take necessary precautions
1.5
Complete prefunctional checklist
1.6
Specify test participants and roles/responsibilities
Component Testing
2.1
Static inspections
2.2
Reheat coil water flow
2.3
Strainer cleanliness
2.4
Sensor calibration
2.5
3-way valve check
2.6
Actuator calibration checks
2.7
Control programming
System Testing: Sequence of Operations
3.1
Normal cooling to heating sequencing
3.2
Heating coil valve control loop stability
3.3
Space temperature stability
3.4
Discharge air control
3.5
CO2 Control/Demand-controlled ventilation
3.6
Occupancy sensor control
3.7
Heating coil valve leakage
3.8
Unoccupied and override control
3.9
Night low limit and morning warm-up
3.10 Night high limit and morning cool-down
3.11 Special fan powered TU guidelines
3.12 Special guidelines for constant volume TUs: Hospital operating rooms
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3.13 Interactions of TUs with air handler fan speed control
4.
5.
Trend Analysis
4.1
Heating coil control loop
4.2
Space temperature control
4.3
Excessive reheat of overcooled zones
Automated Testing
5.1
Semi-automated testing
5.2
Automated testing
1. Preparation
1.1
Create 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.
1.2
Develop a sampling strategy. For most commercial applications it 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 is tested. To avoid testing all the features of the TU
when only one element or feature has failed, the TU test can be 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 features of 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:
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Sub-Section
I. Static Inspections
II. Sensor calibration
IV. Actuator calibrations
V. Programming
VI. Functional tests
1.3
Terminal Units
% Failed of
1st Sample
% Failed of
2nd Sample
Determine 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.
1.4
Take 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.
1.5
Complete prefunctional checklist. Verify that the system is ready for functional
testing. Prefunctional checklist items include, but are not limited to, the following:
1.6

Central air handling unit and respective terminal units have been balanced

Air handler is fully operational, including all safeties and interlocks

For TUs equipped with hot water reheat, check:

o Hydronic piping is flushed and clean
o Heating water valve is balanced
o Pumps and heating plant are operating
All control sequences are programmed per design intent
Specify 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
Commissioning Provider
Mechanical Contractor
Control Contractor
Test, Adjust, and Balance Contractor
Existing Building Project
Commissioning Provider
Building Operating Staff
Controls 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 provided in section 3.1.
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2.1
Terminal Units
Static 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:
o
2.2
Smooth, round, straight duct for 3 duct diameters for velocity pressure
sensor, 2 duct diameters at minimum
o Smooth, round, straight duct for 3 to 5 duct diameters for single point
electronic sensors
o 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.
Reheat coil water flow (check only on half of 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.
2.3

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.
Strainer 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.
2.4

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.
Sensor 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
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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 used
for the original calibration or to use a thermometer that has been adjusted against the
contractor’s prior to testing.
 Hold the calibrated test instrument within 6 inches of the site sensor.

Verify that the sensor 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 ±0.1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 an individual terminal unit functional
test.
It is important to use digital thermometers that are sufficiently accurate, or the
calibration process can reduce the sensor accuracy. With an RTD space sensor, if the
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.
2.5
3-way valve check (complete on all units with 3-way valves in the sample).
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. 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:
o
o
o
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).
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These readings should immediately provide indication of a malfunctioning valve.
2.6
Actuator calibration 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”) 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. 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.
2.7
Control 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 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.
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).
Compare these 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 and enhancing system performance
through control modifications. Below are some common parameters that should be
verified and documented.

Auto TU diagnostics. In the control system diagnostics,
o
o
o
o
o

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 minimum cooling flow rate setpoints 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
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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 hours in 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 section 3.1 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 this as 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 the actual K-factor value 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. Measure stroke 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.
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
Terminal Units
Auto-zero function schedule. Set and enable.
3. System Testing: Sequence of Operations
These system testing procedures verify that TUs respond according to the sequence of
operations. These tests are performed primarily by changing setpoints and observing
response in the BAS. Normally, every TU in the sample receives full sequence of operation
testing.
3.1
Normal cooling to heating sequencing. The ability of the terminal unit to respond
to zone cooling and heating loads should be verified. Verification can be
accomplished by simulating a call for maximum cooling and then watching system
response as a heating load is simulated.
In order to perform the test, the central air handling unit must be operating at the duct
static pressure setpoint. Simulate a call for maximum cooling by lowering the zone
cooling temperature and heating temperature setpoints 10F and 20F, respectively,
below current zone temperature and observe the following:

Reheat coil valve is 100% closed.

Primary air damper modulates to full cooling position and design maximum
cooling flow rate is achieved.
Continuing from the previous step with the heating temperature setpoint kept at the
same value, simulate a neutral zone condition by raising the cooling temperature
setpoint 10F above current zone temperature. Observe the following:

Reheat coil valve remains 100% closed.

Primary air damper starts to modulate toward the minimum cooling position and
some intermediate cooling flow rate is achieved. The control loop will
eventually drive the primary air damper to the minimum cooling flow rate.
Continuing from the previous step with the cooling temperature setpoint kept at the
same value, simulate a call for heating by raising the heating temperature setpoint
5F above current zone temperature. Observe the following, in order:

Primary air damper modulates to the minimum heating position and the
minimum heating flow rate is achieved. Note that in most applications the
minimum heating and cooling flow rates are specified to be the same value.

Reheat coil valve begins to modulate open. The control loop will eventually
drive the reheat valve to 100% open.

During heating mode, measure the discharge air temperature from the TU and
verify it is not more than 15F above the space temperature in order to minimize
stratification.
If desired, simulate a transition from heating to cooling mode by reversing the test
procedure outlined above. Observe that the heating coil valve fully closes before the
damper starts to modulate open and the air flow increases. Normally, the point where
the heating coil valve closes fully is the space heating setpoint. The space must
continue to warm through the dead band to the cooling setpoint before the TU begins
to increase its flow rate towards maximum.
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Special considerations: The following operating conditions may occur during the
test depending on system type and the actual control sequences specified by the
designer.
 Electric reheat: The minimum heating flow rate may be higher than the
minimum cooling flow rate in order to ensure adequate air flow across the
electric heating elements.

Dual duct terminal units: The sequences for a dual duct system are similar to
those outlined above except that during heating, the minimum cooling flow rate
is maintained continually and the heating air damper modulates as necessary to
maintain zone temperature setpoint.

Series fan-powered terminal units: In a series fan-powered terminal unit, the
fan operates continually when the air handler is ON. The sequence of operations
is similar to those outlined above except that during heating mode, the minimum
cooling flow rate is maintained continually and the reheat coil valve (or heating
air damper in the case of a dual duct system) modulates open as necessary to
maintain zone temperature setpoint.

Parallel fan-powered terminal units: The sequences of operations are similar
to those outlined above for a cooling load. When the zone temperature
approaches the heating setpoint, the primary air damper modulates to the
minimum cooling flow rate and the fan turns on to temper the supply air with air
from the return plenum. If the zone temperature continues to drops below
heating temperature setpoint, then the primary air damper remains at the
minimum cooling flow rate, the fan stays enabled, and the reheat valve modulates
open as necessary to maintain zone temperature setpoint.
3.2
Heating coil valve control loop stability. This can best be verified through
trending. Refer to Section 4 of this document for trending test procedures and
acceptance criteria.
3.3
Space temperature stability. This can best be verified through trending. Refer to
Section 4 of this document for trending test procedures and acceptance criteria.
3.4
Discharge air control. If the TU discharge air is being controlled, adjust its set point
and observe the heating coil valve modulate appropriately.
3.5
CO2 control / Demand-controlled ventilation. The test procedures and acceptance
criteria for verifying CO2 control are provided in the
Demand_Controlled_Ventilation test guidance document.
3.6
Occupancy sensor control. Some TUs are tied to occupancy sensors which limit
operation to periods when the sensor indicates occupancy. If the zone is unoccupied,
the unit may go into “UNOCCUPIED”, an intermediate mode, or the primary air
damper closes completely. Note that this feature should only be tested once the
furniture is in place in order to simulate actual occupant conditions. With the zone
unoccupied for at least 20 minutes (variable depending on how frequently sensor
status is polled), walk by the doorway with the door open and verify that the sensor
did not trip the unit to an OCCUPIED condition, as specified in the control
sequences. Then walk into the room and ensure the sensor enables an OCCUPIED
condition, as specified in the control sequences. Sit in the chair for 10 minutes at the
most remote occupant locations and do nothing more than read. Ensure that the
sensor does not put the unit into an UNOCCUPIED mode. Leave the room and
record the time needed for the TU to return to UNOCCUPIED mode. Ensure both
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the OCCUPIED and UNOCCUPIED control parameters meet the specified sequence
of operations.
3.7
Heating coil valve leakage. The test procedures and acceptance criteria for verifying
valve leakage are provided in the Valve Leak-by test guidance document
(Valve_Leak_By.doc).
3.8
Unoccupied and override control. First verify that the unoccupied schedule and
override duration meets the actual schedule. Reduce the override duration to 5
minutes for testing. Either change the time of day to be within an unoccupied time
period, or adjust the unoccupied period to include current time. Note that the
occupied schedule must be changed as well to prevent the system from executing
both “occupied” and “unoccupied” commands in rapid succession. Ensure the
system enables the “unoccupied” parameters per the design sequence of operation.
Then engage the override button and observe the system reverts back to the
“occupied” conditions per the design sequence of operations. Wait for the length of
the override duration and observe that the unit goes back into UNOCCUPIED mode.
When complete, return all settings to their original values.
3.9
Night low limit and morning warm-up. The night low limit control sequence will
provide heating when the space temperature is below the unoccupied heating
setpoint. To test, put the TU and air handler units into unoccupied mode. This can
be accomplished by either changing the time of day to be within an unoccupied time
period, or adjusting the unoccupied period to include the current time. Note that the
occupied schedule must be changed as well to prevent the system from executing
both “occupied” and “unoccupied” commands in rapid succession. Change the
space unoccupied heating setpoint to be 5F above the current space temperature so it
will go into a night low limit mode. Observe the following:
Non fan-powered units

The central air handler starts with its outside air dampers closed. For dual duct
systems, only the central heating air handling unit is enabled.

Hot water plant starts (if applicable).

Primary air damper modulates to minimum heating flow rate. For dual duct
systems, the primary cooling air damper remains 100% closed.

Reheat valve modulates, electric heating elements are staged ON, or primary
heating air damper modulates as necessary to meet zone temperature setpoint.
Fan-powered units

All central air handling units stay OFF.

Primary cooling air damper remains 100% closed.

Terminal unit fan starts to circulate return air into the space.

If the zone still calls for heating, the following should occur: the reheat valve
modulates, the electric heating elements are staged ON, or the central heating air
handling unit will start and the primary heating air damper modulates as
necessary to meet setpoint.
Once the unoccupied setpoint is met, all systems go back into unoccupied mode.
When testing is complete, return all system parameters to normal.
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3.10 Night high limit and morning cool-down. The night high limit control sequence
will provide cooling when the space temperature is above the unoccupied cooling
setpoint. To test, put the TU and air handler units into unoccupied mode. This can
be accomplished by either changing the time of day to be within an unoccupied time
period, or adjusting the unoccupied period to include the current time. Note that the
occupied schedule must be changed as well to prevent the system from executing
both OCCUPIED and UNOCCUPIED commands in rapid succession. Change the
space unoccupied setpoint to be 10F cooler than the current space temperature so it
will go into night high limit mode. Observe the following:

Primary air damper is driven to maximum cooling flow rate.

In fan-powered systems, the parallel fan stays OFF while a series fan is
commanded ON.

The central air handler starts in full economizer position.

If the economizer is locked out or cannot handle the load, the cooling plant is
enabled.
Once the unoccupied setpoint is met, all systems go back into unoccupied mode.
When testing is complete, return all system parameters to normal.
3.11 Special fan powered TU guidelines.

During the programming check, verify that a delay timer is programmed so all
fan powered units don't start at once (if specified), to minimize peak demand.

In series type fan-powered boxes, the TU fan could be rotating backwards if the
primary air handler is started before the TU fan is started. This would result in
the TU fan running backwards once it is energized. Avoid this condition by
ensuring the TU fan starts before the central AHU fan, keeping the primary air
damper closed until the TU fan is started, or verifying that the TU fan can handle
backward rotation and restart in the proper direction.

In series type fan-powered boxes, make sure during design duct static pressure
conditions with the TU damper fully open that primary air is not pushing out the
return air inlet, rather than drawing in (caused by the primary air volume flow
rate exceeding the fan flow rate).

During static inspections, observe the unit for excessive fan noise or vibration.

During static inspections for parallel boxes, check the balancing report for any
TUs that needed dampers added to the return plenum intake. Pay close attention
to these units since increased velocity across the damper may create excessive
noise.

During functional testing of parallel fan powered units, verify that when the unit
is in heating mode, the backdraft damper is fully open and that as soon as the unit
goes out of reheat mode into cooling mode that the fan turns OFF and the back
draft damper closes.

Refer to the test procedures above for special issues with fan power boxes during
unoccupied high and low limit modes.
3.12 Special guidelines for constant volume TUs: Hospital operating rooms.
Operating Rooms have both supply and return TUs that have a fixed flow differential
in order to maintain a positive pressure in the room. Many of the test procedures
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outlined in this document apply. However, a specific test may be necessary in this
particular application. Put the air handler and exhaust or return fans into a worst case
situation (command other zones to maximum flows, etc.) to try and starve the supply
side, including simulating a dirty TU filter by covering ½ of it with paper. Repeat this
test but starve the return side to verify that the fans have sufficient capacity to meet
the design flows for the TU being tested. This test is less important in control
schemes that vary the flow differential to maintain an actual monitored room
differential pressure. For the fixed differential flow control scheme, do not trust the
differential alone. Test the actual pressure across the door to be sure the room is
actually positive across the door.
3.13 Interactions of TUs with air handler fan speed control. As the space cooling load
fluctuates, the TU dampers respond by modulating. As the dampers close, duct static
pressure increases. The duct static pressure setpoint is exceeded and the system backs
down the fan speed via the variable speed drive. In order to take advantage of the
energy savings potential in this strategy, two conditions should exist.

First, the duct static pressure sensor should be 2/3 to 3/4 down the duct from the
first to the last terminal box takeoff on the most hydraulically restrictive branch.
This can be checked visually as part of the TU test.

Second, when the balancer determines the fixed duct static pressure setpoint
(regardless of where it is located), the most restrictive branch from the air handler
down to the terminal unit should have all balancing dampers fully open. This can
be checked during the terminal unit or air handler tests by asking the balancer to
show you the branch that is fully open. Have the balancer walk you to each
balancing device (you can check their location on the plans) to verify they are all
fully open.
Refer to the AHU_Reset test guidance document for additional issues pertaining to
the terminal unit/air handler interactions, as well as detailed test procedures and
acceptance criteria.
4. Trend Analysis
4.1
Heating coil control loop. To verify if the heating coil valve (HCV) is under
control, trend the HCV command and the space temperature at one to two minute
intervals for 24 hours. The HCV command should not be hunting.
4.2
Space temperature control. Testing terminal units is not complete until trending of
space temperatures has been accomplished. This trending is usually implemented on
more than the small sample of TUs that received manual functional testing. The
number of rooms trended will depend on the size of the project, the critical nature of
the spaces, and the capabilities of the control system and the controls technician. 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 ±1F of setpoint at
all times, except during start-up.
4.3
Excessive reheat of overcooled zones. As stated previously in section 2.5, the
minimum cooling air flow rate setpoint is typically based on minimum design
ventilation requirements for the zone being served. In many instances, actual
occupant and equipment loads are far lower than design load estimates and the zones
tend to be overcooled. Heat is added to the air stream in a reheat situation to
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maintain zone temperature but zones served by cooling-only VAV boxes will simply
become overcooled, possibly leading to comfort complaints. Zone temperature and
setpoint, as well as reheat operation, should be trended to identify zones that are
being overcooled. A possible solution is to discuss the potential for lowering the
minimum cooling air flow rate setpoint with the designer.
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5. Automated Testing
5.1
Semi-automated testing. Some control systems can run reports on various TU
parameters for diagnostic purposes, which can be a valuable troubleshooting tool.
These parameters, such as which TUs are starved for air and which zone
temperatures are not satisfied, can be useful in verifying project performance. Ask
the control technician which reports, if any, are available and use the information to
help troubleshoot operational problems that may arise after initial functional system
testing is complete.
5.2
Automated testing. Some TU controllers and their associated control system can
perform sophisticated diagnostic tests that can eliminate the need to perform many of
the functional tests described in this document. At least one control company has a
program that will test controller calibration, air flow measurement calibration,
damper operation, airflow tracking, reheat functions, and space temperature control.
It does this much like manual testing by changing setpoints and observing and timing
TU responses. A full report and list of deficiencies is automatically provided. This
testing is done after balancing is complete and when the space is unoccupied. It can
be scheduled ahead of time and run all night on up to six TUs at a time before
automatically moving to another set. If automated testing is used, modify any manual
tests and trends accordingly.
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