SECTION 23 3600 (15840) – AIR TERMINAL UNITS

SUPPLEMENTAL INFORMATION

By David P. Rebhuhn, PE (GA PE#32928), NCEES (#18079), CSI Member

General Description:

Air handling units (AHUs) circulate and regulate air as part of an overall HVAC system for an entire building structure. Smaller air handling units for localized use, referred to as "air terminal units," control air-volume, pressure, sound, temperature, and mixing of air within a building structure for a given space. They are used primarily in medium- to high-velocity air systems. Modulation of airflow into the space is accomplished using velocity sensors and dampers located within the terminals as well as fan controls for units equipped with fans.

Air terminal units are available either as "variable-volume" (VAV) or "constant-volume" (CV) units. CV units deliver constant supply air quantities to a given space and usually contain a reheat coil to control air temperature. VAV units vary the volume of air supplied by the AHU to a given space to control air temperature and are pressure-dependent or pressure-independent. Pressure-dependent VAV units use control dampers in response to space temperatures. This causes a variation in air flow into a given space. Pressure-independent VAV units maintain constant airflow into a space regardless of variations in supply air pressure.

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Types of Air Terminal Units:

Bypass terminal units divert unneeded CV conditioned primary air from the AHU to the ceiling plenum based on control signals from the space thermostat. Units are available with reverse- or direct-acting pneumatic or electric thermostats. Thermostats directly control the by-pass damper assuring only the air that is needed is delivered to the space.

In general, the application usually involves a packaged rooftop air handling unit (RTU) where the potential for freeze-up of the cooling coil exists unless constant airflow is maintained. The bypassed air is either ducted back to the RTU or released into the return air plenum above the ceiling. When the space signals for less air volume, the integral bypass damper diverts more air from the supply directly into the return air path, thereby maintaining constant volume to the space. The availability of hot water or electric heating coils for the bypass terminal enables the unit to add supplemental heat to the space on a call for heating.

This application is the lowest first cost option but is energy inefficient when compared with VAV systems that can throttle system air delivery based upon demand. Bypass units are most often used in smaller buildings where the cost for a VAV system cannot be justified.

Air-to-air induction units supply cooler primary air and warmer re-circulated (induced) air from the ceiling plenum or other spaces via return ductwork. Unit-mounted control devices modulate the mixture of air in response to control signals from the space thermostat. This method requires higher inlet static pressure thus requiring more supply fan energy and capacity. These units are specified in Section 238200 (15760) – CONVECTION HEATING AND COOLING UNITS (TERMINAL HEATING AND COOLING UNITS).

Single-duct units with an actuator, flow sensor, and some type of control, are used to modulate primary air supply into a given space. The damper modulates air into a space based on the temperature requirements of the space. This is the simplest terminal unit but is only viable when no reheat or cooling is required and if the dampers can be shut completely. Units operate in a pressure-independent or pressure-dependent fashion. Pressure-independent operation is based on measuring and controlling air volume by opening and closing the unit damper in response to space temperature and air volume. Pressure-dependent operation is based on modulating the unit damper in response to the space temperature where the air volume may increase or decrease due to the changing static pressure in the main duct. A typical control sequence for "cooling only" controls the space temperature by changing the air volume to the controlled space based on the signals of the space thermostat. Either a minimum air flow to the space can be set, or the unit can close off the air flow completely.

Single-duct units are also used for cooling, cooling/heating (when the main air handling unit provides both), or reheat when equipped with a steam, hydronic, or electric heating coil that is installed in the discharge of the unit. Local cooling and reheat capability can be provided without any changes to the main air handling unit.

Reheat is sometimes included to prevent over-cooling when a minimum airflow is required for the controlled space. A typical control sequence may involve minimum cooling, maximum cooling, and reheat air flow. During the cooling cycle, the space temperature is controlled by the position of the modulating damper that regulates the amount of cool air introduced to the space. During the heating cycle and when the space temperature falls below a set-point, the controller increases the flow of reheated air.

  • Steam or hydronic reheating: Typically a 2-way valve will modulate open as long as there is a call for heat. Other control schemes may utilize on/off control or 3-way mixing valves.
  • Electric reheating: The first stage of heat is activated and as the space temperature continues to fall, additional stages of electric heat, if specified, are energized.

Dual-duct units primarily accept cold and warm air from separate supply ducts and mix the air to provide the desired discharge temperature. Another common configuration connects the main air supply and a dedicated ventilation system. Dampers on both ducts mix the air according to instructions received by a thermostat to provide the desired space temperature. Baffling may be required inside the unit or at the discharge to ensure uniform mixing of air streams. These units are commonly used in buildings where a minimum air flow is permissible during changeover from heating to cooling and where a reheat coil is not desired. VAV units have heating and cooling dampers that throttle separately (one of the dampers is always closed). CV units have dampers that throttle air in tandem. Air volume is sustained by modulating the relative amounts of hot and cold air. Air flow monitoring stations, indicating the amount of discharge air volume and cold air volume, are usually implemented. Though more costly than single duct units, the use of direct digital controls quite often makes the cost justifiable when greater accuracy in temperature and air flow is required.

Fan-powered terminal units need to be carefully selected to minimize the generation of radiated sound resulting from over-sizing the unit. Cycling fans may also add to ambient background noise. If possible, locate the units over unoccupied space such as a hallway or closet and limit the air flow volume to no more than 2500 – 3000 cubic feet per minute. Air filters and racks are not provided integral to the units by all manufacturers. Building owners and operators view filters on air terminal units as a high maintenance, low value feature.

Series units are typically used to vary cooling and heating loads in spaces with exterior walls or provide heat during unoccupied conditions when the central AHU has been de-energized. These units have two inlets and operate continuously in conjunction with the AHU. One of the inlets is connected to the supply duct from the AHU. The other inlet supplies return or plenum air from the occupied space. All air delivered passes through the unit fan. The air from the AHU is modulated by an airflow control mechanism based on calls from the thermostat. The unit compensates for this modulation by drawing in return or plenum air from the occupied space. It is important to start series units before the AHU to prevent "dumping" primary air into the return air plenum which may cause the fan to spin backwards. When power is applied to the fan motor in this situation, the fan may continue to operate backwards, causing high noise levels, lower air volume, and possible fan motor overheating. Control sequences allowing variable air flow may be implemented to prevent "over-airing" the occupied space. CV operation is generally more acceptable to occupants used to consistent ambient sound levels and air movement. A typical sequence of operation is where the minimum air volume from the primary AHU is based on occupancy requirements and the maximum air volume is based on the space cooling load. The percent of primary vs. return air volume changes while the total air volume for the space is held constant as the cooling load changes. Activation of the heating coil will occur upon a fall in space temperature below set limits.

Parallel units have fans mounted outside or parallel to the airstream which allows for intermittent fan operation. The fan draws air from the return air or plenum air from the occupied space. A backdraft damper stops the primary air from the AHU from entering the return air supply when the fan is not running. These units are well suited for VAV cooling and CV heating applications. When cooling, primary air from the AHU is allowed to bypass the fan and flow directly into the occupied space. When heating, the unit fan supplies warmer return or plenum air (which may or may not be reheated) which mixes with the primary air, prior to discharge from the unit. A typical sequence of operation modulates the primary air damper from minimum air volume (determined by the minimum space ventilation based on occupancy type) to the maximum cooling volume (established by the cooling load). The volume of primary air varies based on the cooling demand. The fan will cycle on upon activation of the reheat coil when the space temperature falls below the lower dead band limit.

Permanent split capacitor (PSC) motors with lubricated bearings and thermal overload protection are generally standard on fan-powered terminal units. These motors, usually AHRI certified and ETL listed, are designed for use with electronic speed controllers allowing continuously adjustable fan speed. The controls are electronic, matched to operate with the motor, and usually equipped with a minimum voltage stop. The terminal unit will have a single-point electrical and control connection. Electrical components are usually enclosed in a single control box with an access panel. PSC motors are somewhat inefficient because fan noise requires the motor to run at less than full load. When turned down to lower speeds, PSC efficiency falls into the range of 12 to 45 percent.

Electronically commutated (ECM) motors may be available as an option. These DC motors function using a built-in inverter and a magnetic rotor. They function at a high level (65 to 75 percent) of efficiency at a variety of speeds. ECMs have a higher initial cost when compared to PSCs but their energy efficiency reduces overall operating costs. ECMs are not prone to overheating and do not require additional measures to offset the generation of heat, as PSCs often do.

Controls:

Either electric or pneumatic controls are utilized with the bypass terminal due to lowest first cost. Fan powered, dual-duct, and single duct terminal units are usually specified to be provided with pneumatic, electronic, or DDC controls. Controls can be provided by the terminal unit manufacturer or separately. It is vital to ensure that all electronic or DDC control components are compatible with the building temperature control or BAS system (new or existing). The controls may be powered either from the conditioned supply air (system powered) or from a pneumatic or electric source (external).

Reset controllers regulate air volume at a modulating or fixed rate based on demand from the occupied space. Controllers can be specified to be pneumatic (pressure dependent or independent), electric (pressure dependent), or analog/direct-digital electronic (pressure independent). Temperature inputs are required for maintaining comfort levels within a space. Provisions for indication of actual airflow are required in order for the pressure independent controller to reset the airflow control device in VAV applications. Typically, the airflow volume control is available with either automatic or manual adjustment capabilities where automatic operation is enabled by a flow regulator, thermostat, or BAS system.

Sound Attenuation:

Spaces sensitive to sound may require additional attenuation in the ducted air system to prevent noise from being introduced into the space. Spaces such as concert halls, conference rooms, music studios, classrooms, and private offices may benefit from specifying an air terminal unit with an integral silencer. Some manufacturers include sound attenuation in their unit assembly. Others may offer it as an option. The difference between a silencer and a standard duct attenuator is very significant when it pertains to the attenuation of sound. A silencer that is typically attached to a duct may not provide the same amount of sound attenuation that can be obtained with an integrated unit that is manufactured and certified as an assembly. A common issue with combining different components such as a silencer and an air terminal unit is called "system effect." System effect is the additional pressure drop and sound generation resulting from duct elements being installed too close together in less than ideal inlet conditions. The reader may wish to seek out resources addressing system effect to gain a deeper understanding of sound attenuation, silencers, and the impact of system effect on ductwork pressure drop.

Sustainability Design Options:

LEED: There are several IEQ credits that may be obtained resulting from the application of VAV terminal units in HVAC systems.

  • IEQ Prerequisite 1 establishes a baseline for providing a minimum amount of outdoor air to buildings in order to maintain good indoor air quality. It references ASHRAE 62.1 for the building ventilation rates.

  • IEQ Credit 6.2 compliance addresses the controllability of HVAC systems where individual controls are provided for at least 50% of the occupants, in regularly occupied areas.

  • IEQ Credit 7.1 resulting from thermal comfort meeting the criteria with ventilation by mechanical means in addition to natural means.

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How to Specify:

Editing of Section 233600 is designed to start in PART 2, progress to PART 3, and finish up with PART 1.
The various types of air terminal units specified comprise the main articles in PART 2. When a particular air terminal unit is activated for use in the project, the appropriate air terminal unit listed under SECTION INCLUDES will be activated. Related articles and optional text in PARTS 1 and 3 will be activated as well.

When a particular article or paragraph is chosen that contains a reference standard in PART 2 and PART 3 the corresponding standard cited under REFERENCE STANDARDS in PART 1 is activated. If the Consolidated List of Citations option is active, cross sectional links (not visible in the links window) will activate the reference standard in Section 014219 – Reference Standards as well.

When a particular article or paragraph is chosen that cites another section in PART 2 and PART 3, the corresponding Section cited under PART 1-RELATED REQUIREMENTS is activated.

Certain Sections cited under PART 1-Related Sections are not cited in either PART 2 or PART 3 but are listed under RELATED REQUIREMENTS because they include items that might be expected to be found within this Section or include action items important for the completion of the work that are not specified in an obvious location (e.g. isn't obvious from the section title).

All optional text and choices under PART 2 include a fill-in to accommodate any updates that listed manufacturers may offer but are not shown in the choice options. Default options for choices are based upon what would be reasonable for the application. The content of PART 2, including the choices, has been structured to accommodate the listed manufacturers.

An equipment schedule has been provided in PART 3 for projects which are limited in scope or for which drawing equipment schedules have not been provided.

Sample Schedules for Drawings:

Do not duplicate information on Drawings and Specifications. The schedules below are examples that may be used as guides.

SINGLE-DUCT BYPASS AIR TERMINAL UNIT SCHEDULE

TAG

N/A

     

LOCATION (CEILING/ACCESS FLOOR)

N/A

     

MANUFACTURER

N/A

     

MODEL NUMBER

N/A

     

SIZE  

INCH (MM)

     

Unit Air

Max. Primary Air Flow

CFM (L/s)

     

Min. Primary Air Flow

CFM (L/s)

     

Min. Inlet Static Pressure

IN WG (Pa)

     

Min. Discharge Static Pressure

IN WG (Pa)

     

Hot Water Heat

Min. Air Heat Output

MBH (kW)

     

Entering Air Temperature

DEG F (C)

     

Leaving Air Temperature

DEG F (C)

     

Entering Water Temperature

DEG F (C)

     

Leaving Water Temperature

DEG F (C)

     

Water Flow

GPM (L/s)

     

Water Pressure Drop

FT H2O (kPa)

     

Electric Heat

Input

KW

     

Amps

mA

     

Stages

N/A

     

Specify Sound Power Below by either NC rating OR Octave Rating

Sound Power

Radiated

Noise Criterion

       

2nd Octave

dB at Scheduled Static Pressure

     

3rd Octave

     

4th Octave

     

5th Octave

     

6th Octave

     

7th Octave

     

Sound Power

Discharged

Noise Criterion

       

2nd Octave

dB at Scheduled Static Pressure

     

3rd Octave

     

4th Octave

     

5th Octave

     

6th Octave

     

7th Octave

     

 

SINGLE DUCT VARIABLE VOLUME AIR TERMINAL UNIT SCHEDULE

TAG

N/A

     

LOCATION (CEILING/ACCESS FLOOR)

N/A

     

MANUFACTURER

N/A

     

MODEL NUMBER

N/A

     

SIZE  

INCH (MM)

     

Unit Air

Max. Primary Air Flow

CFM (L/s)

     

Min. Primary Air Flow

CFM (L/s)

     

Min. Inlet Static Pressure

IN WG (Pa)

     

Min. Discharge Static Pressure

IN WG (Pa)

     

Hot Water Heat

Min. Air Heat Output

MBH (kW)

     

Entering Air Temperature

DEG F (C)

     

Leaving Air Temperature

DEG F (C)

     

Entering Water Temperature

DEG F (C)

     

Leaving Water Temperature

DEG F (C)

     

Water Flow

GPM (L/s)

     

Water Pressure Drop

Ft H2O (kPa)

     

Electric Heat

Input

KW

     

Amps

mA

     

Stages

N/A

     

Specify Sound Power Below by either NC rating OR Octave Rating

Sound Power

Radiated

(With Fan On)

Noise Criterion

       

2nd Octave

dB at Scheduled Static Pressure

     

3rd Octave

     

4th Octave

     

5th Octave

     

6th Octave

     

7th Octave

     

Sound Power

Discharged

(With Fan On)

Noise Criterion

       

2nd Octave

dB at Scheduled Static Pressure

     

3rd Octave

     

4th Octave

     

5th Octave

     

6th Octave

     

7th Octave

     

 

Dual-DUCT AIR TERMINAL UNIT SCHEDULE

TAG

N/A

     

LOCATION (CEILING/ACCESS FLOOR)

N/A

     

MANUFACTURER

N/A

     

MODEL NUMBER

N/A

     

COLD SIZE

INCH (MM)

     

HOT SIZE

INCH (MM)

     

Unit Air

Max. Cold Air Flow

CFM (L/s)

     

Min. Cold Air Flow

CFM (L/s)

     

Max. Hot Air Flow

CFM (L/s)

     

Min. Hot Air Flow

CFM (L/s)

     

Min. Inlet Static Pressure

IN WG (Pa)

     

Min. Discharge Static Pressure

IN WG (Pa)

     

Arrangement

Cold Duct

LH or RH

     

Hot Duct

LH or RH

     

Specify Sound Power Below by either NC rating OR Octave Rating

Sound Power

Radiated

Noise Criterion

       

2nd Octave

dB at Scheduled Static Pressure

     

3rd Octave

     

4th Octave

     

5th Octave

     

6th Octave

     

7th Octave

     

Sound Power

Discharged

Noise Criterion

       

2nd Octave

dB at Scheduled Static Pressure

     

3rd Octave

     

4th Octave

     

5th Octave

     

6th Octave

     

7th Octave

     

 

FAN-POWERED AIR TERMINAL UNIT SCHEDULE

TAG

N/A

     

TYPE:   PARALLEL (P) or SERIES (S)

N/A

     

LOCATION (CEILING/ACCESS FLOOR)

N/A

     

MANUFACTURER

N/A

     

MODEL NUMBER

N/A

     

SIZE  

INCH (MM)

     

Primary

Air

Max Air Flow

CFM (L/s)

     

Min Inlet Static Pressure

IN WG (Pa)

     

Min Discharge Static Pressure

IN WG (Pa)

     

Fan

Air

Max Air Flow

CFM (L/s)

     

Min Inlet Static Pressure

IN WG (Pa)

     

Min Discharge Static Pressure

IN WG (Pa)

     

Unit Air

(Parallel

Units)

Max Air Flow

CFM (L/s)

     

Min Inlet Static Pressure

IN WG (Pa)

     

Min Discharge Static Pressure

IN WG (Pa)

     

Hot Water Heat

Min Air Heat Output

MBH (kW)

     

Entering Air Temperature

DEG F (C)

     

Leaving Air Temperature

DEG F (C)

     

Entering Water Temperature

DEG F (C)

     

Leaving Water Temperature

DEG F (C)

     

Electric Heat

Input

KW

     

Amps

mA

     

Stages

N/A

     

Fan

Motor

Horsepower

HP

     

Type – ECM of PCS

       

Enclosure / Material

       

Voltage

       

Phase

       

Hertz

       

Specify Sound Power Below by either NC rating OR Octave Rating

Sound Power

Radiated

(With Fan On)

Noise Criterion

       

2nd Octave

dB at Scheduled Static Pressure

     

3rd Octave

     

4th Octave

     

5th Octave

     

6th Octave

     

7th Octave

     

Sound Power

Discharged

(With Fan On)

Noise Criterion

       

2nd Octave

dB at Scheduled Static Pressure

     

3rd Octave

     

4th Octave

     

5th Octave

     

6th Octave

     

7th Octave

     

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