Printer Friendly

Proper specification of air terminal units.

Specifying Air Terminals

An air terminal unit is a relatively easy device to select and specify. However, the parameters that are included in a specification and schedule should be specific and leave little room for interpretation. What may appear as a complete and detailed specification and or schedule may actually be incomplete or unobtainable. The person interpreting the specification in some cases, may not have the same level of product knowledge as the specifying engineer, and may make decisions on the selection that the specifying engineer does not want on the project.

This article presents four areas that are typically included in an air terminal unit specification. Each area will be discussed with suggestions given to improve an air terminal unit specification.

Suggestions are discussed for specifying: pressure, noise criteria (NC), flow rates and insulation.

Specifying Pressure

The industry terms used to define the pressure in an air terminal unit are defined in ASHRAE Standard 130, Methods of Testing Air Terminal Units, and AHRI Standard 880, Performance Ratingfor Air Terminals, and from the ASHRAE Terminology Glossary at ashrae-terminology.

Minimum Operating Pressure: the static or total pressure drop through a terminal at a given airflow rate with the damper/valve placed in its full open position by its actuator while the terminal is operating under steady-state control.

Primary Air: Treated supply air that enters the air terminal unit.

External-Static Pressure Loss: for forced air systems the static pressure loss resulting from airflow through the ductwork and other elements external to the unit.

Maximum Allowable Pressure: maximum gage pressure permitted on a completed system.

Operating Pressure: the pressure occurring at a reference point in a system when the system is in operation.

There are two ways to specify the allowable pressure drop of an air terminal unit. The first is to schedule the inlet or primary pressure, and the discharge or external static pressure associated with each air terminal unit. Although this method clearly defines how each air terminal should be selected to meet the required pressure requirements, it can be time consuming. Using current available air terminal software selection tools can make this an effective method.

The second method of specifying the allowable pressure drop for an air terminal is to specify the operating pressure and the maximum allowable pressure drop for the terminal. This method is commonly used by specifying engineers and, in most cases, produces selections with the required accuracy for a project. This method also only requires the engineer to give these two parameters for all the air terminal units on a project.

It is fairly common to see specifications that do not define one of the two methods of specifying allowable pressure drop for air terminal units. The purpose of specifying the maximum allowable pressure for an air terminal unit is to make sure there is a high enough pressure at the inlet of the air terminal to deliver the required flow by overcoming the pressure of the air terminal, and the downstream external static pressure where applicable. If this pressure is not clearly shown in a specification or schedule, smaller units could be selected that may not be able to deliver the scheduled flow.

If the terminal units require a hot water coil, make sure the pressure drop through the air terminal unit includes the pressure drop through the hot water coil. The pressure drop through the hot water coil can be significant and without accounting for this, the required flow through the air terminal unit may not be obtainable. Regarding electric heat, typically the pressure drop through an electric heater is very low and insignificant when specifying the allowable pressure drop through an air terminal unit. However, there are applications where the pressure drop through the heater should be considered.

Finally, to select the sound produced by a unit at a given capacity requires the operating pressure to be stated. The operating pressure may or may not be the same as the primary inlet pressure. In the case of single duct units, dual duct units and parallel fan-powered units, it is the pressure required to pass through the terminal and through the downstream ductwork. On a series fan-powered air terminal, it is the pressure that is needed to deliver the required primary air to the inside of the unit. Operating pressure is given by the design engineer and is the pressure used to determine sound performance. It may not be the actual pressure differential the installed air terminal will see. Providing the operating pressure to determine sound performance ensures selections for sound are based on the same criteria.

The sound produced by an air terminal unit will increase as the inlet pressure increases. A terminal selected at an operating pressure of 0.5 in. w.g. (125 Pa) will produce lower sound levels compared to the same terminal, at the same flow at 1 in. w.g. (249 Pa). Depending on the type of unit, radiated or discharge or both may increase.

Specifying Noise Criteria (NC)

It is typical for an air terminal unit specification to include a maximum NC level. An example is a typical specification may indicate a maximum air terminal unit NC level of 35. The NC level, without giving the allowable attenuation factors is a meaningless number. There are two sources of sound generated by an air terminal unit. These are:

Radiated Sound Power Level: sound power that radiates from the terminal casing (plus the induction port if present).

Discharge Sound Power Level: sound power that is transmitted from the terminal outlet.

If noise criteria is used by the designer, the following terms need to be understood:

Sound: A physical disturbance, vibration, or frequency transmitted by a solid, liquid, or gas that is capable of being detected by the human ear.

Sound Power Level: The sound power of a source is its rate of emission of acoustical energy.

Sound Pressure Level: The ear's response to sound waves in air and are variations in pressure above and below atmospheric pressure.

NC: A single number for rating the sound of a space. The measured octave bands for a space are compared with the NC curves. These curves were developed with lines that represent constant perceived loudness by the human ear. The NC rating is the value of the highest NC curve touched by measured sound in Octave Bands 1 through 8.

Noise: Unwanted sound.

The actual sound that an air terminal generates is the sound power level and is measured for radiated and discharge sound per the method of test as outlined in ASHRAE Standard 130-2008, Methods of Testing Air Terminal Units. NC is actually a calculation to predict the sound heard by occupants in a space and is based on the response of the human ear. The method of predicting NC from sound power levels is outlined in the AHRI Standard 885, Proceduresfor Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets. AHRI Standard 885 is available at no cost at As of this date, the most current release of this standard is Standard 885-2008 with Addendum 1.

To establish fair competition among air terminal unit manufacturers, AHRI formed the Air Control and Distribution Devices (ACDD) section, which governs the air terminal unit certification program for AHRI. The standard used by the terminal unit manufacturers to obtain their air terminal unit certified data is ANSI/AHRI Standard 880-2011, Performance Rating of Air Terminals.

Because NC is a calculated value based on attenuation assumptions, the air terminal unit manufacturers agreed on the attenuation factors to use when publishing their air terminal unit catalog data. The first agreed-upon factors were given in the ARI Standard 885-90, Appendix E.

Appendix E was modified again in 1998 with the release of Standard ARI-885-98. Once again, manufacturers were required to publish cataloged NC values based on the attenuation for both radiated and discharge sound based on this appendix. The air terminal unit manufacturers revised Appendix E in 2008 with the release of AHRI Standard 885-2008 (Figure 1). Printed air terminal unit catalogs in engineers' libraries may include NC values that were calculated using attenuation factors that are from the outdated AHRI 885 Standard. The same terminal unit will have different cataloged NC rating, depending if the printed catalog uses the procedures outlined in Standards 885-90, 88598 or the current 885-2008. It is recommended that designers check the notes pertaining to the NC calculation in a printed catalog to make sure the values are based on the most current Standard 885.

The purpose of Appendix E is to allow a comparison of sound levels between manufacturers at common operating points and with common attenuation. The actual sound levels on installed units, unless the conditions are exactly the same as shown in Appendix E, will vary. Designers also should make sure they are using NC data obtained by the methods required per Standard 885-2008 or the most current release.

One way to avoid any confusion is to specify sound power levels. Beginning in 2012, terminal unit manufactures were required to add end reflection into the discharge sound power levels for their catalog and certified data. This does not change the attenuation values found in AHRI Standard 885 but it does increase the discharge sound power levels found in the manufacturers' catalogs. More information on end reflection can be found in the June 2012 ASHRAE Journal article, "End Reflection Loss."

Specifying Flow Rates

There are two types of controls for air terminal units; they are pressure dependent controls and pressure independent controls.

Pressure-Dependent Control System: a control system in which the airflow through the air terminal varies with system pressure.

Pressure-Independent Control Systems (Pressure-Compensated Control System): control system in which the airflow through the air terminal is independent of system pressure.

The majority of systems designed today are pressure-independent systems. In a pressure-independent control system, the flow range for an air terminal unit is determined by two factors. The first is the differential velocity pressure as determined by the difference between the total and static pressure values sent by a flow sensor to the controller. The second is the operating range of the transducer that is built into the controller. Typically, a pressure independent VAV controller has a built-in transducer that receives the signal from the air terminal unit sensor and senses the differential across a diaphragm.

The DDC VAV controller uses the pressure differential to compare to a lookup table and reports airflow based on the unit size and the K factor. K factors vary between manufacturers (Equation 2). The minimum and maximum capacity for flow in a terminal unit is set by the operating range of the transducer in the controller and the amplification of the VAV sensor.

VAV controllers generally have an operating range of velocity pressures from 0.03 in. w.g. to 1.0 in. w.g. (7.47 Pa to 249 Pa) and it is this range that sets the minimum and maximum capacities of an air terminal unit's primary air capacity. The minimum velocity pressure of 0.03 in. w.g. (7.47 Pa) has been an industry standard for many years. However, advancements in the accuracy of the transducers in VAV controllers has reduced the minimum for some controllers to a velocity pressure between 0.01 in. w.g. and 0.015 in. w.g. (2.49 Pa and 3.74 Pa), lowering the controllable flow for air terminal units.

One of the major differences in performance between different air terminal unit manufacturers is the amplification that is produced by the flow sensor installed in the unit and the resulting operating range published for that air terminal unit. The amplification of the velocity pressure produced by a sensor is created by directing the airflow around the sensor to create a lower static pressure at the point where the static pressure is measured (Figure 2).

Amplification factor (F): the ratio of sensor output to true velocity pressure. For example, a pressure sensor with a reading of 1.0 in. w.g. (249 Pa) of pressure at a true velocity pressure of 0.43 in. w.g. (107 Pa) would have an amplification factor of 1.0/0.43 = 2.3. F may be calculated from K with the following formula, where A is the nominal duct area in square feet. The nominal duct area is calculated based on the geometry of the duct, not on the actual free area.

Amplification Factor equation:

f = [(4005 x A/K).sup.2] (1)

Flow Coefficient

K = (4005 x A/[square root of F]) (2)

K is used in an air terminal unit VAV controller to calculate actual airflow using the following equation, where cfm is airflow in [ft.sup.3]/min and AP is flow sensor output in in. w.g.:

Determining flow using a air terminal unit flow sensor

Q = K x [square root of ([DELTA]P)] (3)


Q = Airflow Rate (cfm)

[DELTA]P = Sensor Differential Pressure (in. w.g.)

K = K-Factor Calibration Constant (Standard Air)

F = Amplification Factor (Sensor Gain)

A = Nom. Duct Area ([ft.sup.2])

Velocity sensors are not all the same. The amplification of a sensor will vary between manufacturers Table 1 (Page 32). The range at minimum flow (0.03 in. [V.sub.p]) may vary as much as 24%. When specifying air terminal units, a design engineer may want to consider what range of flow is required to meet ventilation standards and make sure the specified product and controller is designed to operate in that range. This has become more significant, especially as engineers look to reduce flow rates to reduce the energy usage of a system. The higher the amplification, the lower the minimum cfm that can be obtained and controlled.

Specifying Insulation

If the insulation specified by the designer is not specific enough, the owner may get something other than what the designer intended and the difference may make the terminals unfit for the application. It may occur that the interpretation may be within the guidelines of a specification, but not what the design engineer was actually requiring for a project. More information on specifying air terminal unit insulation can be found in the September 2010 ASHRAE Journal article, "Specifying Insulation for Air Terminal Units."


Selecting and specifying air terminal units for a project is relatively simple. However, without clearly defining what is required for a project may cause the designer's intent to be misread, resulting in air terminal units that met the engineer's specified requirements, but not what the design engineer ultimately wanted for that project.

It is recommended that the design engineer:

* Clearly define pressures by choosing a pressure method to select air terminal units.

* Use maximum allowable pressure drop to define minimum pressure that will be accepted. This method also requires the designer to state operating pressure to allow the sound levels to be selected. The inlet pressure at the air terminal unit will determine the sound power level of that terminal unit. Without stating the inlet pressure, the sound levels for a terminal unit cannot be determined.

This can be an acceptable method to give the selection requirements for all the air terminal units on a project.

* Inlet and discharge pressure can be given for each air terminal, which would give the requirements for both allowable pressure drop and also at what point to provide sound levels. The schedule for determining the individual performance required for each terminal can be obtain using manufacturers' air terminal selection software.

* State what the noise criteria (NC) should be based on. Currently, the AHRI Standard 885-2008 with Addendum 1 is the standard to be used for calculating NC. Using the latest or most current version will ensure the most consistent noise levels in the occupied space. An alternative is to specify the maximum acceptable power levels for both radiated and discharge sound power levels.

* Check to make sure the specified flow range for an air terminal unit is obtainable. Air terminal units operating ranges for comparable inlet sizes between different manufacturers are not all the same. Specifications should include the minimum and maximum flow for the primary air valve based on obtainable flows.

The flow sensors of varying manufacturers will allow different minimum and maximum flow rates. Designers should make sure to verify both the flow range of the sensor and controller to be included in a schedule.

* Clearly state the requirements for the insulation required for the air terminal units on a schedule. This is commonly overlooked by specifying

engineers but can make differences on performance and efficiency of the systems.


(1.) 2011ASHRAE Handbook--HVACApplications, Chapter 48, Noise and Vibration Control.

(2.) 2009ASHRAE Handbook-Fundamentals, Chapter 8, Sound and Vibration.


David A. John, P.E., is general manager, vice president of ADE Engineered Solutions of Florida in Tarpon Springs, Fla.

TABLE 1 Comparison of obtainable minimum flows (cfm) at 0.03 in. w.g.

INLET SIZE          6 IN.   8 IN.   10 IN.   12 IN.   14 IN.   16 IN.

MANUFACTURER A         81     154      318      433      576      805
MANUFACTURERB          94     171      284      407      563      710
MANUFACTURER C         78     157      249      328      522      665
PERCENT SPREAD ON   17.0%   10.1%    21.8%    24.3%     9.4%    17.4%
  (0.03 IN. W.G.
COPYRIGHT 2014 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE)
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2014 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:John, David A.
Publication:ASHRAE Journal
Geographic Code:1USA
Date:Sep 1, 2014
Previous Article:Performance of HVAC systems at ASHRAE HQ: Part one.
Next Article:Future climate impacts on building design.

Terms of use | Privacy policy | Copyright © 2019 Farlex, Inc. | Feedback | For webmasters