Chapter 11: Distribution

11.1. Theory and Applications

11.2. Commissioning the Distribution System

11.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparations and Cautions

Time Required to Test

11.2.2. Design Issues Overview

11.3. Testing Guidance and Sample Test Forms

11.4. Supplemental Information


11.1. Theory and Applications

The distribution system provides the path between the air handling system and the terminal equipment that distributes the conditioned air. While relatively passive in nature, it can often account for a significant portion of the system’s energy consumption due to the static pressure requirement it imposes on the fan. The few active components in the distribution system can be critical to life safety functions and can impose significant damage on the system if they function inadvertently and without adequate safety measures in place to protect the system.

Subtle differences in the way a duct fitting is fabricated can make significant differences in the pressure losses associated with the fitting. For example, the ASHRAE Duct Fitting Loss Coefficient Tables document five different turning vane designs with loss coefficients that vary from a low of 0.11 to a high of 0.43. Duct fittings and pressure drops are discussed in detail in Section 11.4: Supplemental Information.

 There are two equations associated with evaluating the pressure loss through a duct fitting. As shown in Appendix D, the first is used to evaluate the loss as shown in Equation 11.1.

Equation 11.1 Duct Fitting Pressure Loss

This equation states that the loss through a fitting is a function of an experimentally determined loss coefficient and the velocity pressure associated with the velocity of the air flowing through the fitting. Equation C.5 is used to convert the duct velocity to its corresponding velocity pressure.

Equation 11.2 Velocity Pressure as a Function of Velocity

Notice that the velocity pressure is a function of the square of the velocity. That makes it a powerful relationship in HVAC systems. If you double the velocity through a fitting (to do this, double the flow; flow and velocity are directly related), you will increase the pressure loss through the fitting by a factor of four. The magnitude of the loss will be a function of the loss coefficient, with a smaller coefficient resulting in less pressure loss.

11.2. Commissioning the Distribution System

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

11.2.1. 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. Note that many of the following test requirements pertain to issues that should be addressed and checked during design, submittals, construction, or initial start-up in order to successfully test each system.

Key Commissioning Test Requirements

Most of the energy benefits to be realized from distribution system commissioning and testing are related to minimizing and avoiding pressure drops. There are also significant gains to be realized by ensuring that the systems will be fabricated and installed in as leak-free a manner as is warranted by the project’s requirements. Therefore, testing in the air handling distribution system is generally targeted at the following:

1 Verify during construction observation that construction and installation changes from design do not increase the distribution system pressure drop. For additional information see Section General Rules.

2 Verify that life safety equipment and systems are viable and reliable, including smoke and fire control systems. Proper installation of smoke and fire dampers should be verified prior to performing any functional tests on the distribution system.

3 Verify that pressure relief doors are installed and operate correctly to protect the ductwork if fire/smoke dampers shut down quickly but fans are still slowing down.

4 Verify that leakage requirements have been met and that the intended operating efficiencies are achieved.

5 Verify that pressure drops do not exceed design expectations. Identify and eliminate the root cause of high pressure drops by working with the testing and balancing team to test relevant portions of the system. The pressure drop should be reduced when possible, rather than increasing the operating static requirement of the system—a common solution applied in the field when the motor has capacity available beyond the design brake horsepower requirement.

Access doors should be appropriately sized and located close enough to specific devices to facilitate maintenance and inspection. For example, the access door near a smoke/fire damper should be large enough and close enough so that that the fusible link can be reached for testing and replacement as necessary by maintenance personnel.

Appendix C: Calculations contains detailed descriptions and examples of techniques that can be used to project potential savings due to elimination of pressure drops and savings due to elimination of leakage (leakage calculations to be presented in a future revision of this Guide).

Leakage and pressure drop requirements will generally be controlled by the duct and fitting fabrication techniques employed by the sheet metal contractor. In turn, the specification language, any referenced standards, and details provided on the drawings typically sets technique that will be employed. Tests for life safety related equipment will also need to meet the requirements of the applicable codes and standards, (typically NFPA 90A and B) as well as any local codes or industry related standards that may apply. (State licensing requirements for health care facilities are a good example).

Key Preparations and Cautions


Generally, the cautions employed should follow those outlined in Functional Testing Basics. In particular, testing fire and smoke dampers in active systems places the system at risk for failure due to air hammer and related effects and proper precautions should be taken to minimize this risk.

Releasing and resetting any fire damper should be undertaken with caution due to the sharp sheet metal edges that can be encountered and the high spring forces associated with the closure mechanisms.

Opening access doors on active systems can expose personnel to significant forces created by the operating pressures on the doors, and they should proceed with caution when opening, closing, or going through these doors. They are also at risk for being trapped inside an active unit if the pressures pull the doors closed behind them. Personnel should take appropriate precautions to prevent this including working in teams and making sure that someone on the site knows where they will be working and when they expect to be finished.

Testing safety systems such as permissive interlocks, static switches, and pressure relief doors can place the operating system at risk, the exact level of which is determined by the rigor of the test. Appropriate cautions and controls should be in place when performing these tests to thwart any problems and limit the exposure of the system to damage in the event that the component under test were to fail to perform its intended function.

Ensure all smoke and fire dampers are in the open position prior to performing any functional tests on the distribution system.

Test Conditions

Test conditions will vary with the test being performed.

Most pressure drop testing needs to be performed with design or near design flow occurring in the system or in the branch tested.

Leakage testing needs to be arranged to ensure that only one pressure class is tested at a time and so that leakage from the temporary closure plates is not a significant component of the overall leakage documented by the test or can be accounted for by some means.

Ideally, fire and smoke damper acceptance testing needs to be coordinated to occur after all other systems and equipment in the vicinity of the dampers has been substantially complete. These acceptance tests also need to be coordinated with any external authorities having jurisdiction, such as a City Inspector, Fire Marshall, or Insurance Underwriter to be sure that their criteria will be met and occupancy permitting will not be delayed.

Instrumentation Required

Instrumentation requirements will vary from test to test but typically will include the following in addition to the standard toolkit listed in Functional Testing Basics, Section 11. Basic Tools, Instrumentation, and Equipment:

1 A duct leakage-testing machine. This is often available from the sheet metal contractor, but may require certification if it has not be certified recently.

2 Inclined manometers, Magnehelics, Shortridge Air Data Multimeters., and other instruments capable of measuring and documenting low air static and velocity pressures.

3 Duct fitting pressure loss tables.

4 NFPA Codes 90A and B.

5 Vernier calipers and micrometers (for verification of sheet metal gauges)

6 SMACNA Duct Construction Standards

7 SMACNA HVAC Air Duct Leakage Test Manual

Time Required to Test

Time required to test is highly dependent on the test being performed.

1 Leakage testing typically requires 15 to 30 minutes per test section for the actual test. Preparation time on the part of the sheet metal contractor can be significantly longer than this.

2 Pressure drop testing can be time consuming if a pressure gradient must be established for the entire system, but individual readings will average 5-15 minutes depending on how difficult access is. Developing a system diagram and theoretical pressure gradient can take 4-16 hours depending on the complexity of the system, the availability and reliability the of existing documentation, the level of familiarity with the required calculations held by the analyst, and the amount of field inspection required.

3 Damper acceptance testing can take between 5-15 minutes per damper depending on how difficult access is. Systems with engineered smoke control cycles and numerous operating modes can take much longer, especially if significant coordination is required between several trades to achieve success. This topic will be covered in more detail in Chapter 15: Management and Control of Smoke and Fire.


11.2.2. 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.

Duct aspect ratio and duct size

Large ducts can operate at high velocities even if they are also operating at a low friction rate. High velocities imply high velocity pressures and higher losses through fittings. A poor fitting on a large duct can have pressure loss that is four or five times larger than the same fitting design applied in a smaller duct operating at the same friction rate.

Supporting discussion and example calculations are included in Section General Rules.

Duct fitting design

Subtle differences in the way duct fittings are fabricated can have major impacts of the pressure drop (and therefore energy requirement) associated with the fitting. See Section 11.4.1: Duct Fittings and Pressure Drops, for supporting discussion.

Improvements can often be realized without an increase in first cost and sometimes with a reduction in first cost just by folding the sheet metal a different way. An example of the importance of attention to geometry is presented Chapter 11, Section Duct Fittings. Simply specifying SMACNA standard construction does not adequately address this issue.

Fan inlet and outlet conditions

The arrangement of the inlet and outlet ducts on the fan can have a significant impact on its performance and energy consumption. Refer to Section Fan and System Effects, for an illustration.

Flex duct selection and installation

Flex duct runs and bends can have significantly higher pressure drops than equivalent sheet metal sizes. Use should be limited or other accommodations made. See Section Flex Duct, for details.

Damper locking mechanisms matter

Some manual balancing damper locking designs are more robust than others, and the balancing settings made with them tend to persist longer. The avoided operations and maintenance issues associated with the better locking mechanisms quickly pay for the modest added first cost. For an example, see Section Manual Balancing Dampers.

Fire and smoke damper selections can significantly impact energy use

Using air foil or ‘out of the air stream’ blade designs can reduce the pressure drop through fire and smoke dampers by 50% or more, saving significant energy over the life of the system for a modest increase in first costs. For details, see Section 11.4.2: Fire and Smoke Dampers.

The payback period for installing the increased efficiency dampers can be less than a year if the dampers are specified in the original design and have no retrofit costs associated with them. Appendix C.2.4 illustrates a calculation to estimate the energy savings associated with airfoil blade smoke isolation dampers instead of the conventional design.

Sudden damper closures can generate air hammer effects that can explode or implode duct work

The pressure pulses generated by an air hammer event have rise times that are less than a second and magnitudes that exceed 15 inches w.c. (see Figure 11.8). Systems that are prone to this sort of problem need to be designed to minimize the potential for damage. For details on what systems may be at risk for air hammer and how to avoid this problem, see Section 11.4.3: Air Hammer.

Duct leakage specifications need to be suited to the needs of the project

Duct leakage represents wasted energy. In some cases, the leakage can cause problems with maintaining the desired pressure relationships. A perfectly air tight duct system is a practical impossibility and the leakage rate is a function of a variety of parameters. Design leakage specifications and testing should be tailored to the needs of the project and the operating conditions the duct system will see. Additional information can be found in Section 11.4.4: Duct Leakage.

Duct insulation affects energy consumption and IAQ

Duct insulation can save energy by preventing undesirable temperature changes in the distribution system, especially VAV systems operating at low flow rates. It can also prevent condensation on cold ducts and the indoor air quality and corrosion problems associated with it. Internal insulation can improve the acoustic characteristics of a system but may be more prone to IAQ problems. For this reason, internal insulation is not permitted by some jurisdictions in some applications. For more details on these issues, refer to Section 11.4.5: Duct Insulation.

Providing good indoor air quality starts at design

Appropriate design application of duct liner and insulation can minimize indoor air quality problems due to condensation and microbiological growth. But to be effective, the material specifications need to be supported by specification requirements regarding protection of the ductwork from contamination during construction and under conditions of temporary operation. These specifications should be enforced through construction monitoring. Filtration requirements for temporary operation may actually exceed the requirements for normal operation due to the nature of the dust, the operating status of the systems and the location of the dust source. More details on these issues are presented in Section 11.4.6: Indoor Air Quality.

Leak-free construction, cleanliness, drainage, and equipment access all are design issues that need to be considered for under floor plenums

While simple and attractive in concept, the successful design, specification and construction of under floor plenums can be quite challenging. Advantages are described in Section 11.4.7: Under Floor Plenums.

Achieving leak free construction is difficult, and the leaks that occur tend to be hundreds of small ones instead of a few large ones. Equipment locations that are not carefully considered in light of the proposed floor plan can result in major access problems when service is required. By design, the plenums will be the lowest points on the floor, and thus are subject to contamination problems due to normal debris and flooding if there is a leak above them. These issues are described in Section Leakage, Drainage, Cleanliness and Equipment Access Requirements.

Under floor plenums can implement several different air distribution strategies, all of which require different design and control approaches

Displacement ventilation (see Section and Under floor air distribution (see Section both use under floor plenums as a delivery system for the supply air. However, the design and control approaches associated with these two distribution strategies are quite different and not directly interchangeable. In addition, both approaches are cooling strategies and thus are not readily adaptable to serving perimeter heating loads.

Seismic restraint requirements need to be clearly specified and detailed in the contract documents and submittals

Requirements for longitudinal and lateral bracing need to be clearly defined to allow the field staff to interpret them properly. The information on the contract documents and submittals needs to be supplemented with field observation during construction to verify proper interpretation. Some structural designers may require field testing of the anchor systems to verify load carrying capacity, a process that may involve the commissioning agent. Floating equipment bases and other spring isolated devices need to be sized and snubbed to withstand the design seismic loads and may require field adjustment to achieve the design intent.


11.3. 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

11.4. Supplemental Information

There are many hyperlinks throughout Chapter 11 that reference supplemental information regarding the distribution system. In addition to accessing this information by clicking the hyperlinks, the supplemental information document can be accessed using the link below.

Bevel: Distribution System Supplemental Information