Engineering plays a definitive role when it comes to establishing the functional requirements of an occupied or unoccupied space’s supply air methods and devices. 

Multiple criteria must be considered based on these functional requirements. This is a timely discussion for any consulting firm to have with clients in light of the recent COVID-19 pandemic. 

Ventilation is important in all facilities, especially health care facilities, where people with all sorts of illnesses are enclosed. While air distribution within a space is not often discussed in the media, it is a critical part of the social/physical separation when airborne particulates need to be effectively removed and displaced with appropriate ventilation air in accordance with the applicable codes and standards.

As a result of the coronavirus, the codes, standards, and design guides that cover air distribution will be discussed in greater detail than ever before by project teams integrating all disciplines of design. A great liability will now be thrust upon the professional engineers and firms that design new and retrofit projects. More than ever before, air distribution needs to become a top priority in facility design.



Consistency is of the utmost importance when considering engineering codes and standards terminology. For example, there may be terms in various ASHRAE Handbook chapters that are defined in those chapters but have not been included in the ASHRAE terminology database. Care must be taken to discover where the various terms are used and ensure consistency. The use of these terms may or may not be consistent. Consistency and clarity are part of diligent and prudent communication in contract documents. It may be wise to have a statement in contract documents that provides the source of the definitions of the terms that are used. 

Here are a couple of terms, but there are many more in the ASHRAE terminology database related to air distribution: 

  • Air distribution: transportation of a specified airflow to or from the treated space by ducts or plenums. Air-treatment devices can be added to the distribution system for the purpose of treating the air (e.g., cleaning, heating, cooling, humidifying, dehumidifying, etc.)
  • Air-diffusion performance index (ADPI): A single number rating of the air-diffusion performance of a mixing system at a specified supply air delivery rate, temperature, moisture content, and space cooling load. ADPI is based on air speed and effective draft temperature.


Codes are adopting more and more industry standards, such as ASHRAE Standards 55, 62, 90.1, and 129, which impact air distribution considerations and design criteria. There are many other sources from ASHRAE and other industry-specific organizations that should be consulted when the air distribution part of mechanical engineering design and specifications are done by the design team. It is prudent the professional engineer-of-record be familiar with these codes, standards, and design guides, such as: Std. 70, “Methods of Testing,” and Std. 170, “Ventilation of Health Care Facilities (diffuser groups).” 

Applicable ASHRAE Handbook entries include, 2016 HVAC Systems and Equipment: Chapters 1, 5, 19, 20, and 28; 2017 Fundamentals: Chapters 9-13, 16, and 20; 2018 Refrigeration – various chapters on specific industrial applications; and 2019: HVAC Applications: Chapter 58 – Air Distribution and Chapter 59 Modeling. Other resources include ASHRAE Journal article, January 2020 – RP 1546, ADPI for Heating; and ASHRAE TC 5.03 – Room Air Distribution.



It is essential to establish the required functional design criteria when selecting an air device. It’s also prudent for the entire design team, including the architect, interior designer, and owner of a facility, to clearly understand and agree on the required design criteria related to air distribution. Documentation of the methodology as to why a specific air device was selected is also an important part of the design intent documentation. 

Communicating the design criteria within the contract documents is important to ensure the specifications are clear, concise, complete, and correct in accordance with the recommendations of the Construction Specifications Institute (CSI). The “Performance Specification” method is a good place to start when setting the design criteria. Performance is the best basis of comparison beyond just appearance when the product submittals are reviewed. 

One design criteria that embraces comfort, energy, ventilation effectiveness, and even a healthy environment is an air distribution product’s entrainment ratio. Some products have higher entrainment factors than others. This is specifiable and goes well beyond the aesthetics of a product. Even if a product looks similar to another and meets the desired aesthetics criteria, it may or may not meet the performance and entrainment ratio criteria. The entrainment ratio can be verified and documented.  

Design criteria documentation puts the burden of proof on the manufacturers, where the product performance responsibility should be. The application, selection, and layout of air devices as well as the burden of proof and liability of performance then becomes the responsibility of the engineer; however, the manufacturers need to clearly document the installation guidelines that may differ from the test setup. What is not said is just as important as what is said in a manufacturer’s marketing material and performance criteria documentation. 



There are many types of buildings and spaces within buildings that require different levels of air distribution.

Critical facilities, grow facilities, offices, gymnasiums, etc., each require different considerations for establishing air distribution design criteria. The codes, standards, and design guidelines for space types are always under evaluation. These documents are updated as relevant information and research is deemed necessary. The ASHRAE Handbook has chapters dedicated to many specific spaces and building types but not all.

A natatorium, as shown in Figure 1, is one type of space that has experienced numerous air distribution standards changes over the years. Years ago, when energy was a top priority, there were some who implied a low surface velocity that minimized evaporation rate was the top product selection criteria. In more recent years, there have been others who have suggested that a higher rate of airflow and surface supply air velocity is needed due to the fact that people in the water have their heads (breathing zone) at this point. The rationale being that more ventilation air is needed, and evaporation rate is not as important. This is one example of how the focus on various design criteria changes over time. 

Energy, ventilation effectiveness, health, and comfort are all criteria engineers must take into account in every space type when it comes to air distribution. There are indeed trade-offs, and it is not possible to optimize more than one variable at a time in most situations.


There are multiple types of air distribution devices; whether the type of air distribution device be for wall, floor, ceiling, or duct is a matter of consideration. Each air device was more than likely tested in a specific way during installation, and using a device for another installation surface may or may not offer the same performance. Some products require an adjacent flat surface in order to perform as scheduled in the manufacturer’s catalog whereas others do not. If in doubt, and if not documented by the manufacturer, it may be wise to ask and get the response documented in writing from the manufacturer. 

Textile air distribution is one air distribution device that is now included in the ASHRAE Handbook. Figure 2 shows an example of an office space utilizing textile air distribution in lieu of exposed spiral duct with diffusers. There are many benefits of textile air distribution in performance and appearance. It is good to remember that no two products perform the same way. It is good to have a very well written performance specification for each air distribution air device.


It is important to understand the difference in analysis of diffuser selections that are based on heating versus cooling versus a need to consider both heating and cooling. In general, air devices are tested under isothermal performance. Any condition other than isothermal should be taken into account with appropriate adjustments as deemed necessary and as provided by manufacturers for their specific products. Understanding the adjustment factors needed for air device selections is essential.  

Sometimes air device selections are over-simplified, and adjustment factors are not taken into account by mechanical designers who are taught some basic rule-of-thumb selection criteria for the purpose of expediency in selection for design labor cost control.  

Now is the time for relevant and prudent engineering of air distribution product analysis and selection with proper documentation that is defendable. The “standard-of-care” of the past is not necessarily the same “standard-of-care” over time as codes, standards, and design guides in product testing changes and product improvements are made.



Industry standards exist specifically for the testing of supply air devices. There are limitations to the testing and adjustments needed to account for variances in each application. Professional engineers must clearly document what standards are required by the manufacturers that are allowed to bid projects to ensure the performance is verified and defendable. 

When de-value engineering (cost cutting) and substitutions are part of the discussion, an engineer must maintain a prudent perspective for the sake of the client and stay in compliance with codes and standards. Just because a product looks similar to one that has been properly tested, it does not mean the de-value engineered product or substituted product will perform the same. This is especially essential to understand in VAV air distribution systems and product selections.



The ASHRAE Handbook has three methods of selecting supply air devices. Documentation should be made as to which method is used. The same method may or may not be used on the same project depending on the functional requirements of the air device in the various spaces. Some spaces may require different priorities than others when it comes to air throw, noise, terminal velocity, etc. Critical environments (ORs, labs, etc.) will have different selection criteria than a large open space, like a concert hall or sports arena.

There are engineering design tools well beyond the old 50/100/150 terminal velocity rule-of-thumb procedures that should be used as part of prudent engineering of air distribution. ASHRAE has multiple air distribution product selection methods for consideration. The ADPI is one that has been around for years, and recent research has evaluated ADPI in more depth than has been done in years. It would be wise for professional engineers to review and analyze all methods of air distribution devices, document those procedures, and let the client know the methods being used. Speeding up the design process to minimize the design labor cost within a consulting firm is not a defendable reason to use old rule-of-thumb design criteria alone.



Testing and balancing (TAB) is a critical part of a project when it comes to air performance. There are debates whether a trim balancing damper is needed at the neck of a supply air device, if it is ever actually used, and how it may impact the performance of a supply air device. These are all good topics to discuss during the design phase of a project not at the cost-cutting phase. When it comes to commissioning, the best commissioning can’t occur without a professional TAB process and documentation. Testing is not the same as balancing. It takes both and the methods should be documented by a TAB technician and verified by a professional engineer. TAB and commissioning are both critical to ensure the engineering design intent is ultimately what the owner receives.



The Keep It Simple (KIS) principle is well known and is good to keep in consideration; however, sometimes, lowest first cost can be used to justify the KIS principle in product selections over and above applicable and relevant performance criteria. This can increase liability. Although the “tried-and-true” perspective is sometimes based on not hearing any complaints from a client, the fact is, some clients don’t complain and simply opt to work with another engineer. The original engineer or firm doesn’t know of the problems and complaints, so they think everything is acceptable. It’s always good to ask a client for honest and candid feedback. 

Documentation is key to the Keep It Defendable (KID) principle, even in something as seemingly simple as an air diffuser selection by mechanical designers under the guidance of the engineer-of-record. The rules have changed, and the rule-of-thumb, quick-selection methodology is a risky design process.



The engineer-of-record is ultimately the individual who must guide the design team in the important process of air distribution product selection. Although there are many old “rule-of-thumb” and “tried-and-true” design methodologies, the professional engineer must continually evaluate if the instructions given to the mechanical designer(s) are current and defendable. Tried-and-true is sometimes simply a matter of not getting any client complaints about comfort, but it does not confirm compliance with all codes and standards relevant to proper air distribution design, product selection, and ultimate acceptance for a project.