This website requires certain cookies to work and uses other cookies to help you have the best experience. By visiting this website, certain cookies have already been set, which you may delete and block. By closing this message or continuing to use our site, you agree to the use of cookies. Visit our updated privacy and cookie policy to learn more.
This Website Uses Cookies
By closing this message or continuing to use our site, you agree to our cookie policy. Learn More
This website requires certain cookies to work and uses other cookies to help you have the best experience. By visiting this website, certain cookies have already been set, which you may delete and block. By closing this message or continuing to use our site, you agree to the use of cookies. Visit our updated privacy and cookie policy to learn more.
ACHRNews logo
search
cart
facebook twitter linkedin youtube
  • Sign In
  • Create Account
  • Sign Out
  • My Account
ACHRNews logo
  • Magazine
    • Subscribe
    • Current Issue
    • Digital Edition
    • Archives
    • The NEWS eNewsletter
  • News
    • Breaking News
    • Newsline
    • FYI
    • Distribution Trends
    • Standards & Legislation
    • Manufacturer Reports
  • Products
    • HVACR Industry Store
    • Top 20 Products of 2018
    • Interactive Product Spotlights
  • Multimedia
    • Videos
    • AHR 2019 Videos
    • Podcasts
    • Infographic
    • Photo Galleries
    • Webinars
    • eBooks
    • The NEWS HVACR Quiz
    • Online Poll
  • Refrigeration
    • Refrigerants
    • Refrigerant Regulations
    • Reclaim
    • FROSTlines
  • Service
    • Testing, Monitoring, Tools
    • Duct Dynasty
    • Service 101
    • Components & Accessories
  • Business 101
    • Training & Education
    • Service, Apps & Software
    • Opinions
    • Guest Column
    • The Coach's Blog
  • Residential
    • Home Automation
    • Residential Ductless
    • Residential Geothermal
    • Residential IAQ
    • Residential Cooling Showcase
    • Residential Heating Showcase
  • Commercial
    • Internet of Things
    • Commercial Ductless
    • Commercial Geothermal
    • Commercial IAQ
    • Commercial Cooling Showcase
    • Commercial Heating Showcase
    • Marijuana Market
  • Directory
    • Manufacturers
    • Wholesalers
    • Consultants/Services
    • Manufacturers Reps
    • Associations
    • Parts & Equipment Exporters
    • List/Update Your Company
  • More
    • Editorial Calendar & Editor Assignments
    • Calendar of Events
    • Classifieds
    • Custom Content & Marketing Services
    • Continuing Education
    • Market Research
    • Sponsor Insights
    • eProduct Info
    • White Papers
    • Contests
  • Contact
    • Advertise
    • Submit a Letter
Home » Demystifying the Duct Traverse
HVAC Residential MarketHVAC Light Commercial MarketHVAC Commercial Market

Demystifying the Duct Traverse

Examining the measurement technique

duct traverse

PRACTICE MAKES PERFECT: Learning to perform a proper duct traverse is a skill that requires patience and practice.

October 30, 2017
David Richardson
KEYWORDS airflow management / Duct Dynasty
Reprints
No Comments

One of the most effective ways to measure airflow is through a method known as a traverse. While a traverse offers an accurate means of determining airflow, it is rarely used by HVAC professionals due to its intimidating nature. Let’s look at this time-proven measurement technique and see if we can remove some of the mystery surrounding it.

TRAVERSE BACKGROUND

Traverses have been used by air balancers for decades. For many, they are the gold standard of airflow measurement in commercial and residential systems.

A traverse is a series of measurements used to determine the average velocity of air moving through an opening. Air velocity is measured in a grid pattern through a variety of openings. They typically include ducts, registers, grilles, filters, economizers, and grease filters. For  the purpose of this article, we will stick with the duct traverse.

Average air velocity, by itself, doesn’t provide a lot of information about ducts or the mechanical system. But once you combine it with a simple formula, you can determine airflow. The formula is: area x velocity = cfm.

This formula can be broken down into the following components:

• Area = the inside dimensions of the duct measured, in square feet;

• Velocity = the average speed of air measured in feet per minute (FPM) through the duct; and

• Cfm = the calculated airflow moving through the duct, which is also known as cubic feet per minute.

Once average velocity readings are gathered and multiplied by the inside area of the duct, airflow is calculated. In the old days, these calculations were done manually. Thanks to the advances in test instruments today, they perform the math as you measure.

FIELD APPLICATIONS

The number of places a traverse can be applied are plentiful. You can measure airflow on both the supply and return sides of the system. The easiest duct material to perform a traverse on is sheet metal. Common duct configurations to traverse are rectangular, round, and oval duct. Each has its own method for a proper traverse that we’ll discuss later.

Flexible and fiberglass duct can present challenges if you attempt to traverse them. Two typical obstacles are finding an adequate location for the traverse, and sealing the needed test port sites. I won’t say you can’t do it, but be aware as there are complications.

One of the most common applications for a traverse is to determine fan airflow. Depending on the installation, this is performed as a measurement of a return drop or supply duct. Individual branch ducts can be measured using the same procedure. While this can be time-consuming, it will help you evaluate what’s happening in problem rooms or areas.

You can compare measured traverse airflow to required airflow. Let’s say you have an 8-inch metal duct that feeds a bedroom, and it is intended to deliver 200 cfm of air to the space. You can traverse the duct to see what’s really happening. If traverse airflow is only 100 cfm, you know you’ve got a problem.

Another application is on systems equipped with outside air. They often present challenges in determining how much airflow is being added to the return side of a system. A proper traverse of the outside air duct will leave no doubt to the amount of air coming in.

TEST INSTRUMENTS AND ACCESSORIES

If you don’t have the appropriate instruments, you can’t measure. The good news is you can begin traversing with a minimum investment. The cost for test instruments to perform this level of measurement has dropped dramatically over the years. To traverse, you’ll need the following:

• Hot-wire anemometer;

• Cordless drill with a 3/8-inch bit;

• 3/8-inch test port plugs; and

• Duct jigs, which cut down on time for installing test ports.

Do your research before purchasing any test instruments. You get what you pay for. Each instrument has its own learning curve, so practice, and learn how to operate it before measuring. Something as simple as improper probe position can throw your readings off.

As your skills increase, explore other test instrument options, such as a manometer and pitot tube or velocity grid, to perform traverse measurements at other locations.

FUNDAMENTAL GUIDELINES

There are some fundamental guidelines you need to follow when performing a traverse. If you ignore them, you’ll get some really goofy measurements.

The first fundamental is to find an acceptable test location in the duct. This usually presents the biggest challenge since a “by-the-book” traverse requires a long, straight run with no turns, takeoffs, or transitions in it. The ideal length is 10 duct diameters. To give you an idea of how long this is, if you have a 12-inch round duct, it would require a straight length of 10 feet, or 120 inches, to be traversed by the book.

It can be hard, if not impossible, to find ducts that meet these requirements, so many simply say it can’t be done. With a little practice, successful readings can be obtained on duct lengths as low as five duct diameters. If you’re in a pinch, try measuring on shorter length ducts because that is better than guessing.

Next, you’ll need to install test ports for measurement. Their location is 80 percent downstream the length of duct. This allows air to straighten out and reduce turbulence before the measurement occurs. Going back to the example of the 12-inch round duct that is 10 feet long. Test ports would be located 8 feet, or 96 inches, downstream.

A rectangular duct needs test ports installed in the short side of the duct. A round duct needs two test ports installed 90-degrees apart from each other.

There are two frequently used traverse methods that determine the installation of your test ports and measurements. The first is called the “equal area” method. Ports are installed approximately 2-3 inches apart and measurements are taken in an equal grid pattern inside the duct. The second style is called the “Tchybecheff” method. This method uses a lot of math to calculate test port and traverse sites. It is mainly used when specified by an engineer and requires dedication to master.

Once your test locations are set, you’ll obtain between 12-96 velocity readings inside the duct. A good rule to remember is the more readings you take, the greater your chances are for an accurate traverse. Once the velocity readings are obtained, they are averaged and plugged into the cfm formula along with the inside duct area.

PERFORM A TRAVERSE

To sum up the steps we’ve covered so far, you can break a traverse down into six steps. Some of these steps aren’t needed if your test instrument performs the associated functions. Be sure to read the manufacturer’s instructions first.

• Step One: Find an acceptable test site in the duct you want to traverse;

• Step Two: Determine your test port locations, and install them with a drill;

• Step Three: Use a hot-wire anemometer to measure multiple velocity readings in a grid pattern;

• Step Four: Add up all your velocity readings and divide by the number of readings taken. This is your average velocity;

• Step Five: Determine the inside duct area in square feet. Multiply width and length and divide by 144 to arrive at this measurement; and

• Step Six: Plug the average velocity and duct area (in square feet) into the cfm formula, and voila; you have an airflow reading.

The hardest part is getting started. Make the decision to become a student of traverse measurements, and take the first step. There’s so much more that can be said on this subject — I’m barely scratching the surface. This was just a broad overview to make it a bit more digestible.

To help get you started, I would like to offer a National Comfort Institute (NCI) Rectangular Traverse Report and Procedure along with a technical tip on how to make traverse jigs. Contact me at davidr@ncihvac.com  for more information.   

Publication date: 10/30/2017

Want more HVAC industry news and information? Join The NEWS on Facebook, Twitter, and LinkedIn today!

Subscribe to The NEWS

Recent Articles by David Richardson

Four Steps to Determine the Correct Mixed Air Temperature

Three System-Sizing Superstitions The HVAC Industry Suffers From

Skills to Add to Your HVAC Service Repertoire

Five Reasons HVAC Systems Don’t Maintain Comfortable Conditions

Contractors Can Use Persuasion for HVAC Customers' Good

David-richardson

David Richardson serves as a curriculum developer and trainer at the National Comfort Institute (NCI). NCI specializes in training focused on improving, measuring, and verifying HVAC and building performance. Contact him at 800-633-7058 or at davidr@ncihvac.com.

Related Articles

Duct Dynasty: The Ghost in the Duct System

The Scoop on Branch Duct Takeoffs

Duct Dynasty: Designing the Ideal Duct System

Duct Dynasty: The Building Side of the Duct System

You must login or register in order to post a comment.

Report Abusive Comment

Subscription Center
  • Subscribe
  • Online Registration
  • E-Newsletters
  • Subscription Customer Service

More Videos

Sponsored Content What Is Sponsored Content

Sponsored Content is a special paid section where industry companies provide high quality, objective, non-commercial content around topics of interest to the ACHR News audience. All Sponsored Content is supplied by the advertising company. Interested in participating in our Sponsored Content section? Contact your local rep.

Close
Trane-indoor-air-quality-product-01
Sponsored By
Trane Residential

Protecting Your Customers’ Indoor Air Quality Is More Important Than Ever

From seasonal maintenance to a system upgrade, your business is actually centered around providing the best indoor air experience, with quality at the core.

Popular Stories

Breaking News - The ACHR News

Ingersoll Rand Increases Prices on HVAC products

R-22 Phaseout

Using (or Abusing) the R-22 Phaseout as a Sales Tool

Senate HFC Phasedown Bill

Senate HFC Phasedown Plan Debuts With Broad Support

Breaking News - The ACHR News

Lennox Industries Inc. Announces Price Increase

Troubleshooting-situation

Troubleshooting Puzzle: An Uncomfortable Bedroom

The ACHR News Airpod Giveaway

Events

December 7, 2019

2019 HARDI Annual Conference

The major 4-day event for HARDI members brings together distributors, manufacturers and other vendors ready to drive their businesses forward. 

September 16, 2020

2020 World Congress for Solar Cooling

IASC seeks submissions that represent the best contemporary and innovative technology and thinking about Solar Cooling from a global and diverse range of designers, consultants, practitioners, engineers, architects, educators, thought-leaders, students and other professionals concerned with cooling, people, buildings and energy.
View All Submit An Event

Poll

Holiday Appreciation

What is your preferred way to show employees your appreciation during the holiday season?
View Results Poll Archive

Products

"More & New" HVAC Spells Wealth - book

"More & New" HVAC Spells Wealth - book

See More Products
The NEWS - ACHR - Logo The NEWS - ACHR - Digital Edition
Digital Edition

ACHRNEWS

The ACHR News - December 2, 2019

2019 December 2

Week 1: December 2
In this issue of The ACHR NEWS, we look at unique applications of HVAC technology, and new markets contractors should consider expanding into.

Week 2: December 9
We also examine Variable Refrigerant Flow (VRF) technology, and the benefits that contractors can pass along to their customers through the equipment.

View More Create Account
  • Resources
    • Submit a Letter
    • Submit a Press Release
    • Mechanical Group
    • List Rental
    • Ad Specs
    • Reprints
    • eProduct Info
    • Survey And Sample
    • Privacy Policy
  • Want More?
    • Subscribe
    • Connect

Copyright ©2019. All Rights Reserved BNP Media.

Design, CMS, Hosting & Web Development :: ePublishing