Negative Air Duct Cleaning White Paper: Original Post + NADCA Response


In January, C&R posted a news item, under Industry News, detailing the release of a short white paper called Effects of Negative Air Duct Cleaning on Flex Ducts. This did not appear under editorial content, was not paid, nor did it appear in any print editions of C&R. The entire news item was later removed from the C&R website; you can now read it below.

This week, NADCA released an official response to the white paper. You can read the response below.

For ease of navigation, here are links to both sections:

The purpose of this post is to combine both the original, and the response by NADCA in one place. Further comments and questions should be directed to the authors and NADCA for clarification and response. 

New White Paper Release: Effects of Negative Air Duct Cleaning on Flex Ducting

Publisher’s Note: Below is a short white paper recently released by a group of HVAC and restoration experts. 


In this paper, we investigate the pressure and airflow tolerances of flex ducting as it pertains to air duct cleaning.


Flex ducting is a type of ductwork that consists of a flexible inner core  covered by a helical wire and an outer layer of insulation. It is widely used in residential and commercial HVAC systems because of its low cost, ease of installation, and adaptability to different layouts. Most HVAC ducting in residential and commercial structures since the 1980s consists of flex ducting.

Air duct cleaning is a process of removing dust, debris and other contaminants from the ductwork system. One of the most common methods is negative air duct cleaning, which involves generating negative pressure in the duct system by attaching a powerful vacuum, typically to a central juncture to which all main/trunk and branch ducts are connected. Mechanical agitation removes dust and debris from the inner walls of the ductwork, where airflow generated by the negative air machine then pulls it out of the system.

However, negative air duct cleaning can cause problems for flex ducting. According to the manufacturers of flex ducting, the flex ducting utilized in both residential and commercial HVAC applications is not designed to withstand negative pressures exceeding 1 inch of water column (IWC).

The negative air pressures generated by negative air duct cleaning equipment exceed these pressure limits significantly, compromising the structural integrity of the ducting. Some consequences of this damage are:

  • The flex ducts can collapse or tear due to excessive negative pressure.
  • The helical wire can break or detach from the inner core due to excessive tension or torsion.
  • The insulation layer can compress or peel off due to excessive friction or abrasion.
  • The joints and connections can leak or disconnect due to excessive stress or strain.
  • Even minor adhesive failure caused by negative pressure can lead to humidity migration into the interior of the duct, creating an environment conducive to microbial (ie: mold) growth.


According to the leading manufacturers of negative air duct cleaning equipment, portable machines generate an average of 5 IWC negative pressure, and truck-mounted machines generate 20 IWC negative pressure and higher.


The negative pressure applied by negative air duct cleaning equipment, both portable and truck-mounted, significantly exceeds the tolerable limit for HVAC flex ducting, as specified by the manufacturers of HVAC flex ducting.

This damage can reduce the performance, efficiency, and lifespan of flex ducting and compromise indoor air quality and comfort, as well as safety of the occupants. To avoid these problems, it is advisable to use alternative methods of air duct cleaning that do not create excessive negative pressure in the duct system.


Authored by:

John Miles
Chief Science Officer, Superstratum (mold/mycotoxin solutions)
Chemist, Hexion (flex duct adhesive manufacturing) (former)
34 years experience in applied chemistry

David Hart
Scientific Advisor & Instructor, Microbial Warrior Academy
Founder/CEO, RamAir International
30+ years experience in HVAC cleaning and remediation

Andrew Luckey
Engineer, RamAir International

Reviewed by:

Brandon Black
Product Manager, H&C Flex (flex duct manufacturing)
B.S. Mechanical Engineering

Travis Trassey
30+ years experience in HVAC cleaning and remediation

Brad Smith
President, Artemis Bio-Solutions (biocontamination research and remediation)
14 years experience in remediation, decontamination, cleaning and disinfecting solutions

Wade Kincade
HVAC Project Manager and Installation Instructor, Bend Heating & Sheetmetal
30+ years experience in HVAC

Kristian May

Certified Mechanical HVAC Inspector, MICRO
Certified Mold Inspector

Effects of Negative Air Duct Cleaning on Flex Ducting: NADCA’s Response

By now you’ve likely come across the white paper, Effects of Negative Air Duct Cleaning on Flex Ducts, which was released this past December. For many, this has triggered frustration and confusion as to whether or not NADCA supports the claims within the document, and rightfully so. NADCA was in no way involved with the development or review of this paper and does not support the statement that negative air cannot and should not be used to clean flex duct because of the damage it can cause. It is NADCA’s position that this statement is simply untrue.

NADCA takes the stance that a system with flex ductwork, like all systems, should be assessed and evaluated to determine the cleaning protocol and scope of work. According to ACR, The NADCA Standard, section 1.10, System Components Assessment:

Information collected from the HVAC inspection shall be documented and evaluated to assess the condition of the HVAC system components at the time of the inspection.
The assessment shall include a recommendation on the need for cleaning, a clearly defined scope of work for the cleaning and restoration project, recommended cleaning techniques, a determination of the environmental engineering controls required for the workspace, and any unique requirements.

Therefore, only during the assessment, can the technician determine if cleaning is the best course of action or if replacement is necessary. Simply stating — as the white paper does — that the use of negative air machines for cleaning this type of ductwork isn’t an option is creating the false notion that cleaning flex ductwork in accordance with ACR, The NADCA Standard (which is centered around using negative pressure), is not possible, which is not the case.

After speaking with several of our members about this white paper, it is clear that the process of duct cleaning is rarely the culprit when it comes to damaged flex ductwork and in this case, the negative air machines are being blamed. However, it is more than likely that the negative air being used during the cleaning is not more than what the flex duct was designed to handle. Sure, there are older versions of the material that may not be able to withstand the process and need to be replaced due to already existing damage, but most flex ducts are made with a metal spiral support to allow for contact vacuuming and other cleaning methods.

In response to the white paper, NADCA has convened with several subject matter experts to submit a clear refutation to the published document. Their statements are available below.

Clint Orr, ASCS, CVI
NADCA Second Vice President, Board of Directors
Education & Membership Committee Chairperson
Regina, SK, Canada

“I have been in the HVAC industry for over 20 years. Along the way I have installed, replaced, and cleaned thousands of feet of flex duct. The 2021 edition of ACR, the NADCA Standard, provides the “how to” and guidelines on best practices for the assessment, cleaning and restoration processes related to HVAC systems and their components…” CLICK HERE to continue reading.

Tyler Batchelder, ASCS,CVI
NADCA Board of Directors
Colchester, VT

“In response to the White Paper Effects of Negative Air Duct Cleaning on Flex Ducting, I would make the argument that the duct cleaning process helps identify areas where flex duct has otherwise been damaged or disconnected more so than creating the situation. CLICK HERE to continue reading.

Zachary Ortwine, ASCS, CVI
NADCA Member and Volunteer
Richmond, VA

“The premise of the white paper Effects of Negative Air Duct Cleaning on Flex Ducting appears to be that because the negative air equipment manufacturers designed their equipment to exceed manufacturers maximum-rated negative pressure of flexible ducts that the use of this equipment will result in damaging the flex ducts. However, in my years of field-based experience in residential and commercial projects which involves cleaning flex ducts I have never damaged a flex duct due to negative pressure created by our gasoline-powered negative air machines.” CLICK HERE to continue reading.

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Michelle Blevins

Michelle BlevinsMichelle Blevins is a content creator, marketing consultant, and entrepreneur whose career has been centered around education and fostering relationships within the industry she serves.

A journalist by trade, Michelle is passionate about running a publication rooted in integrity and valuable education. She views her role as owner and publisher of C&R Magazine as a bridge between industry experts and restoration and cleaning contractors.

Since joining the restoration industry, Michelle has made it her business to stay on top of the latest industry trends. She has become a go-to resource for anyone looking to learn more about what’s happening within today’s restoration industry. This has earned her a spot on many industry stages facilitating panels and helpful discussion with industry experts on the biggest topics facing restorers and cleaners in the current market.

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