A Case Of Commercial Requirements Being Applied In Residential Applications


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In 2007, a code change proposal to the International Mechanical Code (IMC) passed the code hearings with little objection (or even notice) from the HVAC community.  One year later, a corresponding code change proposal passed to bring the relevant section of the International Residential Code (IRC) into accordance with the IMC.  Neither was reported in the corresponding “Significant Changes to the International Code” document.  When industry experts took notice, however, it became apparent that this ‘minor’ change would have a significant impact on everyone in the residential HVAC industry.

A Matter of Duct Gauge

Duct gauge is the measure of the thickness of the sheet metal used to make an air duct; each gauge has a corresponding thickness in inches (or millimeters).  What may cause confusion for some is that, while the sheet metal gauge increases, the actual thickness decreases.  So, 12 gauge sheet metal is 0.1090 inches thick, while 30 gauge sheet metal is 0.0125 (or about 1/10 as thick as 12 gauge).

What does duct gauge have to do with the building codes?  Where the codes address duct gauges, the intent is to provide design allowances which ensure safety, sustainability, and affordability. Prior to 2006, the relevant code sections (IMC §M603 and IRC §M1601.1.1) permitted duct gauges that the industry considered acceptable for decades.  Of specific importance is that it permitted the use of 30 gauge sheet metal for round ducts up to 14 inches in diameter.

The code change proposal to the 2009 IMC, however, altered Table 603.4 (Duct Construction Minimum Sheet Metal Thicknesses for Single Dwelling Units) to require 28 gauge – thicker – sheet metal for these ducts.  The reasoning behind the change, according to the submitter, was to bring the table into compliance with the SMACNA HVAC Duct Construction Standard – Metal and Flexible.

Industry Opposition

In 2010, when the affected parties became cognizant of the change to the 2009 i-codes, they pointed out many troubling flaws with its passing.  First and foremost was that IMC Table 603.4 was intended as an exception to the SMACNA standard, as it is a commercial duct construction standard.  To apply its commercial requirements to low-pressure, residential applications was always seen as unnecessary from a safety point of view, as the pressures encountered in commercial duct systems are much greater than in residential – thus the exception.

Financially, however, the code change was deeply troubling.  Requiring that the minimum duct gauge be increased from 30 to 28 (minimum thickness going from 0.013 to 0.016 inches), this small change would have enormous financial repercussions for the industry.  With half a billion tons of sheet metal produced annually, the arbitrary increase in material use would result in increased costs for contractors.  Sheet metal manufacturers would incur considerable costs to shift the bulk of their production from 30 to 28 gauge.  These costs would be passed down to the consumer, who would receive no additional safety or system performance benefits for it.  Not surprisingly, this resulted in the formation of an informal coalition that opposed these requirements and sought to reverse them.  Among the members were the ACCA, the Air Distribution Institute (ADI), the Refrigeration Service Engineers Society (RSES), and the Plumbing – Heating – Cooling Contractors Association (PHCC).

Code Hearings

As per the ICC’s code development process, ACCA submitted a code change proposal in January 2012 to revert the values in the table to the pre-2006 values.  At the code hearings held in Dallas, TX in May 2012, members of the coalition testified before the Mechanical Technical Committee in favor of ACCA’s code change proposal.  Following the convincing testimony of the experts and interested parties, as well as very light opposition to the proposal, the Mechanical Technical Committee voted unanimously to pass the proposal.

Following the hearings, the proposal underwent a public comment period, during which only one comment was received.  This comment submitted by SMACNA sought to alter the proposed table to include limits based on water gauge pressure.  SMACNA’s intent was to ensure that ducts installed in larger homes, or in systems with higher static pressure, had an increased duct gauge to ensure system integrity.  The values in the table submitted by SMACNA are based on equations found in their Residential Duct Construction Standard, which is currently being developed.  Following the testimony on the proposal at the Final Action Hearings (held in October 2012 in Portland, OR), the code change proposal passed as amended by the SMACNA comment.

Going Forward

Because code change proposals to one i-code are not automatically carried over to the corresponding section in another i-code, the code change proposal was resubmitted in January 2013 for the IRC (which is in Group B of the i-codes).  Given that the code change proposal is now officially in the IMC, it is expected to pass without opposition in the hearings for the IRC.  To prevent a similar situation like this from happening in the future, ACCA is placing greater emphasis on the monitoring of the code development proceedings.  We also ask that members and interested parties alert us to any important code issues they encounter by contacting the author (email luis.escobar@acca.org or call directly 703-824-8870).

Portions of this article reproduce excerpts from the 2006 International Mechanical Code and the development record of the 2015 International Mechanical Code, which is not yet available. International Code Council, Inc. Washington, D.C. Reproduced with permission. All rights reserved. Ww.iccsafe.org

Luis Escobar
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Posted In: ACCA Now, Residential Buildings, Technical Tips

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