Interpretation #7332

Original Request

Code Version: 2010
Code: MECHANIC
Code Description: Mechanical
Chapter: 6
Section: 601.4, Balanced Return
Topic: Transfer Ducts

Question

Is it the intent of 2010 FMC, Section 601.4, Exception #1 to apply the 1-1/2 times return duct cross sectional area when using transfer ducts only, or will a ducted return air systems also have to comply with this requirement?

Answer Published 06/13/2014 at 1:55 PM

The exception specifically states "Transfer ducts", so,
this section applies to transfer ducts and not a return air
system.

Commentary

A transfer duct is typically not a "forced" air duct and the sizing should ensure a sufficently low static pressure. If the air is being pulled by the system this sizing may not be required, depending on the design of the system. FBC-M Section 603.2 (and related subsections) deals with the appropriate methods for sizing of ducts. The AHJ, if concerned with a specific design/installation, could require the submission of additional data to substantiate the design/installation. Further, the AHJ may also require information on the grille(s) installed to verify they are installed within the manufacturer's installation instructions (dealing with acceptable face velocity, etc). See FBC-M 603.17.