FBC 504.4 vs 504.8.2

[Greg Hatfield]

Although 5th Ed Residential Mechanical required "smooth interior duct", the FBC added the requirements for (3) mechanical attachments to the dryer exhaust piping. Rumors were that the duct cleaning trade pushed this change due to exhaust ducts coming apart during duct cleaning. However, the Mechanical Code was still "smooth" and shall have no screws as it had been for years.

FBC Mech 6th Ed 504.4 still says shall not connect with screws "that will obstruct the exhaust flow" and 504.8.1 still says smooth interior. However they created a slight conflict by changing 504.8.2 "ducts shall not be joined with screws or similar fasteners that protrude INTO THE INSIDE OF THE DUCT" to "MORE THAN 1/8" INTO THE INSIDE OF THE DUCT". Almost like they thought this was a type error even though they didn't list quantity of (3) screws like Residential Mech does.

Is this a conflict or do they now want all dyer venting mechanically attached? Many inspectors don't enforce the screws but multi-county contractors do it to prevent red tags. Thanks, Greg Hatfield

[Jerry Peck - Codeman]

I'm trying to follow the question, so I'll start in this way:

(underlining in all of the below are mine for highlighting purposes)

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2014 (5th Edition) FBC-Residential
- M1502.4 Dryer exhaust ducts.
- - Dryer exhaust ducts shall conform to the requirements of Sections M1502.4.1 through M1502.4.6.
- - M1502.4.1 Material and size.
- - - Exhaust ducts shall have a smooth interior finish and be constructed of metal having a minimum thickness of 0.0157 inches (0.3950 mm) (No. 28 gage). The duct shall be 4 inches (102 mm) nominal in diameter.
- - - (Jerry's comment: The "smooth interior finish" means the duct shall not be rough inside as that would/could catch and collect lint, leading to clogging the ducts.)
- - M1502.4.2 Duct installation.
- - - Exhaust ducts shall be supported at intervals not to exceed 12 feet (3658 mm) and shall be secured in place. The insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow. Exhaust duct joints shall be sealed in accordance with Section M1601.4.1 and shall be mechanically fastened. Ducts shall not be joined with screws or similar fasteners that protrude more than 1/8 inch (3.2 mm) into the inside of the duct.
- - - (Jerry's comment: The "shall be mechanically fastened" means that the duct section shall be fastened together using some form of mechanical method, screws, rivets, etc. The "shall not be joined with screws or similar fasteners that protrude more than 1/8 inch" inside of the duct means that the duct itself has a "smooth interior" and the mechanical fastening method is to also not catch link, i.e., the fasteners shall not "protrude more than 1/8 inch" inside of the duct. No conflict there as M1502.4.1 is addressing the duct material, M1502.4.2 is addressing the installation and fastening of the duct, in the cases of the discussion, that section is addressing the fasteners.)

2017 (6th Edition) FBC-Residential
- M1502.4 Dryer exhaust ducts.
- - Dryer exhaust ducts shall conform to the requirements of Sections M1502.4.1 through M1502.4.7.
- - M1502.4.1 Material and size.
- - - Exhaust ducts shall have a smooth interior finish and be constructed of metal having a minimum thickness of 0.0157 inches. The duct shall be 4 inches (102 mm) nominal in diameter.
- - - (Jerry's comment: The "(No. 28 gage)" from 2014 was dropped - "28 gage" is only 0.0156, not 0.0157 ... of course, though, that depends on who you ask:
- - - - - https://www.engineeringtoolbox.com/stan ... _1345.html ... says 0.0156
- - - - - https://www.unc.edu/~rowlett/units/scal ... metal.html ... says 0.0149
- - - - - https://www.engineersedge.com/materials ... _13178.htm ... says 0.0149
- - - - - https://www.law.cornell.edu/uscode/text/15/206 ... says 0.015625
- - - - - so ... what is the thickness of 28 gage ... the code now just specifies 0.0157 ... which is thicker than 28 gage ... making consistency for a minimum of 27 gage. )
- - - - Exception: Exhaust ducts may be 4 inches nominal in diameter Schedule 40 PVC when horizontally run beneath the slab.
- - - M1502.4.2 Duct installation.
- - - - Exhaust ducts shall be supported at intervals not to exceed 12 feet (3658 mm) and shall be secured in place. The insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow. Exhaust duct joints shall be sealed in accordance with Section M1601.4.1 and shall be mechanically fastened. Ducts shall not be joined with screws or similar fasteners that protrude more than 1/8 inch (3.2 mm) into the inside of the duct.
- - - (Jerry's comment: No meaningful change.)

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2014 FBC-Mechanical
- 504.6 Domestic clothes dryer ducts.
- - Exhaust ducts for domestic clothes dryers shall conform to the requirements of Sections 504.6.1 through 504.6.7.
- - 504.6.1 Material and size.
- - - Exhaust ducts shall have a smooth interior finish and shall be constructed of metal a minimum 0.016 inch (0.4 mm) thick. The exhaust duct size shall be 4 inches (102 mm) nominal in diameter.
- - - (Jerry's comment: Note the requirement for thicker material in the Mechanical code than is required in the Residential code.)
- - 504.6.2 Duct installation.
- - - Exhaust ducts shall be supported at 4-foot (1219 mm) intervals and secured in place. The insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow. Ducts shall not be joined with screws or similar fasteners that protrude into the inside of the duct.
- - - (Jerry's comment: This section is not saying that the duct is not to be mechanically fastened together, and considering the thicker material, mechanical fastening may have been presumed to have been intended, just not with fasteners which "protrude into the inside of the duct" ... which is a feat to do when mechanically fastening the sections together, so these ducts ended up only being taped together.)

2017 FBC-Mechanical
- 504.8 Domestic clothes dryer ducts.
- - Exhaust ducts for domestic clothes dryers shall conform to the requirements of Sections 504.8.1 through 504.8.6.
- - 504.8.1 Material and size.
- - - Exhaust ducts shall have a smooth interior finish and shall be constructed of metal a minimum 0.016 inch (0.4 mm) thick. The exhaust duct size shall be 4 inches (102 mm) nominal in diameter.
- - 504.8.2 Duct installation.
- - - Exhaust ducts shall be supported at 4-foot (1219 mm) intervals and secured in place. The insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow. Ducts shall not be joined with screws or similar fasteners that protrude more than 1/8 inch (3.2 mm) into the inside of the duct.
- - - (Jerry's comment: The change clarifies that mechanically attached duct is the intent, just don't use "screws or similar fasteners that protrude more than 1/8 inch" into the inside of the duct ... except that, unlike the Residential code, the Mechanical code still does not state "shall be mechanically fastened" - however, 504 - below - tries to address that.)

- 504.4 Exhaust installation.
- - Dryer exhaust ducts for clothes dryers shall terminate on the outside of the building and shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination. Ducts shall not be connected or installed with sheet metal screws or other fasteners that will obstruct the exhaust flow. Clothes dryer exhaust ducts shall not be connected to a vent connector, vent or chimney. Clothes dryer exhaust ducts shall not extend into or through ducts or plenums.
- - - (Jerry's comment: I think that is an ill-fated "intent" to indicate "shall be mechanically fastened" but falls short of that - why doesn't the Mechanical code does not use the same clear and simple wording of the Residential code? Maybe because they presume that mechanical contractors doing work under the Mechanical code are smarter than mechanical contractors doing work under the Residential code, or the mechanical contractor use smarter workers for work under the Mechanical code? Hopefully, that is not the reason.)