HVAC Systems Archives | Roger W. Griffith, P.E.

Flexible Ducts versus Flexible Connectors

January 13, 2025 by Roger W. Griffith, P.E. Leave a Comment

flexible air duct HVAC system — Flexible Air Duct

The mechanical codes cover both flexible air ducts and flexible air connectors. They are similar in appearance, but they are not the same.

A flexible duct conveys air in an HVAC duct system. It typically connects a main trunk to an air inlet or outlet, and the mechanical codes do not limit their length. However, design requirements typically limit the length of these ducts to 6 feet to avoid high pressure drops and kinking in long duct sections.

A flexible air connector connects different duct system components, e.g., connecting equipment to ductwork. Flexible air connectors absorb vibrations and slight misalignments, and they are limited to a length of 14 feet.

The listing label for air ducts is rectangular and states “Listed Air Duct.” The listing label for “flexible air connectors” is circular and states, “Listed Air Connector.” Both are listed to UL 181 Class 0 or Class 1 requirements.

Flexible Ducts: International Mechanical Code (IMC) Requirements, 2024 edition

Definitions

FLEXIBLE AIR CONNECTOR. A conduit for transferring air between an air duct or plenum and an air terminal unit or between an air duct or plenum and an air inlet or air outlet. Such a conduit is limited in its use, length, and location

Chapter 6: Duct Systems

Section 603: Duct construction and installation

603.6: Flexible air ducts and flexible air connectors

603.6.1: Flexible air ducts.

Flexible air ducts, both metallic and nonmetallic, shall be tested in accordance with UL181. Such ducts shall be listed and labeled as Class 0 or Class 1 flexible air ducts and shall be installed in accordance with Section 304.1.

603.6.1.1: Duct length

Flexible air ducts shall not be limited in length.

603.6.2: Flexible air connectors.

Flexible air connectors, both metallic and nonmetallic, shall be tested in accordance with UL181. Such connectors shall be listed and labeled as Class 0 or Class 1 flexible air connectors and shall be installed in accordance with Section 304.1.

603.6.2.1: Connector length.

Flexible air connectors shall be limited to a length of 14 feet.

603.6.2.2: Connector penetration limitations.

Flexible air connectors shall not pass through any wall, floor, or ceiling.

603.6.3: Air temperature.

The design temperature of air conveyed in flexible air ducts and flexible air connectors shall be less than 250⁰°F.

603.6.4: Flexible air duct and air connector clearance.

Flexible air ducts and air connectors shall be installed with a minimum clearance to an appliance as specified in the appliance manufacturer’s installation instructions.

Section 607: Duct and transfer openings.

607.7: Flexible ducts and air connectors.

Flexible ducts and air connectors shall not pass through any fire-resistance-rated assembly.

Chapter 3: General Regulations

Section 304: Installation

304.1: General

Equipment and appliances shall be installed as required by the terms of their approval, in accordance with the conditions of the listing, the manufacturer’s installation instructions, and this code. Manufacturer’s installation instructions shall be available on the job site at the time of inspection.

Gas Appliances in Sleeping Rooms

January 14, 2019 by Roger W. Griffith, P.E. Leave a Comment

Gas appliances can pose a threat to people sleeping, unaware of any impending danger. The threats include oxygen depletion and elevated levels of nitrous oxide or carbon monoxide. For this reason, the fuel gas codes restrict the use of gas appliances in sleeping rooms.

2018 International Fire Code requirements

The 2018 International Fuel Gas Code (IFGC), Section 303 – Appliance Location, contains the following criteria:

303.3 Prohibited Locations. Appliances shall not be located in sleeping rooms, bathrooms, toilet rooms, storage closets, or surgical rooms, or in a space that opens only into such rooms or spaces, except where the installation complies with one of the following:

The IFGC lists six exceptions for the requirement of paragraph 303.3. These six exceptions allow gas appliances in these prohibited locations when specific conditions are met. [Read more…]

Roger W. Griffith appointed to ICC PMGCAC

January 27, 2017 by Roger W. Griffith, P.E. Leave a Comment

Roger W. Griffith, P.E., was appointed to the International Code Council’s Plumbing/Mechanical/Fuel Gas Code Action Committee (ICC PMGCAC) for the 2018-2019 code development cycle.

The ICC PMGCAC is responsible for reviewing and proposing revisions to the International Plumbing Code, International Mechanical Code, International Fuel Gas Code, International Private Sewage Disposal Code, International Swimming Pool and Spa Code, and the mechanical, plumbing, and fuel gas chapters of the International Residential Code. These codes govern how mechanical, plumbing, and fuel gas systems must be designed and installed to protect the health, safety, and welfare of the public.

Appointment to the ICC PMGCAC is by selection — members are chosen based on their technical expertise and professional standing in the industry. Committee members review proposed code changes, evaluate technical evidence, and vote on revisions that become adopted into the next code edition.

Mr. Griffith’s participation in national code development directly informs his expert witness practice. His firsthand understanding of how code language is developed, debated, and interpreted gives him unique insight into the intent behind code requirements — insight that is directly applicable in litigation involving alleged code violations and system failures.

Tools and Resources for Insulation Specifications

November 26, 2016 by Roger W. Griffith, P.E. 1 Comment

When designing mechanical systems, engineers and designers often need help specifying the type and thickness of insulation for ducts and piping. Good resources are always valuable and welcome when time constraints leave little time for insulation specifications.

The National Insulation Association’s (NIA) new redesigned website is a helpful resource for all things insulation, from training, design help, and specifications. Check out their new website and bookmark it for return trips. Enjoy!

For suggestions on future blog posts, make a suggestion.

Duct Smoke Detectors – Location Requirements

May 8, 2015 by Roger W. Griffith, P.E. 18 Comments

Duct smoke detectors, when installed in HVAC systems, automatically shut down the system fans upon detecting smoke. For HVAC systems with airflows exceeding 2,000 cfm, duct smoke detectors are required. This includes cases where multiple air-handling systems use common ducts with a combined design capacity greater than 2,000 cfm.

But when duct smoke detectors are required, where should they be located? Let’s look at the requirements in two of the prominent codes and standards.

[Read more…]

Floor Registers in Commercial Buildings

May 29, 2014 by Roger W. Griffith, P.E. Leave a Comment

Commercial buildings typically have ceiling diffusers or sidewall grilles, but occasionally there is a need to locate air supplies or returns on the floor of a facility. Floor registers are common in residential applications. Before locating them on the floor a commercial building, a careful review of the applicable codes is imperative.

Building codes typically do not prohibit floor registers in commercial buildings, however, the fire codes can restrict their usage. For example, NFPA 101: Life Safety Code, paragraph 9.2.1 states that “Air-conditioning, heating, ventilating ductwork, and related equipment shall be in accordance with NFPA 90A.” NFPA 90A: Standard for the Installation of Air-Conditioning and Ventilating Systems, paragraphs 4.3.7.3.1 and 4.3.8.3.1 state that “Air inlets/outlets shall be located at least 3 inches above the floor, unless provisions have been made to prevent dirt and dust accumulations from entering the system.”