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

Flexible Air Ducts versus Flexible Air Connectors – What Is the Difference?

January 13, 2025 by Roger Griffith Leave a Comment

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 air duct conveys air in an HVAC duct system. A flexible air duct typically connects a main trunk to an air inlet or outlet. The mechanical code does not limit their length. However, design requirements typically limit the length of flexible air ducts to six feet. This limitation avoids high-pressure drops and kinking in long sections of flexible air ducts.

A flexible air connector is designed to connect different duct system components, e.g., connecting equipment to ductwork. Flexible air connectors are meant to absorb vibrations or slight misalignments and are limited in length to 14 feet.

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

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 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 in length to 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 to be conveyed in flexible air ducts and flexible air connectors shall be less than 250⁰F.

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.

Sleeper Sofas and Gas Appliances

January 14, 2019 by Roger Griffith Leave a Comment

Gas appliances could pose a threat to people when they are 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.

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:

There are six exceptions listed for 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 Plumbing/Mechanical/Fuel Gas Code Action Committee

January 27, 2017 by Roger Griffith Leave a Comment

ICC Code Committee meeting

Roger W. Griffith has been appointed to the Plumbing/Mechanical/Fuel Gas Code Action Committee (PMGCAC) of the International Code Council (ICC). The role of this code committee is to review and propose revisions to the International Plumbing, Mechanical, Fuel Gas, Private Sewage Disposal, and Swimming Pool and Spa Codes and the International Residential Code, chapters 12 through 33, which covers the mechanical, plumbing, and fuel gas systems in residential occupancies. I am honored to be appointed as a member of this committee for the upcoming 2018 – 2019 code development cycle.

These important codes define how mechanical, plumbing, and fuel gas systems must be designed and installed to protect the health, safety, and welfare of the public.

Tools and Resources for Mechanical Insulation

November 26, 2016 by Roger Griffith 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 items like insulation and other accessories.

The National Insulation Association’s (NIA) new redesigned website is a helpful resource for all things insulation, from training, design helps, 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 – Locate In Supply or Return Ducts?

May 8, 2015 by Roger Griffith 18 Comments

Duct smoke detectors, when installed in HVAC systems, automatically stop the system fans upon the detection of smoke. For HVAC systems with airflows exceeding 2,000 cfm, duct smoke detectors are required. This includes where multiple air handling systems utilize 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 Griffith 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.”