Answers and Analysis

No. Manual D is not the only approved method of sizing ductwork. ACCA Manual Q may be used for commercial applications. In addition, Section 601.2 states that other approved methods may be used. The Authority Having Jurisdiction would have to determine if the alternate method of sizing would be equivalent to the provisions set forth in the UMC.

Yes, air may be returned from a corridor. The intent of this code section is to isolate a required exit corridors air from any adjoining rooms air supply. The duct system could not serve the corridors and other rooms in anyway that "mix" air that could contain smoke with the air in the corridor. A separate system dedicated to the corridor would be allowed because it is not purposely conveying air to or from adjoining rooms.

No. The Uniform Mechanical Code is very explicit in this regard. Section 602.1 is the correct reference, the paragraph immediately preceding the one cited is pertinent due to the system’s negative pressure. It states, “See Chapter 5 for limitations on environmental air systems exhaust ducts extending into or through ducts or plenums.”

Section 504.1 states, “Exhaust ducts shall not extend through ducts or plenums.” The code does not distinguish between positive or negative pressures in this regard.

Explanatory Note: If the duct system were to become disconnected or disassembled at or near the source of contaminants, these fumes, vapors, contaminants or whatever is being exhausted could possibly be drawn into the return air plenum and recirculated. Per your example, a separately sealed shaft constructed independently of the plenum could possibly be used to route the duct through provided it was first approved by the Authority Having Jurisdiction and is constructed per the Building Code.

1. Yes, an electric water heater could be installed in a dwelling unit plenum under Exception 1, Section 602.2.

2. Yes, wiring and connections in a plenum would be allowed under Exception 5, Section 602.2 assuming compliance with applicable listings, installation instructions, and the National Electrical Code.

Yes, when it meets the requirement of Section 602.2 of the UMC and is installed in compliance with its listing and installation instructions.

1. Yes, residential is the same as a dwelling unit provided it meets the definition in the Building Code. There are Group R structures that do not meet the definition of a dwelling unit, such as a facility that provides only sleeping accommodations.

2. No. Exception 1 only allows the materials that the ducts and plenums are constructed of to be of combustible material. All materials exposed within or passing through the duct shall be noncombustible or meet the flame and smoke index requirements in Section 602.2.

3. Yes

No. Section 602.2 clearly states that material exposed within ducts or plenums shall meet minimum flame-spread index and smoke-developed rating requirements. There are no exceptions that would allow exposed wood within the plenum by adding a fire sprinkler system or smoke detectors.

No. Materials exposed within ducts must be tested in accordance with NFPA 255, ASTM E84, or UL 723, except as indicated in the exceptions. Exception 8 of Section 602.2 which addresses UL 2043 provides approval for loudspeakers and recessed lighting fixtures, including their assemblies and accessories. Exception 8 would not apply to plastic start collars.

Yes. Section 602.4 of the UMC states that "duct joints shall be made substantially airtight by means of tapes, mastics, gasketing, or other means." Closure systems for rigid air ducts and plenums shall be listed in accordance with UL 181A, Standard for Closure Systems for Use with Rigid Air Ducts and Air Connectors. Flexible air ducts shall be listed in accordance with UL 181B, Standard for Closure Systems for Use with Flexible Air Ducts and Air Connectors. (See Chapter 17, Part II.)

Duct material used to vent a domestic range hood in a dwelling unit must use Table 6-9 (2003/2006/2009). This would allow galvanized sheet metal of 30 gauge or aluminum B&S gauge of 26 for round duct less than 14 inches.

The code requirement must always be met, although the duct does not necessarily have to maintain 18 inches of clearance for its entire developed length if there is a path maintained to access all areas of the crawlspace. The intent of the code is to make certain that a duct does not block access to areas of the crawlspace. If a person could easily go around a duct to access a portion of the crawlspace, then that portion of the duct would not be required to have 18 inches of clearance as long as it had a minimum of 4 inches of separation from earth. The substantiation for the code change as printed in the 2004 Report on Proposals states “often ducts are installed in crawl spaces and block all access. It is often necessary to cut air ducts apart to gain access to a crawl space area or tunnel under the duct.”

Yes. Sections 602.1 and 602.5 (2003/2006/2009) allow the use of UMC Standard 6-2, based on the first edition, 1995 HVAC Duct Construction Standards, Metal and Flexible, published by the Sheet Metal and Air Conditioning Contractors National Association. Section 1701.0 (2003/2006/2009) states that UMC Standard 6-2 is declared to be part of the code. The SMACNA Duct Construction Standard addresses supports, hangers and states that the supports contained in the document are not seismically qualified and refers you to the Seismic Restraint Manual for additional reinforcement. The code does not specifically reference the Seismic Restraint Manual; therefore you would have to receive approval from the AHJ to use that manual as a guideline.

It is not the intent of Section 604.2.4 (604.1.4, 2003) to be a seismic requirement, but rather a requirement to minimize horizontal and lateral movement. The AHJ would have the final determination on whether the duct is adequately braced. In seismic zones, there may be a requirement that a licensed engineer design the system to be compliant with the Building Code for earthquake loads which would address the horizontal and lateral load bracing.

Yes, factory made flexible air ducts may be used for this application. They must be listed or approved as Class 0 or Class 1 factory made air ducts and shall be installed in accordance with the terms of their listing and the requirements of UMC Standard No. 6-5. Flexible air connectors are not permitted.

1. Yes, Section 605.0, exception B is for ducts or plenums in conditioned spaces. The space above a ceiling does not meet the definition of conditioned space as set forth in Section 205.0 (2003/2006/2009) of the code.

2. No, heating and cooling ducts installed in return air plenums do not have to be insulated per Table 6-6A or Table 6-6B (2003/2006/2009), since these spaces are classified as indirectly conditioned spaces per footnote C of aforesaid table.

1. Yes. Section 606.5 (605.5/ 2012) requires dampers to be provided with an approved means of access that is large enough to permit inspection and maintenance of the damper and its operating parts. Access points shall be permanently identified on the exterior with labels that read "Smoke Damper." The code section provides for easy identification of the location of dampers for maintenance purposes by workers without access to original plans or as-built.

2. Yes, if you are using a T-bar ceiling panel as access, you must identify that panel.

Yes. There is no prohibition on the location of gas piping in relation to a plenum, except as noted in Section 608.13 (607.14/ 2012), which pertains to using under-floor space as a supply plenum.

No, ducts located in rated corridors are not prohibited by section 602.1, provided there are no outlet openings in the in the ductwork to convey air to or from the corridor. The intent of the code is to isolate required exit corridors air from any other portions of the building.

Yes. FPTU’s are generally part of a VAV assembly that is suspended from the building structure. A fan powered terminal unit would be considered an Air Moving System as defined in Section 203.0 of the Uniform Mechanical Code, and required to meet all of the provisions of this code. Therefore, if the total supplying air system exceeds 2,000 cubic feet per minute then Section 609.0 (2003/2006/2009), 608.0 (2012) would apply.

No, under section 603.3 of the 2012 Uniform Mechanical Code factory made air ducts shall not be used as vertical risers in structures greater than two stories.

The 2015 UMC section 603.5 states the length shall not exceed 5 feet. The code change was propagated to address static pressure loss based upon charts provided by the code change proponent using information from manufacturers of the flexible duct. Maximum length of flexible air ducts and connectors must comply with UL standard 181, as well as manufacturers installation guidelines; both of these requirements would coincide with the 2012 ASHRAE handbook.

Yes, 603.4.1 Length Limitation of the 2015 Uniform Mechanical Code applies to factory made flexible air duct and connectors in general with the exception being residential occupancy installations. The Limitation refers to both the connector as well as the duct. Neither may exceed 5 feet except in a residential occupancy.

Yes, under this section it would the intent of the code to not allow a direct connection of flexible ductwork directly to a diffuser without the use of a rigid elbow or fitting.

1. No, section 603.5 of the 2015 Uniform Mechanical Code limits only flexible air connector lengths are limited to no more than 5 feet.

Section 603.4 of the 2015 Uniform Mechanical Code states that factory duct must meet UL 181. UL 181 addresses requirements which include preformed lengths of flexible or rigid ducts, materials in the form of boards for field fabrication of lengths of rigid ducts, and preformed flexible air connectors. Not all flexible ducts comply with UL 181.

2. No, Section 603.5 of the 2015 UMC only addresses the length of a flexible duct connector that is limited to 5 feet in length maximum.

No, this requirement is not limited to exposed plenum cables and wiring. Section 602.2 of the 2012 Uniform Mechanical Code begins by stating that “materials exposed within ducts or plenums shall be noncombustible or shall have a flame spread index not to exceed 25 and a smoke developed index not to exceed 50.” This requirement would include all exposed construction materials. The wood roof is considered combustible and would not meet the 25/50 flame and smoke index requirements.

1. There is not a specific definition of Factory Made ductwork in the 2015 Uniform Mechanical Code. All ductwork not made onsite or mass produced ductwork could be considered factory made. Under the provisions of section 603.4 of the UMC all Factory Made must comply with UL181. The broad range of products covered under this standard are often of proprietary design. They typically come in flexible or rigid lengths or in sheets or boards for field assembly into a rigid duct. Their composition is limited to metal or mineral materials. Nonmetallic or organic materials may only be used as “binders, adhesives, sealants, or finishes.” The manufacturer must mark every portion of a Factory Made air duct system to show compliance with the standard. The duct system must be installed in accordance with the terms of its listing and the installation instructions shipped with the duct material. The SMACNA-2006 HVAC Duct Construction Standard, which includes directions for the installation of factory-made air ducts, should be consulted. The manufacturer’s instructions should include a complete and detailed description of the methods to be used for joining and splicing the ducts. The materials used for this purpose must comply with the requirements of the referenced standard for closure systems found in UL 181A and UL 181B.


2. An Air Connector is a short length of flexible non-porous material that is inserted between metal ductwork and a metal air diffuser (or air register) to prevent the transmission of vibration between these components of the system.

The markings on the material will identify it as either a flexible air duct or a flexible air connector. Both the flexible air duct and flexible air connector must comply with the requirements of UL 181 for Class 0 or Class 1. Flexible air ducts require more extensive testing (flame-penetration, puncture, and impact tests) than flexible air connectors. This difference in testing is what determines the markings on the material and whether or not it is classified as a flexible air duct or a flexible air connector.

3. This length limitation is not arbitrary. The length limitation of section 603.4.1 in the 2015 UMC applies to factory made flexible air duct and connectors in general with the exception being residential occupancy installations. In this particular section the limitation refers to both the connector as well as the duct. Neither may exceed 5 feet except in a residential occupancy.

No, since all factory-made and flexible air ducts are required to be listed to UL181. Fabric ducts, which are more commonly known as Air Dispersion Systems, are listed and labeled to UL 2518. The 2015 UMC does not recognize UL 2518 in the referenced standards.

Fabric duct would need to be installed per manufacturer’s guidelines and meet the guidelines for designed specific use. Fabric duct must be installed entirely in exposed locations, be utilized in systems under positive pressure and not pass through or penetrate fire-resistant-rated-construction.

No, as defined in Chapter 2 of the 2015 Uniform Mechanical Code, air-moving systems pertain to air-handling units. An air-handing unit is defined as a blower or fan used for the purpose of distributing supply air to a room, space, or area. An exhaust system does not have the same requirements as an air-moving system. Section 608.1 applies specifically to air supply moving systems and not exhaust systems.

However the Authority Having Jurisdiction would determine the occupancy classification of the warehouse. Its use and occupancy would have a direct bearing on whether smoke detection or fire separation is required. Smoke detection required under this type of occupancy maybe required to be interlocked to the fan power source to provide shutdown upon detection of smoke.

1. No, there are other approved systems that are not considered rigid duct systems that can be used. Only factory made flexible air ducts and connectors listed to UL181 are limited to not more than 5 feet in length under section 603.4.1 of the 2015 Uniform Mechanical Code. That section is specific in that the limit of 5 feet of factory made flexible air ducts shall not be used in lieu of rigid ducts. Section 603.5 of the 2015 UMC and UL 181 provide the regulations for flexible duct to be used in a commercial application. In order to be in full compliance with the 2015 UMC both the manufacturer’s instructions as well as the terms spelled out by SMACNA HVAC Duct construction standards for both metal and flexible duct must be adhered to.

2. Yes, under this section it would allow flex duct between the rigid duct and terminal air device, however it is limited to not more than 5 feet in length and shall not be used in lieu of rigid elbows and fittings.

3. Yes, under this section flex duct is allowed to connect the rigid duct to equipment or appliance. Providing it is limited to not more than 5 feet in length and shall not be used in lieu of rigid elbows and fittings.

1) The requirements in section 603.3 of the 2012 Uniform Mechanical Code are based on decisions made decades ago by the Mechanical Code Development Committee incorporating Uniform Mechanical Code Standard No. 10-1 which was based on Standard 181 of Underwriter Laboratories, Inc. Materials were tested for their fire hazard classification under U.B.C. Standard No. 42-1. Because they were only subject to flame-penetration, temperature, puncture along with impact or static load testing, and not flame resistance testing, they are not regarded as equivalent to air ducts or as a substitute for metallic duct materials.

2) Yes, as long as the duct does not penetrate construction where fire dampers are required.

3) Each individual duct. The intent is for the duct not to penetrate the construction where a fire damper is required so that the integrity of any rated assembly is maintained. Fire damper was replaced with fire-resistance-rated construction 2018 - 603.4

Yes. Evaporative cooling systems used to provide supply air to enclosed spaces of buildings fall under the 2018 Uniform Mechanical Code Section 608.1. It requires an automatic shutoff when the velocity of the supply air produced exceeds 2000 Cu. ft. per minute (unless the criteria in one of the exceptions listed in 608.1 is met).

No, this application could be consistent with 2015 Uniform Mechanical Code Section 608.1 exception number (1). As long as this exhaust is controlled by the exhaust hood, no additional smoke detectors are needed. Otherwise any and all independent air-moving systems including a makeup air unit supplying air in excess of 2000 cfm to an enclosed space within a building are required to have an automatic shutoff.

No, Section 602.1 of the 2015 Uniform Mechanical Code states that “concealed building spaces or independent construction within buildings shall be permitted to be used as ducts or plenums”.

According to this section of the code, a building of wood frame construction would be allowed to use the space above the T-bar ceiling as the duct or return plenum. However, Section 602.2 of the 2015 UMC clearly states that combustible material exposed within ducts or plenums shall meet minimum flame-spread index and smoke-developed rating requirements. The requirement for what type of material a plenum may be constructed of is found in the building code. There are buildings of combustible construction that would allow for combustible plenums. The intent is to allow plenums to be constructed of materials that are consistent with the materials that are allowed for the building construction.

In summation, a plenum constructed of combustible materials shall meet minimum flame-spread index and smoke-developed ratings.

(1) Yes section 602.2 (1) of the 2015 Uniform Mechanical Code does allow concealed spaces that serve an individual dwelling unit shall be permitted to be of combustible construction for return-air.
(2) An individual dwelling unit return-air does not have to be ducted as per Section 602.2 exception (1) for the fan coil, but it would need to be filtered from the room to the plenum (311.2). Section 311.3 of the 2015 UMC regulates the location of the return air from prohibited sources. The required Ventilation (outside air) air requirements are proscribed in Table 402.1 of the 2015 UMC.

1. Yes, but only if the fire sprinkler lines and the cast iron pipe are associated with the function of the garage exhaust duct. Noncombustible material may not just pass through an exhaust duct. Section 602.2 of the 2018 Uniform Mechanical Code specifically describes the use of non combustible material in ducts or return air plenums (either ducted, or are part of the building, such as a cavity or concealed space). The listed exceptions and sections 602.2.1 through 602.2.4 pertain to use within the duct. Where required by section 506.8, sprinklers and other fire protection devices shall be installed within ducts having a cross-sectional dimension exceeding 10 inches, with non-metallic fire sprinkler piping to be listed and labeled for plenum use under the provisions of 602.2.2. The hangers and supports of the fire sprinkler piping within the duct are to be listed to UL 2043 if they are non metallic (see also section 206.0 of the 2018 UMC).

In addition to the requirements of the UMC, section 309.4 of the 2018 Uniform Plumbing Code states in part ‘…All materials shall be installed so as not to adversely affect the systems and equipment or the structure of the building…’ therefore cast iron pipes that penetrate through a garage exhaust duct would generally not be acceptable.

2. No, Section 602.2 of the 2018 UMC applies to the construction of the duct and anything within the duct. This section does not only apply to the construction of duct and plenum material; it requires that any material exposed to the airstream in ducts and plenums exhibit specified surface-burning characteristics i.e. the 25/50 flame spread/smoke development rating.

No. Ducts are specifically required to be substantially airtight by SMACNA duct construction standards and are permitted by Section 603.12 of the (2015/2018/2021) Uniform Mechanical Code to be constructed of concrete, clay, or ceramics where installed in the ground or in a concrete slab. PVC is not listed for supply or return air ducts and does not meet the smoke & flame spread rating index for this use. A breach in the duct work would allow the concrete to become the duct work as a backup. The PVC would not be allowed under these installation requirements because it is not listed for this intended use. The UMC is very specific on specifying concrete. If the duct ever rotted away, the concrete would provide a barrier. PVC would not meet the flame spread of conveying ducts. The 2" concrete around metal ducting is a protective sleeve provided for the purpose of reducing the corrosion of the metal ducting. PVC piping would not be allowed as a protective sleeve because of off gassing of the PVC pipe with elevated temperatures within the heating system.

1. As required by section 608.1 of the 2018 Uniform Mechanical Code, air-moving systems supplying air in excess of 2000 cubic feet per minute to enclosed spaces within buildings shall be equipped with an automatic shutoff whether they are serving a common space or not. The key word is enclosed spaces within buildings. If they do not share any of the same supply or return ductwork, and the spaces are separated, or they meet one of the other exceptions in section 608.1 of the 2018 Uniform Mechanical Code, the separate shut downs do not need to be interlocked. The enclosure of each space shall extend from the floor through the interspatial space to the floor/ceiling assembly. The intent of the UMC is to isolate the smoke from a fire in one area.

2. Fire walls, fire barriers, smoke partitions, and other separations are defined in the Building Code based on occupant factor type of building and building use. This generally does not defined in the UMC.

Yes, it meets 2018 Uniform Mechanical Code section 608.1, exception (2) therefore an automatic shutoff is not required.