What materials are suitable for installation in Heating, Ventilation, and Air Conditioning (HVAC) plenums? Let’s first define a plenum. The 2021 edition of the International Mechanical Code (IMC), a subset of the International Building Code (IBC), defines a plenum as “an enclosed portion of a building structure, other than the occupied space being conditioned, that is designed to allow air movement, and thereby serve as a part of an air distribution system.”
Here we note that the occupied space is excluded from the definition, and the portion of the building structure that is designed to allow air movement is the focus. This is the key to understanding when a space in a building can be considered a plenum.
Is a portion of the building structure, for example a space above a suspended ceiling or a mechanical room or closet, designed to allow the movement of air as part of an air distribution system? If air moves through these spaces as opposed to through a duct system, then the space is a plenum.
Open return fan coil unit in a ceiling.
When a plenum is present, there are specific requirements in the code for the materials installed in a plenum. Chapter 6 of the IMC section 602.2.1 Materials within plenums states as follows:
Except as required by Sections 602.2.1.1 through 602.2.1.8, materials within plenums shall be noncombustible or shall be listed and labeled as having a flame spread index of not more than 25 and a smoke developed index of not more than 50 when tested in accordance with ASTM E84 or UL 723.
Exceptions:
The IMC requires that materials within a plenum be noncombustible or have a flame spread index of not more than 25 and a smoke developed index of not more than 50. Why? Since a plenum is part of the “air distribution system” any smoke or byproducts of combustion that are generated will be distributed throughout the area that is served by the plenum. If the plenum is on the return of an air distribution system, then potentially all of the areas served by the respective supply system will receive any smoke that is generated if the system is running. If the plenum were to be on the supply side of a system, then any smoke or byproducts of combustion will be distributed to the area served by the respective supply plenum.
Why does this matter? The National Fire Protection Association (NFPA) tells us that most fire deaths are caused by smoke inhalation and not by actual burns. As an interior structural firefighter and a fire investigator, I have seen this to be the truth. The smoke will often get you before the flames do. So as designers and installers of HVAC systems, we want to minimize the spread of smoke to the best of our abilities.
In my role as a forensic engineer, I often see materials that do not meet the requirements of the IMC regarding flame/smoke spread installed in plenum areas. Big offenders are often PVC piping, data cabling and electrical wiring.
Often plenums are created by installing an open grille in the door or wall of a mechanical room and allowing the return air to pass through the mechanical room to the open return on the air handling unit. This room is by definition a plenum and the requirements of the code with regard to flame/smoke spread apply to this space. Materials installed in this plenum must meet the flame spread of 25 and smoke developed of 50. A repeat offender is the condensate drain line on an air handling unit. PVC is not suitable or listed for use in a plenum. Copper is a much better choice as it will not burn. CPVC can also meet the flame/smoke spread requirements but check the listing to be sure.
Finally, don’t forget wires. Most modern wiring and cabling will have some type of man-made insulation on it. Be sure that all of these meet the flame/smoke spread requirements as well when installed in plenums. Plenums are often found above ceilings in commercials structures where cabling and wiring are found as shown in the photograph with the hydronic fan coil unit. Also, raised computer room/data center floors are often used for both routing of cables and supply or return of conditioning air. Here all items installed in the plenum must be rated for this type of installation.
Example of plenum spaces
While plenums can be advantageous in the design and installation of a building HVAC system, they add a new layer of requirements that must not be ignored for the safety of the occupants of the building.
Chad Jones, PE, CFEI, CVFI, CMSE has a Bachelor of Science in Mechanical Engineering from Clemson University. Chad has over 25 years of engineering experience including mechanical, process, and manufacturing engineering. This work has included equipment design, machine safeguarding, cost estimating and safety compliance. Chad also has over 10 years of commercial, industrial, and residential HVAC and plumbing design experience. A lifelong auto and motorcycle enthusiast, Chad is accomplished in the maintenance, repair, and modification of vehicles and engines. Chad is a Certified Fire and Explosion Investigator, Certified Vehicle Fire Investigator, and IFSAC certified Firefighter II in Greenwood County, South Carolina.
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