In case of a building fire, proper material selection is critical for preserving the safety of building occupants and responding emergency services personnel. As a result, model mechanical codes around the country are restrictive regarding what materials are allowed within a plenum area. 

“In commercial and multifamily construction projects that use piping systems within return-air plenum areas, it is imperative that the materials selected aid in the structure’s fire resistance and don’t impede it,” said Brian Helms, training manager at Charlotte Pipe and Foundry. “The model mechanical codes define specific criteria to ensure that materials in return-air plenums will not propagate flame and smoke.”

What is a plenum?

Offering a combination of functionality and visual appeal, drop ceilings are a popular design feature for commercial buildings. Drop ceilings and raised floors also provide a useful space for locating components of the building’s HVAC system. 

“Although the forced-air supply is ducted, these plenum areas can function as an unducted air return,” Helms said. “This eliminates the need for duct in this portion of the HVAC system.”  

Plenums also provide a location to house various components of the building’s mechanical, electrical, and plumbing systems. This can include air ducts, electrical wiring, fire suppression, and plumbing pipe — all of which are concealed above the drop ceiling to maintain a clean and unobstructed appearance in the occupied spaces below.

Plenums, however, can also pose a significant risk in a fire due to the potential for fire and smoke to spread rapidly through them. 

Understanding Fire-Rated Testing Standards

Plenums are considered part of the building’s air-handling system. In a fire, they can serve as conduits for smoke and flames to spread rapidly throughout the building. Therefore, the materials used for plumbing systems within plenums must meet strict fire safety standards to minimize flame and smoke propagation.

The materials used in commercial construction undergo rigorous testing to evaluate their fire-resistance performance. To accomplish this, ASTM International developed two important testing standards that help determine the suitability of materials, including pipe and fittings, for fire-resistive construction and plenum areas.

1. ASTM E136

As the standard test method for assessing combustibility of materials using a vertical tube furnace at 750 C, ASTM E136 evaluates the combustibility of building elements. This testing method involves exposing materials to 750 C for 30 minutes to determine whether the material is noncombustible, meaning it does not aid in combustion or appreciably add heat to a surrounding fire.

The results define which materials are combustible and which are noncombustible.

Per ASTM E136:

• Cast iron pipe and fittings are defined as noncombustible, meaning plumbing systems that use this material will not sustain combustion and can be used in plenums without restrictions.
• All plastics, including PVC, are combustible materials.

2. ASTM E84

As the standard test method for surface-burning characteristics of building materials, ASTM E84 is a 1950s-era comparison test that determines flame-spread values and quantifies smoke generation. With this test, asbestos is assigned a flame-spread and smoke-developed index of 0, and red oak is assigned an index of 100. Comparative values are then established for the tested materials, with higher numbers indicating faster flame propagation and smoke density.

“While ASTM E84 may be the measuring stick for flame-spread and smoke-developed indexes, the model mechanical codes — both the IMC and UMC — define the benchmarks in suitability for inclusion of combustible materials in return-air plenums,” Helms said. 

For inclusion in return-air plenums, both mechanical codes require that combustible materials exhibit a flame-spread index of 25 or less and a smoke-developed index of 50 or less when tested to ASTM E84.

• Since cast iron is noncombustible, these systems can be installed in return-air plenums without restriction.
• Per ASTM E84, PVC does not meet the 25/50 flame and smoke requirement for plenum applications as defined by the model mechanical codes in the U.S.

“These two testing standards are important for plenum material selection because one influences the other, with ASTM E136 determining if a material is combustible,” Helms said. “If the material is combustible — and all plastics are combustible — then it must meet the flame-spread and smoke-developed index values set forth by the model mechanical codes when tested to ASTM E84 to be allowed in a plenum.”  

Plenums and Plumbing Design Solutions That Save Lives

When it comes to life safety, the materials used in the plumbing systems within plenums can have a significant impact. Since both flame spread and smoke density can profoundly affect occupants’ ability to escape a building fire, the values defined by model codes per ASTM E84 are critical considerations.

In addition to using fire-resistant materials to prevent the spread of fire and smoke, it’s also important to properly seal and encapsulate plumbing systems within plenums per local and model code requirements. This can include using fire-rated sealants, enclosures, and barriers to contain potential fire hazards.

“We are often asked if Charlotte Pipe manufactures plenum-rated plastic DWV pipe,” Helms said. “The answer is yes … and it’s called cast iron.” 

By coupling thoughtful design with materials that are either noncombustible or compliant with model mechanical code requirements, plumbing engineers and contractors can help minimize the risk of fire-related incidents and keep occupants safe.

For additional information regarding how to achieve code-compliant plumbing systems within a return-air plenum, contact the Charlotte Pipe Technical Support team.