IMC insights

• The International Mechanical Code (IMC) has evolved over the course of the past 10 years.
• Since 2015, the IMC has made significant changes to Chapters 4, 5, 6, 10 and 11. Learn about them here.

Professional engineers and designers rely on the International Mechanical Code (IMC) in their day-to-day decision-making process to provide the minimum requirements to design buildings with public health and safety in mind. The IMC is one of many codes published by the International Code Council (ICC).

There are several reasons for selecting the five IMC chapters highlighted here. First, these contain key evolution since 2015. Second, these chapters are important in heating, ventilation and air conditioning (HVAC) systems design. Third, abbreviating to these five maintains the length of the article and leaves other chapters for future review. It is highly encouraged that the reader references the code thoroughly for further information.

Figure 1: Exhaust fan upblast for grease duct. Courtesy: WSP USA Buildings Inc.

Codes have been a part of building construction from centuries ago, a reason for code development has been fire and safety. In construction, materials such as wood, presented a fire hazard. In the 1940s, the International Conference of Building Officials (ICBO), the Building Officials and Code Administrators (now part of ICC) and Southern Building Code Congress International (also part of ICC) develop the first building code. The code released in 1927 was by the ICBO. It was approximately 67 years before the ICC was established.

One crucial factor that led to the development of codes was the number of fires in buildings that occurred in the early years. The vast number of incidents brought experts together to develop guidelines for design and construction to protect the building occupants.

Today there are approximately 12 editions published and seven distinct categories under the ICC, this is not including BOCA, ICBO and others. This article will focus only on the IMC and its evolution since the 2015 edition.

Basic IMC chapter background

The first three chapters on every code provide general guidelines and definitions, which are introductory. These include:

Chapter 1: Scope and administration, Part 1 and Part 2: This pertains to the administration of the code and how it is implemented through a design, and the local districts or authorities that have a role on items that deviate from the code and require approval to move forward.

Chapter 2: Definitions: Each code provide its definitions, updated each edition, to include content within the code changes or new terminology. The definitions under this section are key for the reader to understand, but most importantly to correctly interpret the code intent.

Chapter 3: General regulations: This section of this code is key for the engineer and/or designer due to the amount of information provided and is specific under each code. This chapter pertains to the installation and approval of appliances and/or mechanical systems as dictated by the code requirements under the different chapters.

It consists of key topics:

• Fuel gas appliances, which should follow the International Fuel Gas Code Section 301.6.
• Electrical controls, wiring and equipment or appliance electrical connections, which should follow NFPA 70: National Electrical Code.
• Condensate disposal, which should follow International Plumbing Code (IPC).
• Piping support, which should follow ANSI/MSS SP-58and provides information on specific system support distance depending on the pipe material.

All pipe support must be designed to withstand lateral and dynamic movement. In some instances, depending on the project location, seismic requirements may come into play. Typically, this is dictated by an analysis performed by the structural engineer. The piping and systems support extends beyond the basic code-required spacing and provides some type of additional bracing.

The IMC is under a three-year renewal cycle. However, not all states have adopted the newest edition of the code. Each state has a code adoption process at a local level and a state level. Not all states have a legislative code adoption mandate, which is one reason for the misalignment of code adoption.

Table 1: This shows a summary of United States International Mechanical Code (IMC) adoption per state. Courtesy: WSP USA Buildings Inc.

Refer to Table 1, which presents a summary indicating the number of states that have adopted a specific IMC edition.

IMC Chapter 4 ventilation updates

For HVAC design, the next chapters play an important role. IMC Chapter 4 contains information regarding natural and mechanical ventilation, outdoor air delivery per space and occupancy, recirculation, air transfer and exhaust.

Section 401.5 intake opening protection provides direction regarding intake openings protection not only pertaining to louver size and corrosion resistance, but Table 401.5 provides detailed information for the minimum/maximum louver opening size. One modification from the 2015 to 2018 edition is the footnote to clarify the dimension in relation to the shape of the opening. This is key because in prior editions, the dimension was set from an engineer‘s interpretation.

Section 403 recirculation of air within the code requires the following of specific criteria. For example, when designing swimming pools the recirculation of air should maintain the space at a relative humidity below 60%. Within this section, one evolution within the IMC from 2015 through 2024 is the compliance with ANSI/ACCA 10 Manual SPS: HVAC Design for Swimming Pools and Spas. This standard provides detailed criteria for swimming pool HVAC design.

Furthermore, Group R-2, 3 and 4, are included in sections 403.3 and 403.3.1 to provide specific criteria targeting outdoor recirculation of air.

Every designer understands the importance between the amount of outdoor air in relation to each space and their occupants. Indoor air quality is key and has been directly related to occupant performance. Although the IMC includes detailed information about the outdoor air requirements under Section 403.3, there are other standards that come into play and are implemented in conjunction with section 403.3. These standards include:

• ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality, which provides detailed steps on calculations for outdoor air to meet the critical zone and ventilation effectiveness by providing a specific outdoor air rate (i.e., cubic feet per minute [cfm]/square foot and cfm/person in the space and space type square footage).
• State- and facility-specific codes, such as the Texas Health and Human Services‘ Texas Administrative Code and Facility Guidelines Institute also provide outdoor air requirements in air change rate format.

Typically, the engineer should comply with the most stringent of all applicable codes or standards that impact the specific design.

Other key changes were made to:

Section 404 enclosed parking garages in the 2018 edition, where the option to continuously operate mechanical ventilation in an enclosed parking garage was included. The automatic operation remains as another option as sensed by nitrogen dioxide detectors. The requirement for these detectors to be UL 2075 listed, which requires the detectors to undergo rigorous testing to ensure their proper functionality. This is key to mantain a safety environment for occupants in enclosed parking garages or other facilities with nitrogen dioxide or carbon monoxide.

IMC Chapter 5 exhaust systems updates

IMC’s Chapter 5 provides minimum requirements for exhaust systems serving buildings such as general and hazardous exhaust, smoke control systems, cooking appliances and others as indicated under Section 502. The exhaust requirements may vary depending on the building classification as described under the International Building Code. This comprehensive chapter contains 14 sections of information regarding exhaust systems and where these are required. The following sections have adopted changes since the 2015 IMC edition.

Section 501.5 common ducts includes requirement regarding ducts serving separate dwelling units. In the event these are tied to a common shaft, the shaft should maintain negative pressure. This section was updated under 2024 IMC.

Section 502 required systems: Section 502.4: Stationary Storage Battery Systems was modified to remove the previous requirements/exceptions listed under 2015. The 2018 edition directs the engineer/designer to the International Fire Code (IFC) under section 1206.2.11.3. This reference also applies to Section 502.5: Ventilation of Battery Systems in Cabinets, the 2015 edition indicated an airflow rate of continued exhaust (1 cfm/square foot) while the 2018 edition directs the reader to the IFC.

 Section 502.16 repair garages for vehicles fueled by lighter-than-air fuels is a revised section under the 2018 code edition. As we continue to see changes in technology for vehicle motors with other fuel sources such as natural gas, nitrogen, hydrogen or other. The code adopted new minimum safety guidelines for designers and engineers to have a baseline. Read through the detailed updates applicable to repair vehicle garages with fuels other than oil. In general IMC 2018 has updated to provide detail exhaust requirement for garages depending on their fuel source. The code continues to provide guidelines for the design and operation of the exhaust systems.

Section 502.16.2 exhaust ventilation systems includes an exception for natural ventilation, however, it should be approved.

Section 502.16.2.1 design provides guidelines and states that the mechanical exhaust and supply system for indoor locations should be designed to provide uniform air distribution by specifying low exhaust and high supply along the room walls. The 2015 edition stated specifically low exhaust along exterior walls. For example, most fuels weight more than air. By specifying low exhaust, these fuels may be captured at the source. A quick comparison between methanol (CH₃OH) and air at standard conditions, the first has a density of 791 kilograms/cubic meter while air density is 1.29 kilograms/cubic meter.

The design of large commercial buildings will often include a kitchen that needs hood ventilation and exhaust equipment. This is a key section within IMC Chapter 5 as it includes grease duct design requirements for the specific type of hood and appliance being served. IMC editions 2015, 2018 and 2021 follow similar format for these few sections 506.1,2 and 3; however, the latest edition of IMC 2024 includes a slight modification. Specifically, it provides details regarding grease duct systems and grease duct construction materials under Section 506.3, while moving criteria regarding Type-I Hood requirements to Section 507.

Table 2: International Mechanical Code (IMC) Section 506.5.2 pollution control unit – UL listing modification. Courtesy: WSP USA Buildings Inc.

Section 506.5 exhaust equipment provides guidelines regarding grease reservoirs and exhaust equipment in the 2018 edition; Section 506.5.2 Pollution Control Units is new. This section includes 15 detailed points for equipment compliance serving grease ducts and their respective fans. This section has undergone several updates with respect to the listing of UL references that are key for the engineer or designer when specifying equipment serving a kitchen hood. Reference Table 2, which summarizes the UL listing change and their definitions.

Section 507 commercial kitchen hoods remained the same, except the 2024 edition has included several exceptions for Type I and Type II hoods. As a refresher, a Type I hood captures all smoke and grease from a cooking appliance, so the grease duct system must follow the guidelines under NFPA 96: Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations.

Section 508.1.1 makeup air temperature shows a difference in the statement regarding makeup air and the maximum design setpoint, not to exceed 10°F.There is also a new exception where the makeup air to the hood is not required to be conditioned. The design engineer should be aware of the project location in question, as best practice is to condition the air to be able to maintain the latent load in the space at or below 60%.

Health care design for facilities under IBC classified under I2-type occupancy require HVAC systems to be fully ducted; however, in medical office buildings and/or commercial buildings, among others, the return system may be of plenum type. This constitutes construction cost and labor savings due to eliminating linear feet of sheet metal on the return system.

IMC Chapter 6 duct systems updates

The IMC under Chapter 6 targets duct systems. For projects with plenum systems, Section 602 provides details on minimum requirements such as construction material selection for HVAC and electrical wiring, fire protection and finishes, among others. IMC 2018 edition changed the section 602.2.1.6 to be specific for foam plastic in plenums as interior finish or interior trim. This section continues to include a requirement for flame spread index 25/50. To be “plenum-rated,” an insulation’s flame-spread index must be not more than 25 and a smoke-developed index not more than 50. In addition, in the prior edition, IMC 2015, the requirement was flame spread index of 75 or less and smoke spread of 450 or less.

Section 602.2.1.7 plastic plumbing piping and tubing and Section 602.2.1.8 pipe and duct insulation within plenums are new additions in 2018 IMC. Pipe and duct insulation in plenums both must not exceed the flame spread index 25/50. When reviewing the IMC 2024 edition, note that the section numbering and scope changed.

IMC Chapter 10 boilers, water heaters and pressure vessels updates

Chapter 10 includes changes regarding boilers, water heaters and pressure vessels. The most relevant modification is under Section 1006.6, which focuses on safety and relief valve discharge. Before diving into the code content, it is important to understand the indication of safety relief valves for boilers or other systems under pressure.

Safety relief valves must comply with the ASME Boiler and Pressure Vessel Code Section VIII. There are several types of safety relief valves. Common ones specified may be pilot operated, spring loaded and power actuated among others. Safety relief valves are often specified in steam systems.

Figure 2: Safety relief valve general detail. Courtesy: WSP USA Buildings Inc.

For example, a central plant generates high pressure steam at 120 pounds per square inch (psi) sized for 2,000 pounds/hour. One air handler requires low pressure steam at 15 psi or less. In this instance, a dual pressure-reducing station is specified to protect the system a safety relief valve is required. Typically located upstream of the primary header, the discharge of the safety relief valve should be routed to the exterior of the building and independently piped. No other system relief should tie to the safety relief valve discharge piping (see Figure 2).

Section 1006.6 under the IMC 2015 edition, a few requirements for the safety relief valve were provided, specifically regarding the requirement of pipe diameter matching the valve outlet, material type and discharge to the outside of the structure. On the other hand, low-pressure systems are acceptable for interior discharge as long as the design complies with IPC applicable to the year and project location.

Section 1006.6 in IMC 2018 has evolved to include 13 specific line items and dedicated to temperature or pressure relief valves and their pipe discharge. A few are worth mentioning:

• The discharge piping of a safety relief valve should be independently piped to the exterior of the structure. Therefore, no other system relief may be combined with a safety relief discharge pipe.
• The discharge location should be visible to the occupants. Keep in mind that in the IMC 2024 edition, an addition to this point pertains to adding leak detection to the discharge system if it is not visible to the occupants.
• For the discharge piping materials under the IMC 2018 edition, the recommendation is to follow the IPC under Section 605.4. However, in the 2024 edition the requirement for piping material is indicated in Section 1202.

IMC Chapter 11 refrigeration updates

Finally, the last portion of the code evolution to review is a hot topic: Chapter 11 focuses on refrigeration. This chapter includes any equipment that uses refrigerant as a means of providing comfort for occupants in a building. The design industry has seen a vast evolution that began U.S. Green Building Council certifications.

However, we have seen codes evolve to include environmental perspectives to become more stringent toward protecting the environment. In particular, the International Energy Conservation Code and ASHRAE Standard 189.1: Standard for the Design of High-Performance Green Buildings. For this reason and the impact on equipment pertaining to refrigerants, this chapter is worth noting.

Section 1101.1.1 Refrigerants other than ammonia is an update on the IMC 2024 edition to provide compliance not only with the requirements of IMC but also the IFC and ASHRAE Standard 15: Safety Standard for Refrigeration Systems.

Section 1101.1.2 ammonia refrigerant is an update on the IMC 2024 edition specifically for systems using these types of refrigerants. This states their need to comply with the various ANSI/ASHRAE IIAR 1 through 4 for the different systems and their components. These are separate standards with which the reader should be familiar.

A subject worth further discussing is the transition from previous refrigerants to new ones, as many facilities have older pieces of equipment that may still be under R-134A or older. A phasing process is required so that facilities and the industry embraces the requirement with less cost burden. Moving forward, a heavier weight should be on careful planning and due diligence of the design engineer when working with facilities and their systems that use older refrigerants.

The IMC has evolved since it first publication. Codes that began to protect the building or structure have moved beyond to include public safety and occupant health. Additionally, the format to indicate code changes from one edition to another has also evolved. The IMC 2024 edition uses a bar-code system that takes the reader immediately to the changes.

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