Five Important Changes to the 2021 International Mechanical Code (IMC)

By: Reggie Hucks | Jun 07, 2021

The new 2021 International Mechanical Code (IMC) has been published and is available to adopt. There are 33 significant changes to eight chapters in the Code’s 2021 edition. While the scope of this article does not cover all of the changes, there are five items that should be noted for new construction and renovation. 

Condensate Discharge 

Whenever condensate drains or lines are connected directly to a plumbing system, serious health hazards can occur. For instance, sewer gas could permeate from the drains and fill the entire space of a structure. To alleviate sewer gas build-up and other health-related issues, the IMC has adopted new language regarding the use of condensate lines. In the new language, except where discharging to grade, condensate lines can be located only in floor sinks, floor drains, trench drains, mop sinks, hub drains, standpipes, utility sinks, or laundry sinks. Direct connections to the sewer system are no longer permitted. 

Mechanical Ventilation in Apartments 

Improved language regarding ventilation in apartments (R2 dwellings) can be found in Chapter 4, Ventilation. These dwelling units are required to meet the appropriate air leakage requirements in the International Energy Conservation Code, resulting in minimal air infiltration. Therefore, all dwelling units, such as apartments, must be mechanically ventilated. Openable windows and doors, which typically count as natural ventilation, cannot be the sole source of changing air in the dwelling. 

Manicure and Pedicure Operations 

Point source ventilation with an exhaust inlet within 12 inches of the point of product application is required at manicure and pedicure stations. Under 2021 IMC, the point source exhaust must run continuously when the space is occupied and make up air must be provided for this exhaust. 

Smoker Ovens with Integral Exhaust 

Type 1 grease hoods rated for “extra heavy-duty cooking” are currently required for solid fuel (wood and charcoal) cooking. The 2021 IMC requires a smoker oven to utilize an integral exhaust system that will discharge to the outdoors, instead of a type 1 grease hood and duct. Additionally, the smoker must be listed for use without a type 1 grease hood. 


There are numerous changes to Chapter 11, Refrigeration in the IMC. Here are some of the highlights: 

  • When systems utilize ammonia as a refrigerant, the system must meet the International Institute of Ammonia Refrigeration (IIAR) 2,3,4, and 5 standards instead of complying with the IMC. 
  • New refrigerant flammability classifications have been included in Table 1103.1 (Refrigerant Classification, Amount and OEL), including the A2L (mildly flammable) safety rating classification. 
  • A second means of egress is now required in a machinery room that exceeds 1,000 square feet. The second door must be located a remote distance from the first door and all doors must have panic hardware. 
  • New language for the installation of refrigerant piping has been added and the entire installation section has been rewritten. Details are included about appropriate piping location, prohibited locations, and approved gases for pressure testing.  

While many more changes have been included in the 2021 IMC, ventilation, energy efficient mechanical systems, and new refrigerants are the primary drivers. Since the international codes change every three years, more improvements in energy usage will be expected as we move into the future.

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