If you've ever designed a mechanical system in California and then taken a project in Virginia, you've felt the difference. The Western US largely adopts the Uniform Mechanical Code (UMC), published by IAPMO. Most of the rest of the country uses the International Mechanical Code (IMC), published by ICC. Both govern the same discipline, but they diverge in ways that catch engineers who don't regularly cross the boundary.
Who uses which code
The split is largely geographic. California, Oregon, Washington, Nevada, Hawaii, and several other Western states adopt the UMC (often with state amendments). The majority of Eastern, Southern, and Midwestern states adopt the IMC as part of the IBC family of codes. A few states and jurisdictions adopt neither or use their own mechanical code.
Where they agree
On the fundamentals, the codes are more alike than different. Both require:
Minimum clearances around mechanical equipment for service access. Proper venting of fuel-burning appliances. Condensate drainage from cooling equipment. Protection of ductwork penetrating fire-rated assemblies. Combustion air provisions for appliances in enclosed spaces. Refrigerant safety compliance.
A mechanical system designed to good engineering practice will generally meet the intent of both codes. The differences tend to show up in the specifics: prescriptive requirements, calculation methods, and referenced standards.
Key differences
| Topic | IMC | UMC |
|---|---|---|
| Ventilation standard | References ASHRAE 62.1 and 62.2 | References ASHRAE 62.1/62.2 but also has its own Chapter 4 ventilation provisions |
| Duct sizing method | Defers to ASHRAE and SMACNA | Chapter 6 provides prescriptive duct sizing tables |
| Appliance installation | References manufacturer instructions + code minimums | More prescriptive clearance tables in Chapter 9 |
| Fuel gas | Separate code (IFGC) | Integrated in UMC Chapter 13 |
| Equipment access | §306.5: min clearances for service | §304: similar requirements, different section structure |
| Hydronic piping | Chapter 12: basic requirements | Chapter 12: more detailed prescriptive requirements |
| Referenced standards | Aligns with ICC family (IBC, IPC, IFGC) | Aligns with IAPMO family (UPC, USEC) |
Fuel gas: the biggest structural difference
The most significant difference between the two codes is how they handle fuel gas. The IMC does not cover fuel gas piping or appliance installation. That's handled by the separate International Fuel Gas Code (IFGC). The UMC integrates fuel gas provisions directly into the mechanical code in Chapter 13.
This affects drawing review because the code references change. A gas piping note that cites "UMC Chapter 13" will need to be updated to cite the IFGC when the project is in an IMC jurisdiction. It's not a design change, since the technical requirements are similar, but the documentation needs to match the adopted code.
Ventilation: similar intent, different paths
Both codes ultimately point to ASHRAE 62.1 for commercial ventilation rates. But the UMC also has its own Chapter 4 ventilation tables that provide prescriptive outdoor air rates for common occupancy types. In practice, most engineers in UMC jurisdictions still use the ASHRAE 62.1 ventilation rate procedure. The prescriptive tables are a fallback for simpler projects.
The IMC takes a cleaner approach: Section 401.2 simply requires compliance with ASHRAE 62.1 for most occupancies, with the IMC providing exhaust rates for specific space types in Section 502.
Duct construction and sizing
The IMC largely defers to SMACNA standards for duct construction and to ASHRAE for duct sizing methods. The UMC includes prescriptive duct sizing tables in Chapter 6 that can be used in lieu of engineering calculations for simpler systems.
Referenced standard families
The codes exist within different families of standards, and this affects cross-references. The IMC references the IPC for plumbing, the IFGC for gas, and the IBC for building construction requirements. The UMC references the UPC (Uniform Plumbing Code), keeps gas within its own scope, and references the IBC or state building code for construction.
This matters for coordination. If your plumbing engineer is working to the IPC and your mechanical engineer is working to the UMC, the cross-references won't align neatly. Most jurisdictions adopt the codes as a family, either the I-codes or the Uniform codes, but not always.
Practical advice for cross-jurisdiction work
The good news: the engineering is the same. A properly designed HVAC system works the same way whether you're citing IMC or UMC. The difference is documentation: which code sections you reference, which companion codes you pull in, and which prescriptive provisions you can lean on. For engineers who work nationally, building a reference map of these differences saves time on every new jurisdiction.
Callout supports both the IMC (2024 and 2021 editions) and the UMC (2021 edition) for mechanical drawing review. Select the applicable code for your jurisdiction when setting up a review.