HVAC Code Updates: IMC 2024 and ASHRAE 90.1 Changes

The Mechanical Code Landscape

HVAC installations in the United States are governed by the International Mechanical Code (IMC) or the Uniform Mechanical Code (UMC), depending on the jurisdiction. In addition, energy efficiency requirements for HVAC systems come from the IECC and ASHRAE Standard 90.1. Understanding all of these codes together is essential for HVAC contractors, engineers, and designers.

IMC 2024 Key Changes

Ventilation and Indoor Air Quality

The 2024 IMC includes several updates to ventilation requirements:

• Updated references to ASHRAE 62.1-2022 and ASHRAE 62.2-2022 for minimum ventilation rates in commercial and residential buildings
• New provisions for demand-controlled ventilation (DCV) that allow systems to adjust outdoor air based on occupancy sensors and CO2 levels, improving energy efficiency while maintaining air quality
• Enhanced requirements for filtration in mechanically ventilated spaces, reflecting lessons learned from the COVID-19 pandemic about the importance of air filtration
• Updated kitchen exhaust provisions for commercial cooking operations, including revised hood airflow rates and makeup air requirements

Refrigerant Changes

The refrigerant landscape is shifting rapidly due to environmental regulations:

• The 2024 IMC incorporates updated provisions from ASHRAE 15 (Safety Standard for Refrigeration Systems) to accommodate newer low-GWP (global warming potential) refrigerants
• A2L refrigerants (mildly flammable) such as R-32 and R-454B are now addressed in the code with specific requirements for detection, ventilation, and charge limits
• The phase-down of high-GWP refrigerants under the AIM Act means the industry is transitioning away from R-410A. The code is adapting to allow the safe use of replacement refrigerants

Duct Insulation and Sealing

• Updated duct insulation requirements align with IECC 2024 energy efficiency mandates
• Duct leakage testing requirements have been tightened, with lower maximum allowable leakage rates for both supply and return ductwork
• New construction duct systems in conditioned spaces must meet Seal Class A or B depending on the static pressure class of the system

ASHRAE 90.1-2022 Energy Efficiency Updates

ASHRAE 90.1 is the baseline energy standard referenced by most commercial building codes. The 2022 edition includes significant HVAC-related changes:

Equipment Efficiency

• Minimum efficiency requirements for unitary air conditioners and heat pumps have increased, now aligned with DOE minimum efficiency standards that took effect in 2023
• Variable refrigerant flow (VRF) systems have updated efficiency requirements and are now more explicitly addressed in the standard
• Boiler and furnace efficiency minimums have increased for both commercial and residential-sized equipment used in commercial buildings

Controls Requirements

• Automatic fault detection and diagnostics (AFDD) are now required for certain HVAC systems to identify performance degradation and maintenance needs
• Enhanced economizer requirements, including lower ambient temperature thresholds for economizer activation in more climate zones
• Setback and optimum start controls are required for all HVAC systems serving spaces that are not continuously occupied
• Simultaneous heating and cooling limitations have been strengthened to prevent energy waste in zone reheat systems

Heat Pump Provisions

Reflecting the broader electrification trend:

• New prescriptive requirements encourage heat pump systems over combustion heating in certain climate zones and building types
• Heat recovery requirements have been expanded, particularly for systems that can simultaneously provide heating and cooling to different zones
• Updated performance rating conditions for cold-climate heat pumps that better reflect real-world heating season performance

Staying Current

The mechanical and energy codes are among the fastest-evolving codes in the construction industry, driven by energy policy, refrigerant regulations, and indoor air quality science. HVAC contractors who stay current with these changes have a significant competitive advantage.