ASHRAE Thermal Classes

<p>ASHRAE Technical Committee 9.9 publishes recommended and allowable inlet air temperature and humidity ranges for IT equipment, defining five classes. Class A1 (the most restrictive) allows 15–32°C inlet at low humidity — typical of enterprise data centres. Class A2 extends to 35°C, A3 to 40°C and A4 to 45°C, covering progressively harsher environments.</p><p>Higher ASHRAE class ratings allow operators to raise supply air temperature and extend economiser hours, reducing PUE. Most enterprise and hyperscale equipment is rated A1/A2. Ruggedised and industrial equipment may be rated A3/A4. Understanding the ASHRAE class of installed equipment is essential for designing containment, setting CRAC set points and calculating free cooling hours. Operating above the rated inlet temperature can trigger throttling, cause premature failure and void warranties.</p>