ASHRAE Thermal Guidelines for Data Centers Explained
ASHRAE's thermal guidelines say what air conditions IT equipment should see. They set the target the cooling system is designed to meet, and they have shifted over time towards warmer, more efficient operation.
What they define
The guidelines, from ASHRAE Technical Committee 9.9, group IT equipment into classes by how much environmental variation it tolerates. For each class they give a recommended envelope (the normal target) and a wider allowable envelope.
Where it is measured
Conditions are measured at the equipment air inlet, not in the room average. This is why airflow management and containment matter. See hot aisle / cold aisle containment.
What it means for the design
- Warmer supply air allows more hours of free cooling and better chiller efficiency, which helps PUE. See PUE.
- Humidity limits affect static discharge and condensation risk, and so how humidification is designed.
- The guidelines are a floor for the IT equipment; the manufacturer's specification may be stricter.
Overview of the cooling chain: cooling systems explained. Confirm the current edition and class when you design.
Written by the Vision Matrix Institute editorial team. Worked examples use stated, illustrative assumptions; check them against your project data, the applicable standards and manufacturer datasheets before use.
Frequently Asked Questions
What do the ASHRAE thermal guidelines define?
Environmental classes for IT equipment, and for each class a recommended range and a wider allowable range for temperature and humidity at the equipment inlet.
What is the recommended temperature range?
In recent editions the recommended dry-bulb range for the common classes is around 18 to 27 degrees Celsius at the IT equipment inlet, with humidity limits defined separately. Always check the current edition.
Why do the guidelines matter for design?
They set the supply air conditions the cooling must hold, which drives chiller and airflow design, free-cooling hours and PUE.
Is the allowable range an operating target?
No. The allowable range is for limited periods or conditions; the recommended range is the normal target. Equipment manufacturers' specifications also apply.
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This topic is covered in depth in our Electrical Design – Data Center Specialist program.
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