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Glossary
Data centers

What is ASHRAE TC 9.9 thermal guidelines?

Definition

The ASHRAE TC 9.9 Thermal Guidelines for Data Processing Environments define the temperature and humidity envelopes for air entering IT equipment, grouped into equipment classes.

Who uses it

Data center designers, operators and IT equipment manufacturers. ASHRAE Technical Committee 9.9 publishes the guidelines, and the 90.4-2022 energy standard allows compliance while following the fifth edition.

Envelopes in the fifth edition (2021)

From the ASHRAE TC 9.9 reference card, dry-bulb inlet temperatures:

  • Recommended for classes A1 to A4: 18 to 27 °C
  • Allowable: A1 15 to 32 °C, A2 10 to 35 °C, A3 5 to 40 °C, A4 5 to 45 °C
  • Class H1 for high-density air-cooled equipment: recommended 18 to 22 °C, allowable 15 to 25 °C
  • Temperature change for most equipment: no more than 20 °C in an hour and 5 °C in any 15 minutes

Inlet conditions as a time series

Raising setpoints saves cooling energy only while every inlet stays in range. Aqueduct logs temperature and humidity per zone with minimum, maximum and average bands, and shows supply air temperature from integrated cooling units beside them, so an excursion and the hour it began are on record.

Questions

ASHRAE TC 9.9 thermal guidelines: common questions

The ASHRAE TC 9.9 thermal guidelines are the reference most data centers use to decide how warm and humid server inlet air can be. They define recommended and allowable ranges by equipment class, which lets operators run warmer and save cooling energy with known risk. See ASHRAE 90.4.

No. They are ASHRAE guidance published as a book, binding only where a contract, code or standard references them. Many colocation contracts write the recommended range into service level terms, which gives them force between provider and customer.

The IT equipment manufacturer. ASHRAE leaves it to the maker to judge whether a product needs the high-density class, and operators then design cooling for the most demanding class present in each room.

At server inlets across the room, logged continuously so excursions and their duration are visible. Aqueduct records zone temperature and humidity with minimum and maximum bands and alerts on breaches, which lets operators raise setpoints with evidence, as covered under HVAC energy optimization.

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