ASHRAE Technical Committee 9.9, Mission Critical Facilities, Data Centers, Technology Spaces and Electronic Equipment, publishes the guidelines most of the industry uses to set temperature and humidity conditions in data centers. The core document, Thermal Guidelines for Data Processing Environments, is developed with input from IT equipment manufacturers and defines the conditions at which servers, storage and networking equipment can operate reliably.
The guidelines have reshaped data center design over the past two decades. Earlier practice was to keep rooms cold, often around 20°C (68°F) or lower with tight humidity control. Successive editions widened the recommended ranges, allowing warmer air, less humidity control and far more economizer hours. Understanding what the guidelines actually say, and what they do not, is the starting point for efficient cooling.
Where conditions are measured
The guidelines apply to the air entering the IT equipment, measured at the server inlet. They do not refer to return air temperature, room average temperature or the temperature at the cooling unit. A room can have a comfortable average temperature while individual servers draw hot recirculated air, or a warm hot aisle while every inlet is within range.
This is why containment and airflow management matter so much: they make the supply temperature at the cooling unit a reliable proxy for inlet temperature. Sensors for control and monitoring should be at rack inlets, typically at several heights.
Recommended versus allowable
The guidelines define two kinds of envelope.
- Recommended envelope: the range in which equipment is expected to operate with high reliability and reasonable energy efficiency over the long term. Facilities should aim to operate here most of the time.
- Allowable envelope: the wider range in which manufacturers have tested equipment to function. Operating here for limited periods is acceptable, but extended operation may affect reliability, fan energy and acoustic noise within the server.
Operating in the allowable range for part of the year can be a deliberate strategy to extend economizer hours, accepting a small, quantified change in failure rates. The guidelines include data on relative failure rates at different inlet temperatures to support that decision.
The recommended envelope
For the main air-cooled equipment classes, the widely referenced recommended envelope (fifth edition, 2021) is:
| Parameter | Recommended range |
|---|---|
| Dry-bulb temperature at inlet | 18 to 27°C (64.4 to 80.6°F) |
| Low moisture limit | minus 9°C (15.8°F) dew point |
| High moisture limit | 15°C (59°F) dew point and 70 percent relative humidity |
The upper relative humidity limit is lower, 50 percent, where gaseous contaminant levels are elevated, because humidity accelerates corrosion when pollutants such as sulphur compounds are present. The guidelines describe how to assess contamination using reactivity coupons.
Equipment classes
Air-cooled equipment is grouped into classes with different allowable ranges. Manufacturers state the class their equipment meets.
| Class | Allowable dry-bulb range | Typical equipment |
|---|---|---|
| A1 | 15 to 32°C (59 to 89.6°F) | Enterprise servers and storage in tightly controlled facilities |
| A2 | 10 to 35°C (50 to 95°F) | Most volume servers, storage and network equipment |
| A3 | 5 to 40°C (41 to 104°F) | Equipment specified for wider ranges |
| A4 | 5 to 45°C (41 to 113°F) | Equipment specified for the widest ranges |
| H1 | 5 to 25°C (41 to 77°F), recommended 18 to 22°C (64.4 to 71.6°F) | High-density air-cooled equipment with tightly integrated components |
Each class also has allowable humidity limits, with maximum dew points that rise from A1 to A4. Class H1 was added in the fifth edition to recognize high-density air-cooled equipment, such as some high-performance processors and accelerators, that need cooler inlet air to stay within component limits.
The practical implication is that the facility must be designed for the most restrictive equipment it will house. A facility built for A2 equipment that later receives H1 hardware may need a lower supply temperature in some zones.
Altitude derating
Air density falls with altitude, reducing the cooling capacity of each unit of air moved through a server. The guidelines reduce the maximum allowable dry-bulb temperature above 900 m (about 3,000 ft) of elevation, at a rate of 1°C for every 300 m. Calgary, at over 1,000 m, is just above that threshold. See air density, altitude and HVAC design for broader effects on HVAC equipment.
Rate of temperature change
Rapid temperature changes can stress equipment, particularly tape storage. The guidelines limit the rate of change of inlet temperature, with a tighter limit for tape. This matters during economizer changeover, chiller restarts and recovery from cooling failures, so control sequences should ramp setpoints rather than step them.
Humidity and static electricity
Older practice held relative humidity between about 40 and 55 percent, partly to limit electrostatic discharge. Research sponsored by ASHRAE showed that with proper grounding and ESD precautions, much lower humidity is acceptable, which is why the low limit is now a minus 9°C dew point. This has allowed many facilities in dry climates like Alberta to eliminate or greatly reduce humidification, saving energy and water. See humidity control and static electricity in data centers.
Liquid cooling classes
ASHRAE TC 9.9 also publishes guidance for liquid-cooled equipment, with classes defined by the facility water supply temperature, such as W17, W27, W32, W40, W45 and W+. Higher classes allow warmer water, which reduces or eliminates chiller operation and improves heat reuse potential.
Using the guidelines to save energy
- Confirm the equipment classes of all IT hardware, now and planned.
- Implement containment so supply temperature reliably represents inlet temperature.
- Raise supply air temperature toward the upper part of the recommended range in stages, monitoring inlet temperatures and server fan behaviour.
- Raise chilled water temperature to match, increasing economizer hours. See free cooling and economizers for data centers in Canada.
- Widen humidity control deadbands to the recommended limits.
- Decide, with the IT owner, whether brief excursions into the allowable range are acceptable during extreme weather.
Very high inlet temperatures can increase server fan power, which may offset some facility savings. The optimum is usually found by monitoring total facility and IT power together.
Frequently asked questions
Is the recommended range a requirement?
No. The guidelines are not a code. They represent industry consensus on reliable operating conditions. Service level agreements and equipment warranties may reference them, so check contracts.
Why do some operators still run cold?
Habit, concerns about hot spots without containment, and conservative service agreements. With good airflow management, warmer operation within the recommended range is well proven.
Do the guidelines apply to small server rooms?
Yes. The same equipment limits apply, although small rooms often lack the airflow management that allows warmer operation.
Key takeaways
- The guidelines apply to air entering the IT equipment, not room or return air.
- The recommended envelope is 18 to 27°C inlet with defined dew point and humidity limits.
- Classes A1 to A4 define progressively wider allowable ranges; H1 covers high-density equipment needing cooler air.
- Allowable temperatures are reduced above 900 m elevation.
- Containment plus warmer setpoints within the recommended range unlock large energy savings.
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