Data centre cooling basics: the ASHRAE ranges
ASHRAE's thermal guidelines set the temperature and humidity that IT equipment should breathe at its air inlet. The recommended envelope for classes A1 to A4 is 18 to 27 °C, with moisture between a dew point of -9 °C and 15 °C and a relative humidity cap [1]. Wider allowable ranges exist for each class, but running inside the recommended envelope is what keeps reliability predictable. The UPS battery room needs its own, cooler target, because battery life falls fast with heat.
Key facts
- Recommended (A1 to A4): 18 to 27 °C, -9 °C to 15 °C dew point and 70% RH or 50% RH, depending on the class notes[1]
- Allowable A1: 15 to 32 °C, from -12 °C dew point and 8% RH to 17 °C dew point and 80% RH[2]
- Where to measure: at the IT equipment's air inlet, not the room return or the thermostat
- Battery room: VRLA rated for 5 to 35 °C with an average below 25 °C (CSB)[3]
Measure at the inlet
The ASHRAE ranges describe the air entering the equipment. A room thermostat on a wall, or the return air temperature at a cooling unit, can read several degrees different from the inlet of a server three racks away. Inlet sensors at the top, middle and bottom of a few representative racks give the real picture, and they are the numbers to compare with the envelope.
Recommended versus allowable
The recommended envelope is where ASHRAE expects equipment to run reliably over its life. The allowable ranges are wider, and equipment can operate there, but running hotter or drier for long periods trades reliability or energy against each other in ways the guideline discusses in detail. For most enterprise rooms, the practical target is the middle of the recommended envelope with alarms set before its edges.
Humidity is a dew point problem
ASHRAE frames moisture mainly as dew point, with relative humidity as an upper cap. Too dry raises the risk of static discharge; too humid raises the risk of condensation and corrosion, especially where particulate contamination is present (see contamination).
The UPS and battery room
Batteries are less forgiving than servers. VRLA life halves with every 8 °C rise, by APC’s rule of thumb [4], and CSB rates its VRLA blocks for an average below 25 °C [3]. A battery cabinet that shares a room run at the warm end of the IT envelope will age well ahead of schedule. Where possible, give the battery room its own cooling and its own temperature alarm.
Standards cited
- ASHRAE Thermal Guidelines for Data Processing Environments (2021 reference card)
Common questions
What temperature should a data centre be?
At the IT equipment inlet, ASHRAE recommends 18 to 27 °C for classes A1 to A4 [1].
What humidity should a server room have?
ASHRAE's recommended envelope runs from a -9 °C to a 15 °C dew point, with a relative humidity cap of 70% or 50% depending on the class notes [1].
Where should data centre temperature be measured?
At the air inlet of the IT equipment, at several heights on representative racks, rather than at the room thermostat or the cooling unit's return.
Why does the UPS battery room need its own cooling?
Because battery life falls fast with heat: APC's rule of thumb is that every 8 °C rise halves VRLA life [4].
Sources
- ASHRAE TC 9.9, 2021 Equipment Thermal Guidelines for Data Processing Environments, reference card, Table 2.1, recommended
- ASHRAE TC 9.9, 2021 Equipment Thermal Guidelines for Data Processing Environments, reference card, Table 2.1, allowable A1
- CSB Energy Technology, VRLA Battery User Manual (Operation and Handling, Rev Dec 2024), operating temperature
- Schneider Electric, What is the expected life of my APC UPS VRLA battery? (FA158934), FAQ answer
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