ASHRAE TC 9.9 Set-Point Recommendations: Temperature and Humidity Targets for Australian Data Centres

ASHRAE Technical Committee 9.9 publishes the thermal and environmental guidelines that underpin data centre cooling design worldwide. The fourth edition of *Thermal Guidelines for Data Processing Environments* expanded the recommended supply air temperature range for Class A1 equipment to 18°C–27°C, a shift that carries direct consequences for how facilities set their CRAC unit discharge targets. For operators still running at 20°C supply air, there is measurable energy on the table.

The Four Equipment Classes

ASHRAE TC 9.9 classifies IT equipment into four classes based on the environmental conditions the hardware is designed to tolerate. The classes are not interchangeable. A data centre running a mix of Class A1 servers and Class A4 edge nodes cannot apply a single set-point to both without checking manufacturer specifications.

Class A1

Class A1 covers enterprise servers, storage arrays, and network equipment typically found in purpose-built, controlled data centres. The recommended supply air range is 18°C to 27°C, with a dew point range of 5.5°C to 15°C and a maximum relative humidity of 60%. The allowable range extends to 15°C–32°C, with humidity up to 80% RH at the high end. A1 equipment is designed for stable, well-managed environments. Most Australian co-location and enterprise data centres operate A1 fleets.

Class A2

Class A2 applies to equipment intended for less tightly controlled spaces, including some telecommunications gear and industrial-grade servers. The recommended range is 10°C–35°C, with an allowable upper limit of 40°C. Humidity allowances are broader, permitting up to 80% RH within the allowable band. Operators running A2 equipment have more thermal headroom but should not treat the allowable range as a routine operating target.

Class A3

Class A3 equipment tolerates wider swings still, with a recommended range of 5°C–40°C and an allowable ceiling of 45°C. This class is relevant to ruggedised or industrial computing deployed in environments where precision cooling is impractical. Few Australian enterprise data centres house A3 equipment as their primary load.

Class A4

Class A4 represents the broadest tolerance, with a recommended range of 5°C–45°C and an allowable upper limit of 45°C. This class was introduced to accommodate high-temperature computing deployments and certain edge scenarios. Humidity allowances extend to 90% RH in some conditions within the allowable band.

Recommended vs. Allowable: A Distinction That Matters

The difference between ASHRAE's recommended and allowable ranges is not semantic. The recommended range is where equipment manufacturers design for reliable, long-term operation. Staying within the recommended envelope preserves warranty coverage, minimises thermal stress on components, and keeps failure rates within the bounds that hardware vendors publish in their reliability data.

The allowable range defines conditions the equipment can survive for limited periods without immediate damage. Operating in the allowable zone is not a normal operating mode. It is a buffer for transient events: a cooling unit fault, a hot aisle containment breach, or an unexpected load spike. Treating the allowable range as an extended operating target accelerates component degradation and may void manufacturer warranties.

For Australian operators, this distinction has practical consequences. A facility running supply air at 26°C for Class A1 equipment is within the recommended range. A facility running at 31°C because someone read the allowable figure and treated it as a target is operating outside the design intent of the equipment.

The Energy Case for Raising Supply Air Temperature

CRAC and CRAH unit efficiency is directly tied to supply air temperature. The relationship between supply air set-point and cooling energy consumption is well established: every 1°C rise in supply air temperature at the cooling unit reduces compressor energy by approximately 2–4%, depending on unit type, ambient conditions, and refrigerant circuit design.

Raising supply air from 20°C to 24°C, a shift that remains well within the A1 recommended range, produces a 4°C increase. Applying a conservative 2% per degree figure yields an 8% reduction in compressor energy. At the higher end of published estimates, the saving approaches 16%. For a facility with 500 kW of installed cooling capacity running at 70% average load, an 8% compressor energy reduction equates to roughly 245 MWh per year. At an Australian commercial electricity rate of $0.22/kWh, that is approximately $54,000 annually from a set-point adjustment alone.

The secondary benefit is chiller plant efficiency. For facilities using chilled water CRAH units, a higher supply air set-point permits a higher chilled water supply temperature. Raising chilled water supply from 7°C to 12°C can improve chiller COP by 15–20%, with additional free-cooling hours available in Sydney, Melbourne, and Brisbane climates where ambient wet-bulb temperatures support economiser operation at higher chilled water temperatures.

DX-based CRAC units benefit similarly. A higher evaporating temperature reduces compressor pressure ratio, lowering power draw and extending compressor service life. Liebert, Stulz, and Uniflair units all respond to evaporating temperature changes in predictable ways documented in their performance data sheets.

Why Many Facilities Still Run at 20°C

Despite the published guidance and the energy arithmetic, a large proportion of Australian data centres continue to target 20°C supply air. Several factors drive this conservatism.

First, legacy set-points. Many facilities were commissioned in the early 2000s when ASHRAE guidelines were narrower and operators defaulted to conservative targets that were never revisited. The set-point became institutional knowledge, detached from its original rationale.

Second, mixed equipment fleets. A data centre running hardware from multiple generations and vendors may have equipment with varying thermal tolerances. Without a current audit of equipment classes, operators default to the most conservative figure.

Third, airflow problems. Hot spots caused by poor containment, underfloor bypass, or cable obstruction create localised high-temperature zones. Operators compensate by dropping supply air temperature rather than fixing the airflow issue. This is an expensive workaround: the thermal problem remains, and the energy penalty is permanent.

A thermal audit using computational fluid dynamics modelling or physical airflow measurement can identify whether a facility's hot spots are genuine load issues or containment failures. In most cases, fixing containment and adjusting set-points delivers better outcomes than chasing cold air.

Humidity Control: Dew Point Over Relative Humidity

ASHRAE TC 9.9 shifted its humidity specification from relative humidity to dew point in recognition of a fundamental problem with RH-based control. Relative humidity is temperature-dependent. A space maintained at 45% RH at 20°C has a dew point of approximately 8°C. The same dew point at 27°C corresponds to roughly 28% RH. If a facility raises supply air temperature without adjusting its humidity control strategy, the RH at the equipment inlet will drop, potentially into the range where electrostatic discharge becomes a risk.

The ASHRAE recommended dew point range for Class A1 is 5.5°C to 15°C. The lower limit addresses electrostatic discharge risk; the upper limit guards against condensation on cold surfaces.

For Australian facilities, the practical humidity challenge varies by season and location. Brisbane's subtropical climate creates high ambient humidity loads in summer, driving moisture into the space through infiltration and fresh air intake. Melbourne's cooler winters push humidity down, requiring active humidification to stay above the 5.5°C dew point floor. Sydney sits between the two, with seasonal variation that demands a control strategy capable of both humidification and dehumidification.

Best practice for humidity control:

• Control to dew point, not relative humidity, wherever the BMS and CRAC controls support it
• Set the dew point target at 9°C–12°C for A1 environments, providing margin from both limits
• Calibrate humidity sensors at least annually; capacitive sensors drift and will produce false readings that drive unnecessary humidifier or dehumidifier operation
• For electrode humidifiers, schedule cylinder replacement based on conductivity trends rather than fixed intervals; Brisbane's water chemistry differs from Melbourne's and affects cylinder life materially
• Maintain a minimum 200mm separation between humidifier steam dispersion lances and the nearest cold surface to prevent condensation and microbial growth, consistent with AS/NZS 3666 requirements

Applying Set-Point Changes in Practice

Raising supply air temperature is not a single adjustment. It requires a structured process.

Start with an equipment audit. Confirm the ASHRAE class of every device in the space. Check manufacturer thermal specifications, not just the ASHRAE class label. Some vendors publish tighter requirements than the class default.

Next, address containment. Hot aisle/cold aisle separation, blanking panels in unused rack units, and underfloor grille placement all affect whether a higher supply air temperature reaches equipment inlets within the recommended range. A 24°C supply air temperature at the CRAC discharge can arrive at a poorly contained rack at 32°C or higher.

Then raise the set-point incrementally. A 1°C step every two to four weeks, with inlet temperature monitoring at representative rack positions, allows the facility to identify problems before they become failures. Data centre infrastructure management (DCIM) platforms or standalone temperature logging at rack inlets provide the measurement base needed to validate each step.

Document the final set-point and the evidence base for it. This protects the facility in the event of a hardware failure and provides a baseline for future reviews as the equipment fleet changes.

Set-Point Review as a Maintenance Activity

ASHRAE set-point compliance is not a one-time commissioning task. Equipment fleets change, loads increase, and cooling infrastructure ages. A CRAC unit running with a partially blocked evaporator coil or a low refrigerant charge will not deliver the same supply air temperature at the same energy input as a well-maintained unit. The set-point the facility targets and the temperature the equipment actually receives can diverge without anyone noticing until a thermal event occurs.

Incorporating set-point verification into scheduled CRAC maintenance, alongside coil cleaning, refrigerant charge checks, and sensor calibration, keeps the thermal envelope accurate and the energy savings real.

For facilities across Brisbane, Sydney, and Melbourne looking to review their thermal set-points or assess whether their current cooling configuration supports a higher supply air target, CRAC Services Australia provides thermal assessments and CRAC maintenance programmes aligned with ASHRAE TC 9.9 guidelines. Details are available at [https://crac.services](https://crac.services).