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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsFor most air-cooled IT equipment in ASHRAE classes A1 through A4, target an equipment-inlet dry-bulb temperature of 18–27°C. For qualifying high-density, air-cooled H1 equipment, the recommended range is 18–22°C. Humidity cannot be set safely from relative humidity (RH) alone: use the ASHRAE dew-point envelope together with its RH limit, maintain noncondensing conditions, and verify whether corrosion testing permits the higher 70% RH ceiling. These are equipment-inlet conditions, not a universal thermostat setting for an entire room. Confirm every installed device’s environmental class and manufacturer limits before changing controls.
The two ASHRAE envelopes: recommended and allowable
ASHRAE’s Thermal Guidelines distinguish between a normal operating target and a wider functional boundary. The recommended envelope is the range operators should normally maintain to balance reliability, equipment life and energy use. The allowable envelope is where manufacturers test equipment to verify that it will function. Allowable conditions can support carefully managed excursions, such as economizer or free-cooling operation, but they are not the default setpoints.
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ASHRAE warns that prolonged operation outside the recommended range, particularly near the extremes of the allowable environment, can reduce equipment reliability and longevity. Extended operation outside the allowable range creates a risk of malfunction or catastrophic failure. A control system should therefore treat an allowable limit as an alarm boundary or temporary operating option, not as the temperature or humidity target.
Why the measurement point matters
The published temperature applies at the IT equipment inlet. A return-air sensor, a thermostat on a wall, or a single reading in the middle of a cold aisle may not represent what servers, storage systems or networking equipment actually receive. Temperature can vary substantially between racks and between the top and bottom of a rack. Use measurements from representative equipment inlets when deciding whether the facility is inside the envelope.
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Recommended and allowable temperature ranges by equipment class
The following values are from ASHRAE’s 2021 Thermal Guidelines, reproduced in the 2023 ASHRAE Handbook chapter. They describe air-cooled equipment classes A1–A4 and the high-density H1 class.
| Equipment class | Recommended dry-bulb temperature at inlet | Allowable dry-bulb range | How to use the figure |
|---|---|---|---|
| A1 | 18–27°C | 15–32°C | Keep normal operation in the recommended band; treat 15–32°C as a tested functional boundary, not a target. |
| A2 | 18–27°C | 10–35°C | The wider allowable range may support controlled excursions when equipment and operations are prepared. |
| A3 | 18–27°C | 5–40°C | Verify that every device is actually rated A3 before relying on this boundary. |
| A4 | 18–27°C | 5–45°C | The broadest listed A-class boundary; it still does not replace the 18–27°C recommended objective. |
| H1 | 18–22°C | 15–25°C | For qualifying high-density, air-cooled equipment; use the tighter recommended band. |
Do not infer an installation’s class from the rack’s workload or nameplate power alone. The class is an environmental qualification documented by the equipment supplier. Mixed environments should be controlled for the least tolerant equipment unless physical separation and independent control make different conditions practical.
How to set humidity: dew point, RH and corrosion evidence
ASHRAE expresses the recommended moisture envelope as a dew-point range together with an RH ceiling. For A1–A4 and H1, the published recommendation is −9°C to 15°C dew point and either 70% RH or 50% RH, depending on corrosion-coupon results. The condition must remain noncondensing. A single RH number without its associated temperature and dew point can be misleading because RH changes as air temperature changes.
The RH ceiling is conditional
ASHRAE permits operation up to 70% RH when corrosion-coupon testing shows sufficiently low corrosion in the facility environment: less than 200 Å/month for silver and less than 300 Å/month for copper. These are published thresholds, not an assumption that every data center has achieved them. If corrosion exceeds either threshold, the RH limit should be 50% or lower.
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Pollutants, airborne contaminants and moisture can interact to accelerate corrosion even when a room appears clean. A facility that has not performed representative coupon testing should not claim the 70% allowance merely because its sensors report a low average RH. Coordinate the decision with the environmental monitoring and contamination-control program, and document the test period, coupon locations and results.
Prevent condensation rather than chasing an RH number
Condensation occurs when a surface is at or below the air’s dew point. Keep supply-air, rack-inlet and equipment-surface conditions noncondensing, including during startup, shutdown and rapid changes in cooling output. Dew-point alarms are useful because RH alarms alone may not reveal that a cold surface is approaching saturation. Place sensors where a condensation event would first threaten equipment, not only where the room average is easiest to measure.
A practical procedure for choosing control limits
- Inventory environmental classes. Record the ASHRAE class and the manufacturer’s temperature and moisture requirements for every server, storage array, switch and other air-cooled device. Flag equipment that is liquid-cooled or has a different qualification; the A1–A4 and H1 values do not automatically apply.
- Map equipment-inlet conditions. Install or validate calibrated temperature and humidity probes at representative rack inlets. Sample high, middle and low rack positions and locations near doors, perimeter walls, containment openings and cooling-unit discharge paths. A single room sensor is not sufficient evidence of uniform inlet conditions.
- Choose the recommended target. For a conventional A-class population, design normal control around 18–27°C at the inlet. For qualifying H1 equipment, use 18–22°C. Set deadbands and staging logic so ordinary control movement does not repeatedly drive inlets toward an allowable edge.
- Establish the moisture envelope. Configure dew-point monitoring for −9°C to 15°C and an RH ceiling of 50% unless documented corrosion results support 70% RH. Make the logic noncondensing: a dew-point or surface-temperature alarm should override an RH-only rule.
- Define alarm levels separately. Create early-warning alarms inside the recommended range, action alarms near the selected operating limits, and emergency procedures for allowable-boundary or noncondensing violations. The exact alarm delays should reflect the thermal mass and response time of the facility rather than a generic timer.
- Check airflow and load interactions. Confirm that containment, blanking panels, perforated tiles, fans and cooling-unit controls deliver the measured inlet conditions. Raising a room setpoint does not improve efficiency if bypass air or recirculation leaves rack inlets hot.
- Validate under worst credible conditions. Review hot-day, economizer-transition, maintenance and restart scenarios. Test that alarms, redundant cooling and escalation procedures operate before an inlet crosses an allowable limit.
- Document the decision. Store the equipment-class inventory, sensor calibration records, coupon evidence, chosen limits, alarm delays and approval authority. Revisit the settings whenever equipment, rack density, airflow layout or cooling mode changes.
Temperature strategy: efficiency without sacrificing reliability
Operating closer to the upper end of the recommended band can reduce mechanical cooling demand in some designs and may increase the hours available for outside-air or water-side economization. ASHRAE notes that allowable conditions can enable more free-cooling hours. That opportunity does not make an allowable extreme a suitable permanent target, and no single energy-savings percentage applies to every facility.
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- The installed equipment is documented for the selected class and the OEM limits are no tighter.
- Rack-inlet sensors, not only room sensors, show stable conditions across the recommended band.
- Humidity remains inside the dew-point and RH envelope with no condensation risk.
- Cooling redundancy, alarms and incident response can handle a failed unit or abrupt outdoor change.
- Power, airflow and thermal trends are reviewed after the change rather than assuming a room-average improvement.
When not to raise the setpoint
- Any critical device has an unknown class or a stricter manufacturer requirement.
- Top-of-rack or rear-row inlets already approach the recommended maximum.
- High-density H1 equipment is mixed into an A-class area without separate control.
- Corrosion testing is absent or exceeds the coupon thresholds for the 70% RH allowance.
- Cooling controls produce large oscillations, rapid temperature changes or hot spots during transitions.
The objective is a stable inlet environment, not the highest possible room temperature. A warmer but uneven room can be less reliable than a slightly cooler, well-distributed one.
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High-density H1 equipment needs a tighter target
H1 is intended for qualifying high-density, air-cooled equipment. Its recommended dry-bulb range is 18–22°C, compared with 18–27°C for A1–A4, and its allowable range is 15–25°C. The narrower band reflects the thermal sensitivity and concentrated heat load of this class. Do not apply the A3 or A4 allowable maximum simply because a rack has sufficient electrical capacity; electrical density and environmental qualification are separate questions.
In a mixed room, identify whether H1 racks have dedicated airflow paths or cooling zones. If they share air with ordinary servers, control for H1 inlets or provide separation that demonstrably keeps each population within its own documented limits.
Instrumentation and monitoring design
ASHRAE’s fifth-edition scope explicitly includes facility temperature and humidity measurement. Monitoring should reveal the conditions equipment experiences over time, not just display a comfortable room reading.
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- Measure at the front inlet plane of representative racks, including the highest heat-load racks.
- Include vertical coverage so stratification at the top of a rack is visible.
- Measure both sides of containment boundaries and near likely bypass-air paths.
- Add dew-point or surface-temperature visibility where cold pipes, coils or supply-air streams could create condensation.
- Trend readings long enough to capture cooling-mode changes, overnight load variation and maintenance events.
Calibration and data quality
Use instruments selected for the facility’s required accuracy, operating range and integration system. Maintain calibration records and investigate implausible step changes before changing a control limit. Consumer room sensors can be useful for supplemental awareness but should not be treated as a substitute for calibrated operational monitoring at equipment inlets.
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Altitude and rate-of-change limits
Altitude changes the allowable maximum dry-bulb temperature. ASHRAE derates the allowable maximum for operation above 900 m; operation above 3050 m requires consultation with the IT supplier. Apply the derating to the relevant equipment class and document the altitude used in the calculation. Do not copy sea-level allowable maxima into a high-altitude operating procedure.
ASHRAE’s rate-of-change table contains entries written as changes across specified time windows. A notation such as 5/20 must be read according to that table’s units and interval; it is not a simple instruction to change temperature by five degrees every hour. Use the class-specific row, verify the time window, and configure ramp rates or alarm logic to prevent abrupt transitions during economizer switchover, cooling-unit staging and restart.
Operating scenarios and corrective actions
Rack inlets exceed 27°C while the room average is normal
Check airflow first: blocked intakes, missing blanking panels, containment leakage, overloaded perforated tiles and recirculation can create a local hot spot. Verify the inlet probe and compare several nearby racks. Correct the airflow path or load distribution before lowering the whole-room setpoint, then confirm that the affected inlets return to the recommended band.
RH is below the displayed target but dew point is outside the envelope
Use dew point as the controlling moisture measure and investigate sensor placement and calibration. A low RH reading at a warm location does not prove that colder equipment surfaces are safe. Check for local cold spots and keep conditions noncondensing.
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RH rises toward 70% without corrosion evidence
Operate to the 50% RH ceiling until representative silver and copper coupon results demonstrate the published thresholds. Do not infer permission from a short period of clean sensor data.
Conditions move quickly during free cooling
Review the rate-of-change requirement for the equipment class, confirm ramp controls, and verify that dew point and surface temperatures remain safe. If the transition cannot be held inside the recommended envelope, treat the event as a controlled excursion with operator notification rather than silently changing the normal setpoint.
A new device has an unknown environmental class
Keep the area at the existing conservative limits and obtain the supplier’s environmental specification before changing controls. Unknown classification is not evidence that the device supports A3, A4 or H1 conditions.
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Governance for ongoing reliability
Temperature and humidity settings should be part of change management. Require review when rack density increases, containment is modified, a cooling plant is replaced, a new equipment generation is installed or a facility begins using economizer modes. Trend inlet temperature, dew point, RH, alarms and cooling state together so an apparent efficiency gain can be checked against thermal stability. Keep the recommended range as the normal operating objective and reserve allowable conditions for documented, time-bounded situations with an exit plan.
Roger Schmidt, Ph.D., P.E., a member of ASHRAE Technical Committee 9.9, summarized the contaminant-dependent trade-off: “With little pollutants, a more energy efficient and expanded environmental envelope is available.” The qualification is important: the wider envelope depends on environmental conditions and evidence, not on a thermostat preference.
What to put in the control specification
- Equipment classes covered and any OEM-specific restrictions.
- Normal inlet-temperature target: 18–27°C for A1–A4 or 18–22°C for H1.
- Allowable boundaries used for alarms or temporary excursions.
- Dew-point range of −9°C to 15°C and the selected RH ceiling.
- Corrosion-coupon thresholds and the test records supporting a 70% RH allowance, if applicable.
- Sensor locations, calibration interval and data-retention requirements.
- Altitude, rate-of-change and noncondensing requirements.
- Alarm escalation, redundancy response and the procedure for returning from an allowable excursion.
The Bottom Line
Set normal controls from measured equipment-inlet conditions: 18–27°C for A1–A4 air-cooled equipment, 18–22°C for qualifying H1 equipment, and a −9°C to 15°C dew-point envelope with a 50% RH ceiling unless corrosion-coupon evidence supports 70% RH. Use allowable ranges only as managed boundaries, account for altitude and rate-of-change limits, and verify each device’s class and OEM requirements before pursuing warmer operation.
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