For ASHRAE Classes A1–A4, the recommended equipment inlet temperature is 18–27°C (64.4–80.6°F). Recommended moisture conditions combine a dew point of −9 to 15°C (15.8–59°F) with a relative humidity ceiling of either 50% or 70%, depending on the contamination conditions described below. Class H1 high-density air-cooled equipment has a narrower recommended temperature range of 18–22°C (64.4–71.6°F). These are conditions at the equipment air inlet—not cooling-unit supply temperatures, return temperatures, or room averages.
The recommended envelope is the normal design and operating target. A class’s broader allowable envelope describes conditions under which appropriately classified equipment is expected to function; it does not establish equivalent reliability, unrestricted performance, or suitability for every installed configuration.
Reference Edition and Equipment Class
The tables below are an editorial summary of the air-cooling limits in ASHRAE’s Thermal Guidelines for Data Processing Environments, Fifth Edition, Revised and Expanded, using the official TC 9.9 reference card, Tables 2.1 and 2.2. The reference card identifies these as the 2021 thermal guidelines. ASHRAE’s May 31, 2022 errata states that its corrections were incorporated into the revised and expanded edition.
One correction materially affects this reference: the H1 allowable temperature range is 15–25°C, not 5–25°C. The 2023 ASHRAE Handbook—HVAC Applications, Chapter 20, provides useful operating context, but its displayed H1 row retains the lower value corrected by the errata. Use the corrected reference card for that limit.
Select the class from documentation for the actual equipment model and configuration. A facility cannot make an A1 product suitable for A4 conditions simply by adopting A4 environmental limits. Likewise, H1 is a manufacturer-designated class, not a classification automatically assigned to every high-density rack.
For an existing installation, retain the edition required by the project specification or contract when evaluating that obligation. Identify any comparison with the edition used here separately rather than silently replacing the project’s reference.
Recommended Inlet Temperature and Humidity
All conditions in each row apply together. DP means dew-point temperature; RH means relative humidity. Conditions must be noncondensing.
| Equipment class | Recommended inlet dry-bulb temperature | Recommended dew-point range | Additional RH ceiling |
|---|---|---|---|
| A1, A2, A3, A4 | 18–27°C / 64.4–80.6°F | −9 to 15°C / 15.8–59°F | 50%, or up to 70% where supported by the contamination assessment below |
| H1 | 18–22°C / 64.4–71.6°F | −9 to 15°C / 15.8–59°F | 50%, or up to 70% where supported by the contamination assessment below |
Source: ASHRAE TC 9.9 reference card, Table 2.1 for A1–A4 and Table 2.2 for H1.
When the 70% RH ceiling applies
ASHRAE links the expanded 70% RH ceiling to corrosion testing. Its guidance identifies copper corrosion below 300 Å/month and silver corrosion below 200 Å/month as conditions supporting operation up to 70% RH. Where corrosion exceeds these thresholds, the guidance calls for RH to be reduced to 50% or lower because catalyst-type pollutants may be present.
Do not treat 70% RH as an unconditional default when contamination has not been assessed. For an initial comparison without supporting corrosion data, use the 50% ceiling rather than assuming the expanded envelope applies; the site may require a lower limit based on contamination or equipment requirements.
The 15°C maximum dew point still applies when the 70% ceiling is justified. Raising the permitted RH ceiling does not remove the dew-point boundary.
Allowable Operating Limits by ASHRAE Class
These limits apply to powered-on equipment. The temperature maxima shown are before altitude derating. RH and dew-point limits are simultaneous constraints, not alternatives.
| Class | Allowable inlet dry-bulb temperature | Minimum moisture conditions: meet both | Maximum moisture conditions: meet both |
|---|---|---|---|
| A1 | 15–32°C / 59–89.6°F | DP ≥ −12°C (10.4°F); RH ≥ 8% | DP ≤ 17°C (62.6°F); RH ≤ 80% |
| A2 | 10–35°C / 50–95°F | DP ≥ −12°C (10.4°F); RH ≥ 8% | DP ≤ 21°C (69.8°F); RH ≤ 80% |
| A3 | 5–40°C / 41–104°F | DP ≥ −12°C (10.4°F); RH ≥ 8% | DP ≤ 24°C (75.2°F); RH ≤ 85% |
| A4 | 5–45°C / 41–113°F | DP ≥ −12°C (10.4°F); RH ≥ 8% | DP ≤ 24°C (75.2°F); RH ≤ 90% |
| H1 | 15–25°C / 59–77°F | DP ≥ −12°C (10.4°F); RH ≥ 8% | DP ≤ 17°C (62.6°F); RH ≤ 80% |
| B | 5–35°C / 41–95°F | RH ≥ 8% | DP ≤ 28°C (82.4°F); RH ≤ 80% |
| C | 5–40°C / 41–104°F | RH ≥ 8% | DP ≤ 28°C (82.4°F); RH ≤ 80% |
Source: ASHRAE TC 9.9 reference card, Tables 2.1 and 2.2, including the corrected H1 lower temperature limit. All rows require noncondensing conditions.
Classes B and C address equipment environments such as telecommunications spaces, offices, and weather-protected light-industrial locations. They are not substitute operating envelopes for A-class servers. The reference card does not assign B and C the shared A1–A4 recommended envelope.
The allowable envelope is not exclusively an emergency operating range, but neither is it an automatic continuous-operation target. ASHRAE notes that equipment power and thermal management features may activate within allowable conditions. Check the manufacturer’s supported temperature duration, configuration restrictions, and performance behavior before planning sustained operation outside the recommended envelope.
How to Read the Humidity Limits
A temperature-and-RH rectangle does not fully describe these environmental envelopes.
For the recommended A1–A4 range, an inlet condition must satisfy:
Here, (T_{\mathrm{in}}) is inlet dry-bulb temperature in °C, (T_{\mathrm{dp}}) is dew-point temperature in °C, and (RH_{\mathrm{ceiling}}) is the applicable 50% or 70% limit. For H1, replace the temperature interval with 18–22°C. These expressions restate the recommended boundaries; they do not calculate dew point from RH.
For allowable A-class and H1 conditions, the lower moisture boundary requires both the −12°C minimum dew point and the 8% minimum RH. At the upper boundary, both the class-specific maximum dew point and maximum RH must be met. ASHRAE describes this as using the more restrictive moisture limit at each boundary.
This distinction matters because RH changes with dry-bulb temperature. Dew point is a more consistent indicator of moisture content across a facility where air temperatures vary. A single fixed RH band cannot replace the dew-point limits.
Example: temperature inside the range, moisture outside it
Assume an illustrative A2 equipment-inlet record reports:
- Dry-bulb temperature: 24°C.
- Dew point: 16°C.
- RH: below the applicable recommended ceiling.
The temperature is inside the recommended A1–A4 range, but the dew point exceeds its 15°C ceiling. The condition is therefore outside the recommended envelope despite the acceptable temperature and RH.
It may still be inside the A2 allowable moisture envelope because 16°C is below A2’s 21°C maximum dew point. That judgment also requires the allowable RH limits, altitude-adjusted temperature limit, noncondensing conditions, and the equipment’s own restrictions to be satisfied. This is a boundary-comparison example, not a site measurement or equipment qualification.
Measure the Air Entering the Equipment
The relevant measurement boundary is the air entering the IT equipment. ASHRAE identifies the cool-air inlet and hot-air exhaust as separate equipment interfaces; it also documents equipment with front-to-rear, side-to-side, and other airflow arrangements. A sensor at the nominal “rack front” is only useful if that is where the equipment actually takes in air.
| Measurement location | Appropriate interpretation |
|---|---|
| Actual equipment air intake | Compare against the applicable inlet envelope |
| Cold aisle or rack-front monitoring point | Useful for trending; establish whether it represents the actual intakes |
| Cooling-unit discharge or supply plenum | A cooling-system control condition, not direct evidence of equipment inlet conditions |
| Equipment exhaust, hot aisle, or cooling-unit return | An air leaving/return condition, not an inlet-envelope measurement |
For a practical assessment, record the rack, equipment intake, sensor position, timestamp, dry-bulb temperature, RH, and measured or calculated dew point. Include equipment class, operating state, site elevation, instrument accuracy, and calibration status.
Survey multiple occupied heights and locations rather than relying on one room sensor. Check unusual airflow arrangements and locations exposed to recirculated exhaust. Treat this as a measurement plan to establish representative conditions—not as a claim that a particular sensor count demonstrates ASHRAE conformity.
Evaluate each relevant inlet and time interval. A room average can conceal a local excursion, while an isolated reading cannot establish behavior throughout a load cycle or cooling-system transition. Where a reading is close to a boundary, retain unrounded data and account for sensor uncertainty before making an inside/outside determination.
Altitude, Temperature Changes, and Equipment Restrictions
Derate the maximum allowable temperature at altitude
ASHRAE specifies the following reductions above 900 m elevation. These modify the maximum allowable dry-bulb temperature, not the dew-point limits.
| Class | Reduction above 900 m |
|---|---|
| A1, A2, B, C | 1°C per 300 m |
| A3 | 1°C per 175 m |
| A4 | 1°C per 125 m |
| H1 | 1°C per 500 m |
For a preliminary comparison within the reference card’s altitude range:
-\frac{\max(0,h-900)}{K}
where:
- (T_{\max,h}) is the altitude-adjusted allowable maximum in °C.
- (T_{\max,\mathrm{base}}) is the class maximum from the allowable table in °C.
- (h) is site elevation in meters.
- (K) is the class-specific number of meters per 1°C reduction from the table above.
The reference card lists a maximum elevation of 3,050 m and requires consultation with the IT supplier above that elevation. It also states that, above 2,400 m, the derated allowable temperature takes precedence over the recommended temperature for A1, A2, B, C, and H1. Do not extrapolate the equation above 3,050 m as an equipment approval.
Check temperature variation over the specified windows
For non-tape IT equipment, the reference card specifies no more than 5°C variation within any 15-minute period and 20°C within an hour. Variation means maximum minus minimum inlet temperature within the relevant window—not simply the difference between its first and last samples. These are temperature-change limits over defined windows, not a single constant ramp rate.
Use time-series data with sampling intervals suitable for detecting the changes being assessed. Checking only whether each reading falls inside the static envelope does not evaluate these temporal limits.
Keep special operating conditions separate
Tape products require a more stable, restrictive environment; use the tape system’s specifications rather than the general A-class row alone. Similarly, powered-off installed equipment and equipment in shipping packaging are different operating states. The powered-on table above should not be used to establish storage or transportation limits.
Low-humidity operation also depends on electrostatic-discharge controls. ASHRAE’s low-RH notes include grounded conductive or static-dissipative mobile equipment and grounded wrist straps during hardware maintenance, and identify circumstances in which higher humidity may be needed. An allowable 8% RH boundary is not an instruction to reduce a facility’s humidity to 8%.
Noncondensing air limits do not by themselves show that every surface is protected from condensation. ASHRAE notes, for example, that liquid circulated within a rack must remain above dew point to avoid condensation concerns. Check exposed cold surfaces and equipment transition requirements separately when they are relevant.
Turn the Reference Table into an Operating Decision
Before changing a temperature or humidity target:
- Identify the supported environmental class and restrictions for every affected equipment model and configuration.
- Select the applicable reference edition and calculate any altitude adjustment.
- Establish actual inlet conditions using temperature and moisture data from representative locations and operating periods.
- Classify conditions as inside the recommended envelope, outside recommended but inside documented allowable limits, or outside those allowable limits.
- Set control and alarm thresholds inside the applicable boundaries, allowing for measured spatial variation, control behavior, and instrument uncertainty. Do not assume a universal margin without supporting site data.
For mixed equipment, use the intersection of the supported envelopes unless separate environmental zones are provided. A zone containing H1 equipment cannot adopt the A1–A4 recommended upper temperature of 27°C solely because other servers support it. Likewise, a wider allowable temperature range does not establish that raising the inlet target will reduce total energy: ASHRAE directs users to consider multiple server and facility metrics when evaluating total cost of ownership.
The table supplies environmental reference boundaries. The operating decision still depends on the installed equipment, representative inlet measurements, and the conditions attached to those boundaries.