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 conditional relative humidity ceiling of 70% or 50%. Manufacturer-designated Class H1 equipment has a lower recommended inlet temperature range of 18–22°C (64.4–71.6°F). These are combined environmental envelopes—not independent temperature and humidity settings.
Check conditions at the equipment’s air inlet, identify the applicable equipment class, and assess temperature, relative humidity, and dew point together. The allowable range is broader than the recommended range, but it describes equipment functionality under specified conditions, not equivalent reliability or an unrestricted normal operating target.
Reference Basis and Applicable Edition
The reference values below use ASHRAE’s Thermal Guidelines for Data Processing Environments, Fifth Edition, Revised and Expanded, specifically the official TC 9.9 reference card bearing ©2021, 2024:
- Table 2.1: air-cooling environmental specifications.
- Table 2.2: high-density air-cooled server specifications.
- Associated notes covering humidity, altitude, temperature changes, and equipment operating state.
Keep the edition with any saved table, operating procedure, or printed reference. ASHRAE’s humidity guidance has changed between editions; an older humidity card should not be combined with newer temperature limits and labeled as one universal ASHRAE envelope. For an existing installation, establish which edition the equipment documentation, project specification, or contract actually adopts before changing operating limits.
Recommended Inlet Conditions
These values apply to powered-on equipment and noncondensing conditions. The relative humidity ceiling does not replace the dew-point ceiling: both must be satisfied.
| Equipment class | Recommended inlet dry-bulb temperature | Recommended dew point | Additional RH ceiling |
|---|---|---|---|
| A1–A4 | 18–27°C / 64.4–80.6°F | −9 to 15°C / 15.8–59°F | 70% only under the corrosion-testing conditions below; otherwise use the lower-humidity guidance |
| H1 | 18–22°C / 64.4–71.6°F | −9 to 15°C / 15.8–59°F | Same corrosion-dependent condition |
Source: ASHRAE TC 9.9 reference card, Tables 2.1 and 2.2. H1 is a separate manufacturer-designated class; a high rack power density alone does not establish that the equipment belongs to H1.
When the 70% RH Ceiling Applies
The expanded 70% RH recommendation depends on corrosion evidence. ASHRAE identifies corrosion rates below:
- 200 Å/month for silver coupons.
- 300 Å/month for copper coupons.
The ASHRAE Handbook explains that facilities using this expanded envelope should test with silver and copper coupons at least twice a year, once in winter and once in summer. Where corrosion exceeds the thresholds, the reference card directs RH toward 50% or lower. Facilities without routine testing should consider the lower-humidity limits rather than assuming that 70% RH is an unconditional recommendation. The 15°C maximum dew point still applies.
A lower RH target is not, by itself, proof that a contaminated environment is suitable. ASHRAE notes that certain pollutant mixtures can produce significant corrosion even at 50% RH.
Allowable Operating Limits by Equipment Class
The following table separates temperature, RH, and dew-point limits so that each can be checked explicitly. All conditions in the selected row apply simultaneously; altitude and equipment-specific restrictions may reduce the usable envelope.
| Class | Inlet dry-bulb temperature, °C | Inlet dry-bulb temperature, °F | Minimum moisture conditions | Maximum RH | Maximum dew point |
|---|---|---|---|---|---|
| A1 | 15–32 | 59–89.6 | Dew point ≥ −12°C and RH ≥ 8% | 80% | 17°C / 62.6°F |
| A2 | 10–35 | 50–95 | Dew point ≥ −12°C and RH ≥ 8% | 80% | 21°C / 69.8°F |
| A3 | 5–40 | 41–104 | Dew point ≥ −12°C and RH ≥ 8% | 85% | 24°C / 75.2°F |
| A4 | 5–45 | 41–113 | Dew point ≥ −12°C and RH ≥ 8% | 90% | 24°C / 75.2°F |
| H1 | 15–25 | 59–77 | Dew point ≥ −12°C and RH ≥ 8% | 80% | 17°C / 62.6°F |
Source: official SI reference card, Tables 2.1 and 2.2. The lower dew-point limit of −12°C equals 10.4°F. These are operating conditions, not shipping or storage limits.
For H1, the official SI reference card specifies a 15°C allowable minimum. The online 2023 ASHRAE Handbook’s I-P table displays a conflicting 41°F minimum. This reference uses the explicitly identified SI reference card rather than combining the two entries. Retain that source identification when transferring H1 values into a project document, and resolve any differing requirement against the applicable equipment documentation.
Select the Class From the Equipment Documentation
ASHRAE describes A1 as typically associated with tightly controlled, mission-critical environments. A2, A3, and A4 provide progressively broader environmental envelopes for equipment designed for those conditions. These descriptions do not assign a class to an installed server, switch, or storage system.
Use the manufacturer’s environmental specification for the actual model and configuration. Record restrictions associated with installed components, supported operating modes, and altitude. In a mixed-equipment zone, establish conditions that satisfy every affected device; an A4-rated server does not extend the operating limits of adjacent A1 or H1 equipment.
Classes B and C describe other equipment environments, including office, telecommunications, and protected light-industrial spaces. Their existence does not make them substitute operating classes for data center equipment.
Read RH and Dew Point as Intersecting Limits
Relative humidity changes with air temperature. Dew point provides a more consistent indication of moisture content across a room with different air temperatures, which is why ASHRAE recommends monitoring moisture using dew point rather than relying on RH alone.
Apply the limits as follows:
- At the dry boundary, satisfy both the minimum dew point and minimum RH. The more restrictive minimum governs.
- At the wet boundary, satisfy both the maximum dew point and maximum RH. The more restrictive maximum governs.
- For the recommended envelope, maintain the recommended dew-point range and the applicable corrosion-dependent RH ceiling.
For A1–A4 and H1 allowable operation, the −12°C dew-point boundary and 8% RH boundary intersect at approximately 25°C. Below that temperature, the dew-point boundary sets the minimum moisture level; above it, the 8% RH boundary becomes more restrictive. Consequently, “8–80% RH” is not a complete description of the A1 envelope.
Use a psychrometric method or instrument appropriate to the measured conditions when deriving dew point from temperature and RH. Record whether dew point was measured directly or calculated, and keep the input readings from the same location and time.
A Temperature Reading Alone Cannot Establish an Envelope Check
Consider an A1 inlet reading of 28°C. It is above the 27°C recommended maximum but below the unadjusted 32°C allowable maximum. That supports only a temperature comparison—not a conclusion that the equipment is within its complete allowable envelope. Humidity, dew point, altitude, transient behavior, and the model-specific limits still need checking.
Similarly, a reading within 18–27°C does not establish recommended operation if moisture conditions fall outside the recommended envelope.
Measure the Air Entering the Equipment
The environmental reference point is the equipment inlet—not the cooling-unit discharge, room thermostat, hot aisle, or server exhaust. ASHRAE identifies the inlet as the common design point for air-cooled equipment.
For a usable inlet survey:
- Identify the actual intake location and airflow direction for each equipment type.
- Measure in the air stream entering the intake, following the manufacturer’s measurement instructions where provided.
- Check different intake heights and positions where recirculation or uneven supply could create local hot spots.
- Record temperature and RH together, with the corresponding dew point, timestamp, location, and equipment operating condition.
- Retain local extremes rather than using a room average to represent every device.
Do not assume that the cabinet front is always the intake. ASHRAE describes side-flow equipment and top-of-rack switches whose installation or fan direction can cause them to draw hot return air. Sensor placement must follow the actual airflow path.
When a reading approaches a boundary, account for sensor accuracy, calibration status, placement, and display rounding before assigning a status. A displayed value at the limit provides no demonstrated operating margin.
Adjust for Altitude and Temperature Changes
Altitude Reduces the Allowable Maximum Temperature
The reference card lists a maximum elevation of 3,050 m, with supplier consultation required above that elevation. Above 900 m, reduce the allowable maximum inlet temperature using the class-specific relationship below.
Where:
- (T_{\max,z}) is the altitude-adjusted allowable maximum inlet temperature, in °C.
- (T_{\max,0}) is the allowable maximum in the class table, in °C.
- (z) is site elevation, in meters.
- (D) is the elevation increase, in meters, corresponding to a 1°C reduction.
| Class | (D), meters per 1°C reduction |
|---|---|
| A1 and A2 | 300 |
| A3 | 175 |
| A4 | 125 |
| H1 | 500 |
These relationships come from notes to ASHRAE Tables 2.1 and 2.2 and should not be extrapolated above 3,050 m without supplier consultation. For A1, A2, and H1 above 2,400 m, ASHRAE explicitly states that the derated dry-bulb limit takes precedence over the recommended temperature. A more restrictive equipment-specific rating still needs to be respected.
Check Temperature Excursions Over Time
For non-tape IT equipment, the reference card specifies:
- No more than a 5°C temperature change within any 15-minute period.
- No more than a 20°C temperature change within an hour.
For each window, calculate the maximum inlet temperature minus the minimum inlet temperature. These are limits on the change within a time window, not simply an hourly slope between two endpoint readings. A temperature excursion can therefore be missed by checking only the beginning and end of an hour.
Tape products require a more stable environment. ASHRAE provides separate typical tape conditions and temperature-change guidance; use the tape product’s environmental documentation rather than treating the general A-class envelope as sufficient.
Noncondensing Conditions and Equipment State
All operating envelopes above require noncondensing conditions. Meeting room-air temperature and humidity limits does not establish that every cold surface is above the local dew point. ASHRAE specifically identifies dew-point control as a condensation concern for liquid circulating within racks.
Where cold surfaces or liquid-cooling components are present, compare their relevant temperatures with local dew point and follow the equipment’s condensation-control requirements. Air-inlet classes do not specify the permitted coolant supply temperature.
Also distinguish powered-on operation from installed-but-powered-off conditions. ASHRAE’s “product power off” category refers to equipment removed from its original shipping container and installed but not in use, such as during maintenance. It is not a packaged shipping or warehouse-storage specification.