Cooling
ASHRAE Envelope Reference
Recommended range and A1–A4 allowable equipment classes for server inlet temperature, relative humidity and dew point, plus what each class actually means for equipment risk. Use alongside the ASHRAE Envelope Checker.
Last updated: June 2026
Terms
- ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers
- The US industry body that publishes the standard temperature and humidity ranges data-center equipment is designed to handle. When a chart says "ASHRAE recommended range", it means the safe zone most server makers design for.
Recommended & Allowable Envelopes
| Envelope | Temperature | Relative humidity | Dew point | Use case |
|---|---|---|---|---|
| Recommended | 18 – 27 °C | 8 – 60 % | -9 – 15 °C | Normal efficient operation target — best balance of reliability and energy use |
| A1 | 15 – 32 °C | 8 – 80 % | -12 – 17 °C | Enterprise IT equipment — most servers, storage |
| A2 | 10 – 35 °C | 8 – 80 % | -12 – 21 °C | Broader allowable range — most modern volume servers |
| A3 | 5 – 40 °C | 8 – 85 % | -12 – 24 °C | Extended allowable range — ruggedised/edge equipment |
| A4 | 5 – 45 °C | 8 – 90 % | -12 – 24 °C | Wide allowable range — high-temperature-tolerant IT (rare) |
"Recommended" is a tighter target band inside the wider "allowable" classes — operating inside Recommended maximises hardware life and energy efficiency; operating within Allowable (A1–A4) avoids equipment damage but may shorten component life expectancy at the extremes.
Recommended vs. Allowable — What It Actually Means
| Operating inside… | Risk level | Typical consequence |
|---|---|---|
| Recommended | Lowest | Optimal reliability; manufacturer warranties fully apply; best long-term hardware life |
| Allowable (A1–A4) | Low–moderate | No immediate equipment damage expected, but may affect failure rates and warranty terms at sustained extremes — check vendor documentation |
| Outside Allowable | High | Risk of thermal shutdown, throttling, condensation (high dew point) or electrostatic discharge issues (low humidity) |
Why Dew Point Matters as Much as Humidity
Relative humidity alone doesn't predict condensation risk — dew point does. The same relative humidity reading means very different absolute moisture content at different temperatures.
High dew point (above the envelope's upper limit) risks condensation on cold surfaces — corrosion and short-circuit risk.
Low dew point (below the envelope's lower limit, paired with low RH) increases electrostatic discharge (ESD) risk to sensitive electronics.
Frequently Asked Questions
Which class should I design for?
Design the room's normal operating point inside "Recommended" wherever practical — it's the best balance of energy efficiency and hardware longevity. Use the wider A1–A4 allowable classes as your safety margin for cooling-system upset conditions (e.g. economiser transitions, brief unit outages), not as the everyday target.
What's the difference between A1 and A2?
A2 has a wider allowable temperature range (10–35 °C vs A1's 15–32 °C) and a higher allowable dew point. Most modern volume servers (2010s onward) are rated A2, which gives more flexibility for free-cooling/economiser hours. A1 is the more conservative legacy enterprise class — check individual equipment datasheets rather than assuming.
What about X1 and X2?
X1 and X2 aren't standard published ASHRAE classes — they're commonly used as placeholders for a custom, project-specific or vendor-specific envelope that doesn't match a standard A-class exactly. If your equipment vendor supplies a custom operating envelope, use the checker's custom-envelope option rather than forcing it into the nearest A-class.
Does altitude change the envelope?
Allowable temperature ranges typically need derating above roughly 950 m (3 100 ft) — equipment cooling fans move less mass per rotation in thinner air, so effective cooling capacity drops even at the same air temperature. The ASHRAE Envelope Checker applies an altitude derating factor automatically.