How Warm Can a Data Center Run? – ASHRAE Thermal Guidelines
Plenty of older server rooms are cold enough to need a jacket. For years the instinct was to keep server rooms as cold as a meat locker, just in case. That habit costs real money, because every degree of extra cooling is paid for in compressor and fan power. So how cold does the room actually need to be? How warm can a data center run before the servers start to suffer?
Under the ASHRAE thermal guidelines, a data center can run server inlet air anywhere from 64.4°F to 80.6°F (18°C to 27°C) in the recommended range, and most enterprise servers can tolerate even warmer air for limited periods. Those numbers apply to air entering the front of the equipment, not the room average. Wider "allowable" ranges run from 89.6°F (32°C) for Class A1 up to 113°F (45°C) for Class A4. Running warmer saves cooling energy, but server fans spin faster and failure rates creep up, so the setpoint is a trade-off.
What Are the ASHRAE Thermal Guidelines?
The ASHRAE thermal guidelines are a set of temperature and humidity ranges for IT equipment published by ASHRAE Technical Committee 9.9, the group that covers mission critical facilities. The document is called Thermal Guidelines for Data Processing Environments. It first appeared in 2004 and has been revised several times since, most recently in a fifth edition published in 2021.
The guidelines matter because the people who build servers sit on the committee. IT manufacturers agree on what their hardware should see, and facility operators get a common target. Before that, every vendor had its own spec sheet, and operators played it safe by running cold.
They are guidelines, not code. No inspector will fine you for running 82°F at the inlet. But warranties, service contracts and lease agreements often reference the ASHRAE ranges, so in practice they carry a lot of weight.
Recommended vs Allowable: What Is the Difference?
The recommended range is where ASHRAE suggests you operate most of the time for reliability and efficiency. The allowable range is where the equipment is tested to function, but you would not want to sit there all year.
Think of recommended as cruising speed and allowable as the redline. Brief trips into the allowable zone during a heat wave or a cooling failure are fine and expected. Living there pushes up fan power, failure rates and noise.
| Class | Recommended inlet | Allowable inlet | Typical equipment |
|---|---|---|---|
| A1 | 64.4–80.6°F (18–27°C) | 59–89.6°F (15–32°C) | Enterprise servers, storage |
| A2 | 64.4–80.6°F (18–27°C) | 50–95°F (10–35°C) | Volume servers, workstations |
| A3 | 64.4–80.6°F (18–27°C) | 41–104°F (5–40°C) | Hardware rated for wider ranges |
| A4 | 64.4–80.6°F (18–27°C) | 41–113°F (5–45°C) | Hardware rated for the widest ranges |
| H1 | 64.4–71.6°F (18–22°C) | Up to 77°F (25°C) | High-density equipment |
Notice that the recommended band is the same for A1 through A4. The classes differ in how far the hardware can stretch, which is what lets an operator lean on outside air for more hours of the year.
What Is the H1 Class?
H1 is a newer class for high-density equipment that cannot run as warm as ordinary servers. It arrived with the fifth edition in 2021. Packing more power into each chip and each chassis leaves less thermal margin, so some hardware needs cooler inlet air to stay within its limits.
The Uptime Institute's analysis in Data Center Dynamics on the 2021 ASHRAE update pointed out the tension: H1 sets a recommended range of 64.4°F to 71.6°F (18°C to 22°C) and an allowable top of 77°F (25°C), which works against the long push to run rooms warmer. If you mix H1 gear into an A1 room, the coolest requirement wins for that zone.
How Much Humidity Is Allowed?
ASHRAE expresses the recommended humidity range mainly in dew point, with a relative humidity cap on top. The low end is a dew point of about 15.8°F (-9°C). The high end is a dew point of 59°F (15°C) and a relative humidity limit, which the 2021 edition tied to air quality.
The reason is corrosion. Humid air combined with sulfur compounds or other corrosive gases attacks copper and silver on circuit boards. In clean air, the fifth edition raised the recommended humidity limit to 70% RH. Where corrosive gases are present, it calls for a lower limit. Sites near refineries, paper mills, livestock operations or busy highways should test corrosion rates with copper and silver coupons before assuming the higher limit applies.
Low humidity has its own risk, static discharge. Research behind the 2015 edition found the risk was smaller than once believed, which is why the low end dropped and many sites stopped spending energy to add moisture in winter.
Why Not Just Run Hotter and Save Money?
You can save money running warmer, up to a point. Then the servers start spending the savings for you. As inlet air warms, server fans speed up, and fan power rises steeply with speed.
The fan affinity laws make the point. Power varies roughly with the cube of speed. Here is a labelled example:
- A server fan running at 50% speed draws a baseline amount of power, call it 1 unit.
- Warmer inlet air pushes it to 60% speed, a 20% increase.
- Power rises by 1.2 × 1.2 × 1.2 = 1.73, so about 73% more fan power.
Spread across thousands of servers, that extra fan load shows up on the IT meter, which can make the facility's power usage effectiveness look better while total energy barely moves. ASHRAE's own reliability data also shows failure rates rising gradually with inlet temperature. The sweet spot for many sites sits in the upper half of the recommended range, often around 75°F to 80°F, but the right answer comes from measuring your own fan power and chiller power together.
How Do Warmer Setpoints Change the Cooling Plant?
Warmer setpoints let the cooling plant work less and use outside conditions more. Chillers run more efficiently when they make warmer water, and economizers can cover more hours of the year before compressors need to start.
That shift opens the door to cooling methods that cannot reach very low temperatures. An evaporative cooling data center design, for example, can only cool air to a few degrees above the outdoor wet-bulb temperature. With an 80°F inlet target instead of 65°F, those systems carry the load across far more of the year in dry climates.
Air management is the other half. Hot and cold aisle containment keeps exhaust air from looping back to server inlets, so you can raise the supply temperature without creating hot spots at the top of racks.
What Happens When the Cooling Fails?
When cooling fails, inlet temperatures climb fast, and the allowable range becomes your buffer. A room running at 65°F has more headroom before servers hit their limits than one running at 80°F. That is the honest argument for running cooler, and it deserves a real answer rather than a shrug.
The answer is usually ride-through planning. Chillers take minutes to restart after a power blip, so many sites keep chilled water storage tanks, put pumps and fans on UPS power, or both. With those in place, a warm setpoint costs little in risk. Without them, a few degrees of extra margin might be the only thing standing between a utility trip and a room full of servers shutting themselves down.
ASHRAE also limits how quickly inlet temperature should change, because rapid swings stress components and storage media. A cooling unit cycling hard on and off can cause more trouble than a steady, slightly warmer room.
Where Should You Measure Temperature?
Measure at the server inlets, not at the cooling unit return or a thermostat on the wall. ASHRAE's ranges describe the air entering the equipment. A room can feel cool while the top servers in a poorly contained rack breathe hot exhaust.
A practical approach is sensors at the bottom, middle and top of the front of selected racks, especially at row ends and near the farthest point from the cooling units. Watch the top sensor. That is where recirculation shows up first.
Run Warm, Not Hot: Using the ASHRAE Thermal Guidelines Well
Most rooms are still colder than they need to be. The guidelines give you permission to run inlets near 80°F; the only real exceptions are high-density H1 gear and sites with no cooling ride-through.
The useful next step is to check the class rating of your hardware, put sensors at the inlets and raise setpoints a degree or two at a time while you watch total facility power, not just the chiller. Stop when the server fans start eating the savings. That point is different for every room, and finding it is one of the cheapest efficiency projects an operator can run.