Cooling is the single most common reason a datacenter cannot use the capacity it already paid for. Most halls run well below their design density not because the chillers are undersized, but because the air is going the wrong way.
How much cooling does a rack need?
Start with a simple truth: essentially all the electrical power a server draws becomes heat. A 5kW rack produces about 5kW of heat, so you need roughly 5kW of cooling. There is no clever exception to this.
Typical densities today:
- 3 to 7kW per rack: ordinary enterprise workloads, comfortable with standard air cooling
- 8 to 15kW per rack: dense virtualisation, needs proper containment to work reliably
- 15 to 25kW per rack: difficult on air alone, wants in-row or rear door cooling
- 40kW and above: liquid cooling territory, typically GPU and AI training racks
- 60 to 130kW+: direct-to-chip liquid cooling is the only practical option
CRAC vs CRAH: what is the difference?
A CRAC (Computer Room Air Conditioner) uses refrigerant and its own compressor to cool air, like a large dedicated air conditioner. It suits smaller rooms with no chilled water infrastructure.
A CRAH (Computer Room Air Handler) uses chilled water from a central plant, with a valve controlling flow through a coil. At scale it is generally more efficient, because a central chiller plant is more efficient than many small compressors.
Rule of thumb: a single server room usually gets CRAC. A real datacenter with a chilled water plant gets CRAH.
Containment: the cheapest win available
Without containment, cold supply air and hot exhaust air mix. That mixing is what destroys cooling capacity. You end up over-cooling the whole room to keep a few intakes happy, which is expensive and still leaves hot spots.
Cold aisle containment encloses the cold aisle, so cold air can only escape through the equipment. Hot aisle containment encloses the hot aisle and returns exhaust straight to the cooling units. Either works, both dramatically outperform an open room.
Containment only works if the rack is sealed too. Blanking panels in every empty U, brush strips on floor cutouts, and side sealing are not optional details, they are the mechanism.
In-row, rear door and liquid cooling
As density climbs, you move the cooling closer to the heat.
In-row cooling places cooling units between the racks, shortening the air path and handling roughly 15 to 30kW per rack.
Rear door heat exchangers replace the rack’s rear door with a water cooled coil, cooling exhaust as it leaves. They retrofit well into existing halls with chilled water and handle around 20 to 40kW.
Direct-to-chip liquid cooling puts cold plates onto the CPUs and GPUs themselves, removing heat at the source and supporting the highest densities. This requires a CDU and a properly commissioned loop.
Why does my server room have hot spots?
Almost always airflow, not capacity. The usual suspects, in the order we find them:
- Missing blanking panels, letting hot exhaust recirculate straight into intakes
- Unsealed floor cutouts, dumping cold air where no equipment is
- No containment, so supply and return mix freely
- Vented tiles in the wrong aisle, or too many of them
- Cable bundles blocking the rear of the rack
- Racks arranged without hot and cold aisles at all
The point is that these are cheap to fix. Before anyone quotes you a new chiller, fix the airflow. We have recovered substantial capacity in rooms whose owners were convinced they needed more plant.
Temperature and humidity targets
Modern equipment is far more tolerant than datacenter folklore suggests. ASHRAE’s recommended envelope for inlet air is roughly 18 to 27°C, and most current hardware is rated to operate well beyond that.
Measure temperature at the equipment intake, not in the middle of the room or at the cooling unit return. The intake is the only place the number means anything. Running the whole hall at 18°C to fix one hot rack is a very expensive way to avoid installing a blanking panel.
Frequently asked questions
How much cooling does a datacenter rack need?
Match cooling to the IT load: a 5kW rack needs roughly 5kW of cooling, because nearly all electrical power becomes heat. Standard air cooled halls handle about 5 to 15kW per rack, and above 20kW you should consider in-row, rear door or liquid cooling.
What is the difference between CRAC and CRAH?
A CRAC unit uses refrigerant and its own compressor to cool air. A CRAH unit uses chilled water from a central plant. CRAH is generally more efficient at scale; CRAC suits smaller rooms without chilled water infrastructure.
What temperature should a datacenter be?
ASHRAE recommends an inlet air temperature of roughly 18 to 27°C. Measure at the equipment intake rather than in the room, since the intake is the only reading that reflects what the hardware actually receives.
Why does my server room have hot spots?
Almost always airflow rather than cooling capacity. Missing blanking panels, unsealed floor cutouts, no containment and badly placed vented tiles let cold air bypass equipment and hot exhaust recirculate. These are usually cheap to fix.
At what density do I need liquid cooling?
Air cooling is practical to roughly 15 to 20kW per rack, difficult between 20 and 40kW, and effectively finished above 40kW. Dense GPU and AI racks at 60 to 130kW require direct-to-chip liquid cooling with a CDU.
What is hot and cold aisle containment?
Containment physically separates cold supply air from hot exhaust air, so they cannot mix. Cold aisle containment encloses the cold aisle; hot aisle containment encloses the hot aisle. It is usually the cheapest and largest efficiency gain available.
Read next: our guide to CDUs and HVAC for datacenters. For installation, see datacenter cooling and HVAC or request a site survey.