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Choose data center cooling for the heat your IT equipment produces, the conditions it must operate in, and the facility that has to remove that heat—not by treating air and liquid as an either-or choice. Air cooling remains viable when airflow and inlet conditions are controlled. Rear-door heat exchangers, cold plates, and immersion can target higher heat loads, but may leave residual heat for room air systems and require changes to facility loops, controls, and maintenance.
How do I assess data center cooling options?
Start with the facility and its operating requirements, then compare candidate systems against the same load, reliability, energy, and water assumptions. The U.S. Department of Energy’s (DOE) 2024 Best Practices Guide for Energy-Efficient Data Center Design cautions that no single design guide can specify the most energy-efficient design for every data center. A useful assessment therefore begins with site-specific evidence rather than a generic density threshold or a single efficiency ratio.
- Map present and planned heat loads. Record server and rack loads, including planned GPU or other high-density deployments. Identify whether heat is concentrated in a few racks or spread across the room; localized hot spots may call for a targeted solution rather than a facility-wide conversion. DOE’s 2024 guide uses racks observed at 60 kW in 2013 and recently surpassing 125+ kW to illustrate the rise in high-performance-computing density and direct liquid cooling. Those figures are context, not a universal point at which a site should switch.
- Confirm equipment conditions. Check each equipment manufacturer’s recommended and allowable inlet temperature and humidity conditions, as well as applicable current standards. Design for the recommended operating range, not just the broader limits within which equipment has been tested to function.
- Trace the whole heat path. Identify where heat leaves the IT equipment, what transfers it to the facility system, and how the facility ultimately rejects it. Include residual room heat, heat exchangers, cooling distribution units (CDUs), chillers, cooling towers, dry coolers, pumps, and fans as applicable.
- Compare normal and failure operation. Review controls, redundancy, failover sequences, loss-of-flow response, sensor drift, leak response where relevant, and maintenance requirements. Establish who will operate and service the system and what skills or procedures are needed.
- Evaluate the site’s climate and resources. Assess the potential operating hours for air-side or water-side economizing, outdoor-air quality and humidity, water availability, and the maintenance demands of the proposed heat-rejection method.
- Compare like with like. Use a documented baseline and expected operating conditions for energy, water, reliability, and serviceability. Record measurement boundaries and periods so that ratios from unlike facilities or operating states are not mistaken for a direct comparison.
What are the main air- and liquid-cooling options?
“Liquid cooling” describes several different ways of collecting heat, not one interchangeable technology. Some options work alongside conventional air cooling, and even a liquid-cooled IT loop does not by itself determine how heat is rejected outdoors.
| Option | How it removes IT heat | What to assess |
|---|---|---|
| Room air cooling | Server heat enters room air; computer room air handlers (CRAHs) or computer room air conditioners (CRACs) and the facility plant remove it. | Supply and return airflow, inlet conditions, containment, fan controls, room heat distribution, and the facility plant’s response to changing loads. |
| Rear-door heat exchanger | A rack-mounted exchanger transfers heat from server exhaust air to liquid. Designs may use additional fans (active) or rely on server fans (passive). | Fan impact, pressure drop, loop temperature, water chemistry, compatibility with the rack, and how much remaining heat the room system must handle. |
| Cold plates | Liquid-channel plates replace standard fin-based heat sinks on chips. Plates may also serve memory or other heat-producing components; a CDU commonly interfaces with the system. | Which components are cooled, what heat remains in room air, fluid requirements, serviceability, and the connection to facility cooling. |
| Single-phase immersion | Electronics sit in nonconductive dielectric fluid, which is circulated to carry away heat without boiling. | Equipment support, fluid handling, maintenance practices, heat rejection, and whether the operating model fits the facility. |
| Two-phase immersion | Dielectric fluid boils below component maximum temperatures; vapor carries heat to a heat exchanger and condenses back to liquid. | Equipment support, fluid handling, maintenance, heat-exchanger requirements, and operational fit. |
These descriptions follow DOE’s 2024 guide. They are architectural distinctions, not a ranking: actual performance and compatibility depend on the complete system and site.
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When is air cooling still a good choice?
Air cooling can remain suitable when the equipment’s inlet conditions can be maintained and supply air reaches the equipment without excessive mixing with hot exhaust. Hot-aisle/cold-aisle layouts, containment, coordinated fan controls, and appropriate supply temperatures can improve air delivery and reduce unnecessary airflow. In some facilities, conventional raised-floor delivery may respond poorly to dynamic heat loads, so airflow distribution should be evaluated under actual operating conditions rather than assumed from the room layout.
The DOE 2024 guide reproduces ASHRAE thermal guidance listing a recommended dry-bulb range of 64.4–80.6°F (18–27°C) for its A1–A4 summary under low-pollutant conditions. This is a recommended range in that guide, not a substitute for checking the relevant equipment class, manufacturer requirements, current standard, humidity conditions, or site contaminants. Allowable operating boundaries are broader in some cases and should not be treated as the recommended continuous operating target.
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When should a data center consider switching from air to liquid?
Consider liquid cooling when measured or planned heat loads make the existing air path difficult to manage, or when a liquid-based approach better fits the equipment and facility’s heat-rejection options. High-density racks are a reason to evaluate direct liquid cooling, not an automatic switch trigger. A targeted rear-door exchanger or cold-plate deployment may address a subset of racks while the rest of the room remains air-cooled.
Before deciding on a conversion, establish whether the proposed system can connect to a suitable facility loop and what additional equipment or controls it requires. A CDU commonly transfers heat between the IT liquid loop and facility cooling. The assessment should include flow-loss and switchover behavior, maintenance and staff capability, the residual room-air load, and the effect on the facility’s chillers, cooling towers, or dry heat rejection. If those requirements are not resolved, higher rack density alone does not establish that a particular liquid system is the better choice.
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Is liquid cooling more energy- or water-efficient than air cooling?
Not automatically. Direct liquid cooling moves heat from IT equipment into a recirculating liquid loop instead of first moving it into room air, and liquid can transport more heat than air. Pumping can also use less energy than moving large volumes of air with fans. But the facility still has to reject the heat, and the result depends on the complete configuration: pumps, heat exchangers, chillers, cooling towers, dry coolers, economizers, controls, and local conditions all matter. Evaporative cooling towers can consume water even when the IT equipment is liquid-cooled.
Use power usage effectiveness (PUE) and water usage effectiveness (WUE) as facility indicators, not as standalone proof that one IT cooling architecture is superior:
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- PUE = total facility annual energy use divided by IT equipment annual energy use.
- WUE = annual site water use in liters divided by IT equipment annual energy use in kWh.
Report the measurement boundary and period, and compare systems at comparable IT loads and operating conditions. A favorable PUE or WUE does not by itself describe reliability, lifecycle impacts, or the suitability of a system for a particular facility.
DOE’s Federal Energy Management Program (FEMP) 2019 cooling-water guidance describes air-side economizing, which uses suitable cool outside air, and water-side economizing, which uses a heat exchanger to bypass or reduce chiller operation when conditions allow. Climate, outdoor-air contaminants, humidity, water availability, maintenance, and control sequences affect whether these approaches are appropriate. FEMP also cites a possible 20% reduction in chiller energy from hot/cold-aisle practices that enable higher chilled-water temperatures and reduced airflow; that is a potential result, not a guaranteed saving for every data center.
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FEMP’s 2019 guidance describes a PUE of 2.0 as average and values approaching 1.0 as highly efficient. These are contextual reference points, not targets or guarantees for an individual site. The same guidance reports PUE 1.06 and WUE 0.7 for the National Laboratory of the Rockies data center’s specific thermosyphon hybrid installation; those site-specific values are not a direct comparison of air cooling with liquid cooling alone.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What controls and operational risks should the assessment include?
Cooling performance depends on control behavior as well as equipment selection. DOE’s 2024 guide recommends maximizing efficiency across changing ambient conditions and IT loads, coordinating CRAH/CRAC units, and monitoring supply conditions. It also warns that over-controlling humidity or allowing adjacent units to counteract one another can undermine operation.
- Check sensor placement and calibration, including whether inlet conditions are monitored where they matter.
- Review how setpoints and fan or pump speeds respond to variable IT and outdoor conditions; look for units working against each other.
- For hybrid arrangements, identify every control loop and who monitors and maintains it.
- Document alarms, failover sequences, loss-of-flow response, and recovery steps before relying on a new cooling path.
- Include water treatment and fluid handling where relevant, along with leak response and service procedures appropriate to the selected architecture.
These checks help surface operational requirements that an energy ratio or equipment specification cannot capture on its own.
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