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Data centre cooling methods compared

Updated 23 October 2026 · SEO Dons Editorial

What are the main ways to cool a data room?

The methods differ in where the heat is picked up, so the table runs from the room down to the chip. CIBSE Journal's cost model sets out three approaches for large halls: air cooling by chiller and CRAC units, indirect air cooling air handling plant, and chilled-water cooling from free-cooling hybrid cooling towers with chiller assist.

MethodHow it worksWhere it fits
Room-based CRAC or CRAH unitsCold air to the room, often through a floor void; hot air returned to the unitRooms and halls; see the CRAC versus CRAH guide
Aisle containmentKeeps hot and cold air from mixingAny air-cooled hall
In-row coolersUnits between racks; STULZ lists CyberRow at 11 to 58 kW, up to six cabinets eachTargeted cooling of high-density racks
Rear-door heat exchangersWarm exhaust air passes through a liquid-cooled door, which carries the heat awayUpgrading existing air-cooled halls
Chilled water with free coolingAmbient air cools the warm return water, with adiabatic cooling added in warmer monthsYear-round loads; see the free cooling chillers guide
Evaporative and adiabaticWater evaporation assists heat rejectionSites that can manage the water and legionella duties
Direct-to-chipCold plates on high-heat parts such as CPUs, liquid to a coolant distribution unitHigh-density racks
ImmersionServers submerged in dielectric fluidRacks pushing beyond 100 kW, which CIBSE expects to make it increasingly attractive

CIBSE says traditional air layouts with cold aisle and hot aisle approaches are not terribly efficient and can leave hot spots. STULZ describes containment's aim as preventing hot and cold air from mixing, and says its in-row airflow works like a virtual containment while still integrating easily into hot and cold aisle containments.

How does rack density decide the method?

Chips set the heat. CIBSE Journal notes a modern CPU might draw more than 300 W and a high-end AI GPU more than 700 W, and that conventional air systems can support rack densities up to approximately 25 to 30 kW while emerging workloads frequently exceed 100 kW per rack. For denser air-cooled servers, fan power can reach 10% to 20% of server power. Cooling is estimated at between 20% and 50% of a data centre's total energy use.

  1. Measure the real load per rack now, and agree the planned load with the IT team.
  2. If racks sit within the air-cooling range, concentrate on airflow, containment and a chilled water or DX plant with free cooling.
  3. If a few racks exceed it, consider rear-door or in-row cooling for those racks, or direct-to-chip for the heaviest.
  4. Where whole halls will exceed it, plan the liquid loops, space and water quality at the design stage.
  5. For small rooms, start from the server room cooling requirements guide and the server and comms room air conditioning page.

What do liquid cooling options need?

CIBSE lists three scalable approaches: direct-to-chip, immersion and rear-door heat exchangers. Direct-to-chip attaches cold plates to high-heat parts such as CPUs, with warmed liquid circulated to a coolant distribution unit (CDU) in a closed loop, but it may still need air cooling for power supplies and hard drives. Immersion submerges entire servers in a non-conductive dielectric fluid. Two-phase designs can achieve two to three times the efficiency of single-phase ones but raise costs and maintenance demands, and need environmental consideration where fluorinated refrigerants are used. Rear-door units replace the rack door with a liquid-cooled heat exchanger and suit legacy halls seeking incremental improvements.

Two fluid loops are involved: the technology cooling system serving the IT equipment, and the facility water supply, linked by CDUs. ASHRAE's TC 9.9 classes (W17, W27, W32, W40, W45, W+), as CIBSE describes them, set facility water supply temperatures, and the number is the maximum supply temperature in degrees Celsius. Warmer water means less mechanical cooling and more free cooling.

Retrofits need care. CIBSE says many older halls were designed to keep water away from racks, service level agreements may exclude water near racks, and space is needed for CDUs, manifolds and piping. It describes a hybrid approach as common, and a survey it cites found 22% of data centres using direct liquid cooling and a further 61% considering it. The CIBSE Journal cost model adds that water-cooled rack systems work well but carry an inherent risk of leaks.

Where do evaporative and adiabatic systems fit?

The Energy Technology List says evaporative cooling is increasingly popular in data centres, but that it consumes a continued source of water. CIBSE lists increased water consumption among the drawbacks of air systems with evaporative cooling, and notes the UK has no specific restriction on data centre water use, with general regimes limiting use in water-stressed areas. HSE says anyone with a cooling tower or evaporative condenser should put in place measures to prevent or control legionella. The cooling towers versus dry coolers guide compares the heat rejection options, and the legionella requirements guide covers the duties.

How is efficiency reported?

Power usage effectiveness (PUE) is the total energy used by a data centre divided by the energy used by the ICT equipment, so a facility with equal overhead and IT energy has a PUE of 2.0, according to The Green Grid's glossary. CIBSE Journal says most traditional data centres run at about 1.6, while advanced liquid-cooled facilities are reaching 1.05 to 1.15, and pairs it with water usage effectiveness (WUE): about 1.5 litres per kWh for historic sites against 0.1 or less for modern liquid-cooled ones. WUE is the companion metric for water. The 2025 EU Code of Conduct best practice guidelines are a public reference list of efficiency measures, and ASHRAE publishes Standard 90.4-2025, an energy standard for data centres.

If you want help specifying cooling for a data room, the server and comms room air conditioning page explains the process, and you can send an enquiry through the site's form.

Sources

Standards and guidance are revised, so confirm the current text before relying on a figure. Manufacturer figures are examples from one maker, not a ranking.

Common questions

Is liquid cooling replacing air cooling?

Not wholesale. CIBSE Journal says many operators run a mix, with liquid cooling on high-density or GPU racks and air cooling for conventional equipment, and direct-to-chip can still need some air cooling.

Does liquid cooling use less water?

CIBSE says it can: typical WUE for historic data centres is about 1.5 litres per kWh, while modern liquid-cooled sites can reach 0.1 or less. Results depend on the heat rejection method.

What do the ASHRAE W classes mean?

In CIBSE Journal's description, the number is the maximum allowable facility water supply temperature in degrees Celsius, so W27 means supply at or below 27 °C. Warmer classes need less mechanical cooling.

Can liquid cooling be added to an existing data room?

Yes, with planning. CIBSE says direct-to-chip can be retrofitted with relatively modest infrastructure changes and run alongside CRAC and CRAH units, but water near racks, contract terms and space for CDUs and pipework all need checking.

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