CRAC or CRAH units for server rooms and data centres?

What is the difference between a CRAC and a CRAH?

The Energy Technology List (ETL), the UK government's list of energy-efficient products, covers close control air conditioning equipment that controls temperature, and optionally humidity, in rooms with heat-generating equipment such as servers. Its categories separate DX units, which cool by evaporating a refrigerant in their indoor heat exchangers, from chilled water cooled units and dual mode units that can do both. To qualify, a unit needs a compressor or a chilled water coil with fittings for an external chilled water circuit.

Manufacturers map the labels onto those types. The Airedale glossary defines a CRAC as a computer room air conditioning unit and a CRAH as a computer room air handling unit, and STULZ's US page offers an air conditioner (CRAC) or air handler (CRAH) across a range with direct expansion (DX) units that have scroll compressors and chilled water (CW) units that it calls CRAHs. So a CRAC is the DX type and a CRAH the chilled water type.

The trade uses the terms loosely. A 2019 CIBSE Journal cost model calls chilled-water downflow units "Crac" units, so confirm what is inside the casing before comparing. The table uses the manufacturer STULZ's published ranges as examples.

	CRAC (DX)	CRAH (chilled water)
Where the cooling is made	Compressor in the unit	In a chiller elsewhere, piped to a coil in the unit
Heat rejection options	Air-cooled, water-cooled or hybrid free-cooling versions (STULZ CyberAir 3PRO DX)	Through the chilled water loop to the chiller and its heat rejection
Example published range	22 to 150 kW (CyberAir 3PRO DX)	22 to 260 kW (CyberAir 3PRO CW)
ETL minimum EER	4.00 for air-cooled DX	18.00 for chilled water cooled
Where F-gas duties sit	In the unit's refrigerant circuit	In the chiller

The F-gas row follows from the definitions. GOV.UK includes stationary air-conditioning equipment in its leak-check duties, so a CRAC's circuit is covered by the intervals in the chiller maintenance guide. Refrigerant is also a CRAC-side choice. STULZ lists its DX units with R407C, R410A, R134a and R513A, with GWPs from 1,774 down to 631, and R454C (GWP 148) on its hybrid free-cooling versions, and it notes that R513A is in the A1 safety class, neither toxic nor flammable. The refrigerant GWP table compares the options.

When does a CRAC suit, and when a CRAH?

• A CRAC suits a room with no chilled water on site, or one that needs a self-contained unit. Hybrid free-cooling DX versions exist: the ETL requires an integral free cooling coil to give at least 90% of the unit's DX cooling capacity.
• A CRAH suits a hall that already has, or will have, a chilled water plant. The CIBSE cost model describes chilled-water units fed either by a roof chiller sized for full peak load with no free cooling, or by free-cooling towers with chillers running in parallel at peak times.
• A hybrid suits a site that wants both. STULZ lists chilled water units with a redundant air-cooled or water-cooled DX system, and the ETL has dual mode categories.

For small rooms the deciding question is usually consequence of failure, covered in the server room cooling requirements guide and the server and comms room air conditioning page. Free cooling is a heat-rejection choice that either type can use: STULZ separates direct free cooling, where outside air is conveyed inside the building, from indirect, where it does not penetrate the interior. The free cooling chillers guide counts the hours available in each UK region.

How do they fit with containment, in-row and rear-door cooling?

Both are room-based. CIBSE Journal says traditional layouts with chillers, CRAC units and cold aisle/hot aisle approaches are not terribly efficient and can leave hot spots in the hall, which is why cooling is brought closer to the racks. The ETL draws the same line, separating room units from close coupled products that serve the local area around the equipment or the equipment itself.

STULZ's CyberRow in-row units list 11 to 58 kW each, can cool up to six server cabinets and integrate easily into hot and cold aisle containments. As density rises, CIBSE notes that air cooling needs more space and more CRAC and CRAH units, and that direct-to-chip liquid cooling can operate alongside traditional CRAC/CRAH units without wholesale redesign. The data centre cooling methods guide compares all of these approaches.

How do redundancy, monitoring and maintenance differ?

N+1 counts units. The Uptime Institute describes Tier II as redundant capacity components for power and cooling, a list that includes cooling units and pumps, while Tier III adds redundant distribution paths so that any part can be shut down for maintenance without affecting IT. For chilled water, the pipework, pumps and chiller form a shared path that every CRAH depends on. Air-cooled CRACs each have their own refrigerant circuit, so they do not depend on a shared chilled water loop, although water-cooled versions share a condenser loop and all units share power and the room. Ask a designer which common elements remain behind the unit count.

Monitoring and control differ too. STULZ's chilled water range lists standby management, differential pressure control and filter control management. For upkeep, a CRAC adds a refrigerant circuit to maintain, while a CRAH moves that work to the chiller plant.

How are the units rated, and does ecodesign apply?

To be listed, ETL close control units need a sensible heat ratio of at least 0.9, and must be able to vary capacity with load. The ETL's energy efficiency ratio (EER) is cooling capacity divided by the electric power the product absorbs. For DX and dual mode units that power includes the indoor and outdoor heat exchangers. The chilled water figure covers the unit's own power, so the chiller's consumption comes on top, which is why 18.00 and 4.00 cannot be compared directly. ASHRAE publishes Standard 127-2020 as the method of testing for rating air-conditioning units serving data centres.

The ecodesign regulation 2016/2281 defines air-based and water-based cooling systems for the thermal comfort of human beings. That wording points to comfort cooling, so the ETL close control category is the more relevant published benchmark for IT-room units; any scope question is one for the manufacturer's declaration. If you want help specifying server room or data hall cooling, the server and comms room air conditioning page explains the process, and you can send an enquiry through the site's form.

Sources

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

• HVAC Glossary of Terms, Airedale, manufacturer's own page; checked 22 October 2026.
• CyberAir computer room air handlers and air conditioners, STULZ USA, manufacturer's own page; checked 22 October 2026.
• Close Control Air Conditioning Equipment, Energy Technology List, UK Government; checked 22 October 2026.
• CyberAir 3PRO DX, STULZ, manufacturer's own page; checked 22 October 2026.
• CyberAir 3PRO CW, STULZ, manufacturer's own page; checked 22 October 2026.
• CyberRow, STULZ, manufacturer's own page; checked 22 October 2026.
• Cost model: data centre cooling, CIBSE Journal, posted October 2019; checked 22 October 2026.
• Module 254: Liquid cooling in data centre applications, CIBSE Journal, posted October 2025; checked 22 October 2026.
• ASHRAE Data Center Resources, ASHRAE; checked 22 October 2026.
• Uptime Institute Tier Classification System, Uptime Institute; checked 22 October 2026.
• Commission Regulation (EU) 2016/2281, article 2, legislation.gov.uk, revised 09/01/2017; checked 22 October 2026.
• Checking F gas equipment for leaks, Environment Agency and Defra, GOV.UK, published 21 August 2019; checked 22 October 2026.

Common questions

Is a CRAH more efficient than a CRAC?

The ETL figures are not like for like. A DX unit's EER includes indoor and outdoor power, while a chilled water unit's covers only the unit itself, so the chiller must be added. Compare whole systems, not unit labels.

Can one unit work as both?

Yes. The ETL has dual mode categories combining DX and chilled water, and STULZ lists chilled water units with a redundant DX system.

Do CRAH units contain refrigerant?

A chilled-water unit has a coil with fittings for an external chilled water circuit rather than a compressor, according to the ETL. The refrigerant is in the chiller that makes the water.

Do these units control humidity?

The ETL says close control equipment controls temperature and optionally humidity. A sensible heat ratio of at least 0.9 means most capacity goes into removing heat, not moisture.

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