Reliable cooling is the backbone of every high-density data centre, ensuring equipment performs at its best. Two main types of precision cooling systems are used in data centres: CRAC (Computer Room Air Conditioner) and CRAH (Computer Room Air Handler). These systems provide reliable cooling for mission-critical environments.
But understanding their differences is key to selecting the right system for your site’s layout, load and infrastructure.
What Are CRAC and CRAH Units?
Before comparing the two, it helps to understand what each type of unit does
Definition of a CRAC (Computer Room Air Conditioner)
A CRAC unit cools the air in a server room using refrigerant and a compressor. It operates much like a traditional air conditioner, but with precision controls and continuous monitoring to maintain temperature and humidity within tight tolerances.
- Typically uses direct expansion (DX) technology.
- Ideal for smaller or stand-alone data centres without a central chilled-water plant.
- Provides independent operation, making it simple to install and manage.
Definition of a CRAH (Computer Room Air Handler)
A CRAH unit, by contrast, uses chilled water supplied from a central chiller plant to cool the air passing through a cooling coil. Instead of a compressor, CRAHs rely on a fan and heat-exchange coil, which circulate conditioned air through the data hall.
- Requires a chilled-water supply from a building chiller or cooling plant.
- Offers greater scalability for large or multi-room facilities.
- Generally more energy-efficient when integrated with free cooling systems.
How They Work: Key Technical Differences
Both CRAC and CRAH systems achieve the same goal - removing heat from IT environments - but they do it in different ways.
Cooling Mechanism in a CRAC
- Operates on a refrigerant and compressor cycle (similar to HVAC split systems).
- The system absorbs heat from the return air and expels it through air or water-cooled condensers.
- The evaporator coil cools and dehumidifies the air before it returns to the data hall.
- Best suited for small to medium-sized spaces or edge computing sites that require quick installation.
Cooling Mechanism in a CRAH
- Uses chilled water circulated through coils to absorb heat from the air.
- The fan motor forces air across the cooling coils, transferring heat into the water circuit.
- Often integrated with economiser (free-cooling) chillers, allowing significant energy savings in cooler climates.
- Best suited for larger data centres where chilled-water infrastructure already exists.
Comparing Performance, Efficiency & Use-Cases
Understanding how each system performs in different environments helps determine which is the best fit for your data centre.
Ideal Scenarios for CRAC Units
- Sites without an existing chilled-water plant.
- Smaller or modular data centres that require independent control.
- Locations needing precise temperature and humidity management in isolated rooms.
- Easier to deploy as part of a stand-alone system, often with lower upfront cost.
Ideal Scenarios for CRAH Units
- Large, centralised facilities with existing chiller systems.
- Operations targeting high energy efficiency and sustainability goals.
- Environments where load variability demands flexible, scalable cooling.
- Often paired with containment solutions and liquid cooling for optimised airflow and performance.
Efficiency, Scalability and Infrastructure Demands
- CRAC units are typically simpler but less efficient for large-scale environments because they rely on compressors.
- CRAH units benefit from chiller plant efficiency, variable-speed fans, and free-cooling integration.
- CRAHs generally provide lower total cost of ownership over time in large installations.
Things to Consider When Choosing Between CRAC and CRAH
Choosing between CRAC and CRAH systems depends on several practical factors unique to your facility.
Existing Facility Infrastructure
- CRAC: Works independently - ideal when no chilled-water plant exists.
- CRAH: Relies on chilled water - best where central cooling infrastructure is in place.
Size and Load of Server Room or Data Centre
- For small to medium loads, CRACs offer simplicity and reliability.
- For large-scale loads or tier-level data centres, CRAHs deliver better energy economics and redundancy options.
Maintenance, Lifecycle Costs and Operational Risks
- CRACs may require more frequent compressor maintenance and have higher power use per unit.
- CRAHs reduce moving parts within the unit itself, simplifying maintenance, but depend on the reliability of the chiller plant.
- Consider the total lifecycle cost, including chiller operation, water treatment and controls.
Quick Summary of When Each System Makes Sense
Both systems can achieve precise temperature control and high reliability. The right choice depends on existing infrastructure, energy goals and long-term operational strategy.
Before committing to either option, assess your environment with a STULZ precision cooling expert.
Learn more about STULZ precision cooling solutions, air-handling units, and chiller systems.
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FAQs
- What is the difference between a CRAC and a CRAH unit?
A CRAC unit uses refrigerant and a compressor to cool air, while a CRAH unit relies on chilled water from a central plant. CRACs are ideal for smaller sites, and CRAHs suit large facilities with existing chiller systems. - How does a CRAC unit work?
CRAC units operate like traditional air conditioners using direct expansion (DX) technology. They cool air through an evaporator coil and compressor cycle, maintaining precise temperature and humidity in data centres. - How does a CRAH unit work?
CRAH units use chilled water to absorb heat from the air via cooling coils. A fan circulates air across the coils, transferring heat to the water circuit. They’re more energy-efficient when paired with free-cooling chillers. - Which system is more energy-efficient: CRAC or CRAH?
CRAH systems are generally more efficient because they use chilled-water cooling and variable-speed fans, reducing compressor use. CRAC systems consume more power but are simpler to install in smaller facilities. - When should you choose a CRAC system?
Choose CRAC units for small or modular data centres that don’t have a chilled-water plant. They offer independent operation, easier setup, and precise environmental control in isolated rooms. - When is a CRAH system the better option?
CRAH units are best for large data centres with existing chiller infrastructure. They offer scalable performance, higher energy efficiency, and better integration with free-cooling and containment systems. - What are the maintenance requirements for CRAC and CRAH units?
CRAC systems need regular compressor and refrigerant checks, while CRAH systems require coil cleaning, water treatment, and fan inspections. Both benefit from routine sensor calibration and control optimisation. - Can CRAC and CRAH systems work together?
Yes. Some hybrid cooling setups combine CRAC and CRAH systems to optimise performance. This approach can balance redundancy, improve energy use, and adapt to varying data-hall loads or expansion needs.