CRAC and CRAH equipment can look similar on a data hall floor, but the heat is removed in different ways. CRAC commonly refers to a computer-room air conditioner using direct expansion refrigeration, while CRAH refers to a computer-room air handler with a chilled-water coil.
The better choice depends on the overall plant, not on a single unit’s efficiency label. A CRAH does not run without a functioning chilled-water system; a CRAC still needs heat rejection and electrical power.
What to include in the decision
- Compare the full heat-transfer path
- Plant scale changes the cost comparison
- Evaluate common-mode failures
- Use climate and controls in the evaluation
Compare the full heat-transfer path
A typical DX CRAC circulates room air across an evaporator; a refrigerant circuit and external condenser or heat-rejection arrangement carry the heat away. A CRAH circulates room air across a chilled-water coil supplied by a central plant.
Both can be deployed with variable-speed fans, monitoring and aisle containment. Equipment-specific configurations vary, so use the submittals and design diagrams rather than product-name shorthand alone.
Plant scale changes the cost comparison
CRAC can be straightforward for smaller sites or modular expansions where a central chilled-water plant is not practical. CRAH can make sense when a common chiller or water loop serves many data halls and economies of scale matter.
Compare total installed CAPEX including condensers, chillers, piping, controls, pumps and cooling redundancy. Looking only at the in-room unit price omits much of the real system.
Evaluate common-mode failures
Distributed DX units and central plants have different failure domains. A failed shared pump, chilled-water header or control network may affect multiple CRAHs, while many smaller refrigerant units can create maintenance and leak-management burdens.
For either architecture, test what happens when one compressor, chiller, pump, power source or controller becomes unavailable. N+1 on a diagram is not proof of maintained thermal capacity at design conditions.
Use climate and controls in the evaluation
Economizers, higher supply-water temperatures and intelligent fan controls can change the relative operating cost. An air-handler connected to a plant with effective waterside economization may operate differently from one served by conventional constant-temperature chilled water.
ASHRAE describes airside and waterside economizer methods and the importance of temperature classes and reliable changeover. Model annual conditions rather than assuming a fixed PUE benefit.
Illustrative equipment versus system boundary
Consider a hypothetical 1,200 kW design heat load. Comparing the purchase price of ten in-room CRAH units with ten CRAC units is incomplete because the CRAH option needs a sized water plant and distribution, whereas the CRAC option needs refrigerant heat rejection. Ask vendors for a like-for-like estimate from room air inlet through final heat rejection, with the same design ambient temperature and redundancy.
When a hybrid arrangement is justified
Existing data centers frequently combine equipment generations. A conventional room may retain DX units while an expanded hall uses CRAHs connected to a new plant. The hybrid design may reduce near-term disruption but introduce two maintenance programs and potentially two sets of spare parts.
Calculate full-system N+1 capacity under the design summer condition and with the largest critical component out of service. Include whether the facility can maintain adequate temperatures during a chiller trip, pump trip, DX condenser fault or controls outage.
If density is increasing, confirm whether the room cooling can handle the residual air load after direct-to-chip systems remove most of the heat. Room-level cooling and server-level liquid cooling are not mutually exclusive.
Worked thermal load and plant boundary example
A heat load of 500 kW is approximately 1,706,000 Btu/h, or 142.2 refrigeration tons using 12,000 Btu/h per ton. This conversion describes a thermal duty, not a vendor equipment schedule: redundancy, airflow, design conditions and continuous performance must still be determined.
A DX-based CRAC option may quote room units with compressors while a chilled-water CRAH option may quote air handlers without including chillers, towers or pumping. To compare them fairly, include the complete heat-rejection chain, controls, electrical distribution, water treatment where relevant and maintenance staffing.
A project may favor DX in a small retrofit to avoid building a central water plant, but a multi-hall campus may justify chilled-water infrastructure. Neither conclusion is a general efficiency guarantee; consider location, partial-load operation, climate and available heat-rejection equipment.
Equipment-versus-system comparison
| Question | CRAC (typically DX) | CRAH (typically chilled water) |
|---|---|---|
| Heat-transfer route | Refrigerant circuit plus condenser | Coil, pumps, chiller or other water-side plant |
| Scope often omitted | Condensing unit, distribution or piping | Chiller, pumps, cooling towers and treatment |
| Maintenance exposure | Compressors, refrigerant and condenser | Valves, pumps, central plant, water risk |
| High-density applications | Check available airflow and capacity | Check residual air load and water temperature |
Reliability acceptance test
Define the required inlet-temperature envelope at design outdoor conditions, normal maximum IT load and a credible failure of the largest cooling component. Confirm whether each option can meet that envelope after the failed element is isolated. A unit-count N+1 label without a full-loop failure analysis is not enough.
Use DOE’s energy-efficient data center guidance for the whole system perspective, including IT environmental conditions, air management and cooling. The final operating values must be approved for the installed IT equipment and site design.
Questions to ask before signing or building
- Is the chilled-water plant already installed or part of the new budget?
- What heat-rejection components, pumps and controls are included?
- What failures can remove multiple units simultaneously?
- What annual weather and IT-load profiles were used for energy estimates?
Methodology: illustrative figures are deliberately separated from sourced claims. Utility rates, commercial charges and engineering performance must be validated against site documents. Publication date: October 19, 2026.
Research and technical references
- US DOE FEMP: 2024 data center efficiency design guide — technical or provider documentation, not a project quotation
- ASHRAE Handbook: Data Center Economizers — background reference, not a project quotation
- ASHRAE: Cooling Towers and Waterside Economizers — background reference, not a project quotation