What is a CRAC unit?
CRAC stands for Computer Room Air Conditioner. It is a precision cooling unit with its own refrigerant circuit. A compressor inside or beside the unit drives refrigerant through a direct expansion (DX) coil, which absorbs heat from the room air. The refrigerant then carries that heat to an outdoor condenser or dry cooler, where it is rejected to outside air.
Because a CRAC contains everything it needs to cool, it can run on its own without a central chiller. That makes it the common choice for server rooms, comms rooms, edge sites and smaller data halls. People often call any precision cooling unit a CRAC, even when the unit is technically a chilled water CRAH, because the service category is familiar.
A CRAC removes mostly sensible heat (temperature) and holds humidity inside a tight band, which is what IT equipment needs. The refrigerant work means a CRAC must be installed and serviced by an ARC-licensed refrigeration technician under AS/NZS 5149, the standard for the safety of refrigerating systems.
What is a CRAH unit?
CRAH stands for Computer Room Air Handler. It has no compressor and no refrigerant circuit of its own. Instead, the room unit is a fan coil: warm return air passes across a chilled water coil, and the heat is carried away in the water and returned to a central chiller plant. The chiller, not the room unit, does the refrigeration work.
A CRAH relies on a chilled water system being present. That central plant typically produces chilled water at around 7C supply and 12C return for traditional designs, or warmer (for example 18C supply and 24C return) in higher-efficiency builds that target free cooling. Modulating chilled water valves and variable-speed (EC) fans match the cooling output to the live IT load.
Because the compressor work is centralised, a CRAH unit is mechanically simpler in the room. The trade-off is that the site needs the chiller plant, pumps, pipework and water treatment to make a CRAH work. That plant is what makes chilled water efficient at scale and what makes it harder to justify for a small room.
CRAC vs CRAH side by side
The table below sets the two unit types against the factors that drive the decision: how each one moves heat, where the compressor and coil sit, the room size each suits, redundancy, capital and running cost, and the maintenance focus.
| Factor | CRAC (DX) | CRAH (chilled water) |
|---|---|---|
| Full name | Computer Room Air Conditioner | Computer Room Air Handler |
| Cooling medium | Refrigerant in a DX circuit | Chilled water from central plant |
| Compressor location | Inside or beside the room unit | At the central chiller, not in the room unit |
| Coil type | Direct expansion refrigerant coil | Chilled water coil |
| Heat rejection | Outdoor condenser or dry cooler | Cooling tower or chiller condenser |
| Best room size | Small to mid-size rooms and edge sites | Large data halls with central plant |
| Redundancy | N+1 across paired room and outdoor units | Plant-level plus unit-level redundancy |
| Capex | Lower at small scale, no chiller plant needed | Higher upfront (chiller plant), better per kW at scale |
| Opex and energy | Higher per kW once the room is large | Lower at scale, central chiller plus free cooling |
| Maintenance focus | Refrigerant circuit, compressor, condenser | Valves, actuators, water coil, water delta |
| Key standards | AS/NZS 5149, AS/NZS 1668.2 | AS/NZS 1668.2, AS/NZS 3666 (water side) |
How a CRAC moves heat (DX)
In a DX CRAC, the heat path is self-contained. Warm return air enters the unit, passes a filter, and crosses the refrigerant coil. The compressor moves the refrigerant to an outdoor condenser, where the heat leaves the building. Cooled air returns to the racks. The whole cycle runs inside the CRAC and its paired outdoor unit.
This is why a CRAC suits sites with no chilled water plant. You install the indoor unit, run refrigerant pipework to the outdoor condenser, and commission it. The cost stays low for small rooms, and lead times are short. The limit is scale: as total load grows past roughly 50 to 100kW, many small compressors become less efficient and harder to maintain than one central chiller.
How a CRAH moves heat (chilled water)
In a CRAH, the room unit only handles air and water. Warm return air crosses the chilled water coil, the water absorbs the heat, and the warmed water returns to the chiller plant. The chiller rejects the heat through its own condenser, a cooling tower or a dry cooler. No refrigerant runs through the room unit.
Centralising the refrigeration is what makes chilled water efficient at scale. One large chiller usually outperforms many small distributed compressors, and a free cooling economiser can make chilled water from cool outdoor air for part of the year without running the compressor at all. That is why large data halls favour CRAH units fed by central chiller plant.
When to choose a CRAC
A CRAC is the practical choice when the site does not have, and cannot justify, a central chiller plant. That covers most server rooms, comms rooms, edge cabinets and smaller computer rooms.
- Total IT load is small to mid-size (roughly under 50 to 100kW).
- There is no existing chilled water plant, and building one is not warranted.
- You need fast deployment with low upfront cost.
- The room is a standalone site, an edge location or a single comms room.
- You want each room independent, so a unit fault is contained to that room.
The trade-off to accept with a CRAC is energy cost as the room grows. Several small compressors use more energy per kW of cooling than a well-run central chiller, and there is no free cooling economiser the way a chilled water plant has.
When to choose a CRAH
A CRAH is the right choice once a site is large enough to support central chiller plant, or already has one. The upfront cost of the plant is offset by lower running cost and better efficiency at scale.
- Total IT load is large (commonly above 100kW) and likely to grow.
- A central chilled water plant already exists or is planned.
- Energy efficiency and low PUE are priorities, including free cooling.
- The design targets concurrent maintainability or a higher uptime class.
- The hall mixes standard and higher-density rows fed from common plant.
The trade-off with a CRAH is the dependency on the central plant. The chiller, pumps, pipework and water treatment all have to be designed, maintained and made redundant. Done well, that plant is the reason chilled water wins at scale. Done poorly, a plant fault can affect many rooms at once, which is why redundancy and concurrent maintainability matter.
Maintenance and compliance differences
The maintenance work differs because the cooling medium differs. On a CRAC, the service focus is the refrigerant circuit: compressor health, refrigerant charge and leak checks, superheat and subcooling, and the outdoor condenser. Refrigerant handling requires an ARC refrigerant handling licence and falls under AS/NZS 5149 for refrigerating system safety.
On a CRAH, the refrigerant work moves to the chiller plant. The room unit service focuses on the chilled water valve and actuator, coil condition, fan performance, condensate management and the temperature delta across the water coil. Where the heat rejection uses a cooling tower or condenser water, AS/NZS 3666 applies for the control of microbial growth, including Legionella, in those water systems.
Both unit types fall under AS/NZS 1668.2 for the mechanical ventilation that serves the room, and both should be designed to hold an ASHRAE TC 9.9 thermal envelope at the rack intake. CRAC Services maintains both DX CRAC and chilled water CRAH units across Australia, with ARC-licensed technicians and a written service report on every visit.
Practical tip
Many Australian sites run a mix: DX CRAC in smaller standalone rooms and chilled water CRAH in the main data hall. The two are not rivals so much as the right tool for different room sizes.