Cooling
A CRAC (computer room air conditioner) contains its own refrigeration compressor; a CRAH (computer room air handler) has no compressor and cools the air with chilled water supplied by a chiller. Both sit in or beside the IT space and push cold air to the equipment. The difference is where the refrigeration happens, and that changes cost, capacity, maintenance and how the system fails.
Where the refrigeration cycle is. LBNL's master list of data centre energy measures puts it in one line: unlike CRAHs and air handlers, CRACs contain on-board refrigerant compressors. Schneider Electric's paper on cooling technologies describes the CRAH side: in a chilled water system the refrigeration components move from the computer room unit to a water chiller, which produces water at about 8-15 °C and pumps it to the CRAH units. The CRAH draws warm room air through its chilled water coils and sends the heat back to the chiller in the warmer return water.
A CRAC instead carries its own compressor and evaporator coil. In the most common arrangement, which Schneider Electric calls an air-cooled CRAC DX system, refrigerant runs from the indoor unit to a condenser outdoors or on the roof. DX stands for direct expansion.
| CRAC (DX) | CRAH (chilled water) | |
|---|---|---|
| Refrigeration | Compressor inside the unit | Chiller plant elsewhere in the building |
| Heat leaves the room by | Refrigerant to a condenser, or glycol to a dry cooler | Chilled water back to the chiller |
| Typical room | Small and medium rooms; Schneider Electric calls the air-cooled CRAC the staple | Large rooms and buildings with a chiller plant |
| Cost | Lowest overall cost for the air-cooled type | Highest capital cost below 100 kW of IT load |
| Capacity per footprint | Lower | Greater heat removal capacity, fewer parts |
| Distance limits | Refrigerant piping cannot run long distances reliably | Chilled water loops can run very long distances |
The table follows Schneider Electric's White Paper 59. The headline is simple: CRACs suit rooms that stand on their own, CRAHs suit buildings that already run, or justify, a chiller plant.
In a stand-alone server room or UPS room without a building chiller, and in small and medium rooms generally. Schneider Electric lists the air-cooled DX system's advantages as the lowest overall cost and the easiest maintenance, and its limits as refrigerant piping that must be installed in the field and cannot run long distances reliably and economically. Where the condenser cannot be close, a glycol-cooled CRAC sends its heat to a glycol loop and an outdoor dry cooler instead.
For the smallest rooms there are also self-contained units with the whole refrigeration cycle in one box; Schneider Electric notes these are limited in capacity because of the space the refrigeration components and air ducts take.
When there is, or will be, a chiller plant, and when the IT load is large. Schneider Electric states that chilled water CRAH units generally cost less, contain fewer parts and have greater heat removal capacity than CRAC units of the same footprint, but that chilled water systems have the highest capital cost for installations below 100 kW of IT load. A CRAH also moves the compressors, and their maintenance, out of the IT space.
A CRAC fails one unit at a time: a compressor, a fan, a refrigerant leak. Each unit is its own refrigeration system, so redundancy is a matter of having one more unit than the load needs. A CRAH depends on the chilled water plant: a chiller, a pump or the water supply can take several CRAHs down together, so the plant's own redundancy decides the room's.
Either way, the UPS room feels it. ASHRAE's paper on power equipment notes that during an HVAC failure the IT equipment draws more power as its fans speed up, which raises the load on the UPS just as the room starts to warm. The battery strings then sit in rising heat; the UPS battery life guide explains what that costs.
Yes. LBNL's table of cooling unit maintenance shares several items between the two, such as fan belts, air filters and sensor calibration, while refrigerant charge and the condenser coil apply to CRACs and the chilled water valve actuator to CRAHs. The full list is in the cooling maintenance guide. Cooling design, installation and maintenance are delivered through specialist cooling subcontractors. On the power side, UPS service and battery work across Canada covers the equipment the cooling protects.
Whichever the building can support reliably, sized for the UPS losses as well as any IT load in the room. Schneider Electric's cooling-load worksheet gives the UPS heat as a fixed part tied to the UPS rating plus a part proportional to the load; the server room cooling sizing guide shows the arithmetic. A battery room also needs the narrow temperature range the battery maker specifies, which ASHRAE stresses because battery life is very sensitive to temperature.
Computer room air conditioner: a cooling unit with its own refrigeration compressor.
Computer room air handler: a unit that cools air with chilled water from a separate chiller, with no compressor of its own.
For small rooms, an air-cooled CRAC: Schneider Electric gives it the lowest overall cost, while chilled water systems carry the highest capital cost below 100 kW of IT load.
Yes, if the chilled water is available around the clock. A chiller that shuts down at night or for maintenance leaves the room without cooling.
Usually. DX means direct expansion; Schneider Electric notes that any system using refrigerant and an evaporator coil can be called DX.
Sources, checked 2026-10-04:
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