CRAC vs CRAH units

Computer room air conditioners (CRACs) and computer room air handlers (CRAHs) are the traditional workhorses of data center cooling. They look similar: both are floor-standing cabinets, usually placed around the perimeter of the room or at the end of rows, that draw in warm air, cool it and discharge it into a raised floor, a contained aisle or the open room. The difference is where the cooling comes from.

A CRAC contains its own refrigeration compressor, like a large precision air conditioner. A CRAH contains only a chilled water coil, fans and controls, and depends on a central chilled water plant. That one difference shapes efficiency, scalability, redundancy, free cooling options and maintenance.

How a CRAC works

A CRAC is a direct expansion (DX) refrigeration system. Inside the cabinet are a compressor, evaporator coil, expansion valve and fans. The heat absorbed by the evaporator must be rejected outside the room, by one of several methods:

• Air-cooled: refrigerant piping runs to a remote outdoor condenser.
• Water-cooled: a condenser in the unit rejects heat to a condenser water loop and cooling tower.
• Glycol-cooled: a condenser rejects heat to a glycol loop and outdoor dry cooler. Many glycol units add a second coil, often called an economizer or free cooling coil, that uses cold glycol directly when outdoor temperatures are low enough.
• Pumped refrigerant economizer: in cold weather, a refrigerant pump replaces the compressor and circulates refrigerant to the outdoor condenser.

Because each CRAC is a self-contained cooling system, small rooms can be cooled without a central plant.

How a CRAH works

A CRAH has a chilled water coil, a control valve, fans (now commonly EC fans or plenum fans with variable speed) and controls. Chilled water comes from a central plant of chillers, pumps and heat rejection equipment. The CRAH controls airflow and modulates the chilled water valve to maintain supply air temperature or rack inlet temperature.

Because the refrigeration happens in the central plant, the CRAH itself is simpler, and the plant can use large, efficient chillers with waterside economizers. See chilled water plant design for data centers.

Side-by-side comparison

Efficiency considerations

Efficiency differences come mostly from the refrigeration and heat rejection, not from the cabinet.

• Compressor efficiency: large central chillers are typically more efficient than many small DX compressors, especially at the warm chilled water temperatures that containment allows.
• Free cooling hours: a chilled water plant designed for warm supply water, such as 15 to 20°C (59 to 68°F), can run without chillers for a large share of the year in most Canadian cities. CRACs with glycol economizers or pumped refrigerant can also achieve substantial free cooling, but integrated partial economizer operation is often more limited.
• Fan energy: modern units of both types use variable speed EC or plenum fans. Fan energy depends more on airflow control and containment than on unit type.
• Latent cooling: older units often ran coils cold enough to dehumidify unnecessarily. Raising supply temperatures and coil temperatures avoids wasted latent cooling in both types.

ASHRAE Standard 127 and AHRI Standard 1360 define test methods and ratings for computer room air conditioners, and energy codes, including the NECB and ASHRAE 90.1, set minimum efficiencies for this equipment. Check the adopted code edition.

Redundancy and resilience

With CRACs, each unit is independent. N+1 redundancy in the room means one spare unit. Failure of one compressor affects only one unit. Electrical supply and controls redundancy still matter.

With CRAHs, redundancy must be considered at two levels: the room units and the central plant. Chillers, pumps, cooling towers, piping paths, valves and the plant's power supply all need appropriate redundancy, and a single pipe failure can affect many CRAHs unless the distribution is looped or dual-path. Chilled water storage tanks can provide thermal ride-through while chillers restart after a power interruption. See data center cooling redundancy for the full picture.

Water and refrigerant in the white space

Some operators prefer to keep water out of the room, which favours air-cooled CRACs. Others prefer to keep refrigerant out, which favours CRAHs. In practice, both risks are managed through design: leak detection under raised floors, drip trays, isolation valves, piping routed outside the white space where possible, and compliance with CSA B52 for refrigerant charge and machinery room requirements. As liquid cooling becomes common, water near IT equipment is increasingly accepted with appropriate safeguards.

When to use which

CRACs typically suit:

• Small server rooms and edge sites without a central plant. See server room and telecom closet cooling.
• Facilities that need to add cooling in small increments.
• Buildings where chilled water is unavailable or unreliable.

CRAHs typically suit:

• Medium and large data centers where plant efficiency and free cooling justify the central plant.
• Facilities planning for liquid cooling, since the same plant can serve coolant distribution units.
• Buildings with an existing reliable chilled water system.

Many facilities use both: CRAHs for the main halls and CRACs for small support spaces such as UPS rooms or network rooms that must stay cool if the plant is down.

Frequently asked questions

Are CRACs and CRAHs being replaced by other systems?

In large new facilities, fan walls, air handlers and in-row coolers are increasingly common, and liquid cooling is growing for high densities. Perimeter CRACs and CRAHs remain widespread in existing and medium-sized facilities.

Can a CRAC be converted to a CRAH?

Generally no. It is usually replaced. Some manufacturers offer dual-cooling units with both a DX circuit and a chilled water coil, which suit transitions or extra redundancy.

What supply temperature should CRAHs and CRACs deliver?

With containment, supply temperature can be set within the ASHRAE recommended inlet range, commonly in the low to mid 20s °C, which improves efficiency for both unit types.

Key takeaways

• CRACs have their own compressors; CRAHs use chilled water from a central plant.
• CRACs suit small rooms and incremental growth; CRAHs suit larger facilities with efficient plants.
• Efficiency depends mainly on refrigeration, heat rejection and free cooling hours.
• CRAH redundancy must include the central plant and distribution, not only room units.
• Leak detection, piping routes and CSA B52 compliance manage water and refrigerant risks.