CRAC vs CRAH: what is the difference?
Both sit on the data hall floor and both move room air. What actually differs is where the heat goes after it leaves the coil.
The options
- CRAC (Computer room air conditioner) — A room-air unit with its own direct-expansion refrigerant circuit.
- CRAH (Computer room air handler) — A room-air unit with a chilled-water coil fed from a central plant.
A CRAC and a CRAH look similar on the data hall floor and do a similar job in the room: pull warm air in, pass it across a coil, push conditioned air back toward the equipment inlets. The airflow role is close enough that the two names are often used interchangeably, including by people selling the equipment.
The difference is thermodynamic, not architectural. A CRAC carries its own refrigeration: a compressor, a direct-expansion coil and a path to reject heat outdoors. A CRAH carries no refrigeration at all — it is a coil and a fan section, and the refrigeration work happens somewhere else, in a chiller plant that also serves every other CRAH on the loop. Everything else that differs between them follows from that one fact.
Side by side
| CRAC | CRAH | |
|---|---|---|
| What removes the heat | A direct-expansion refrigerant circuit inside the unit. | A chilled-water coil; the refrigeration happens at a central plant. |
| Where the compressor sits | In the unit, on the data hall floor. | In the chiller plant, outside the white space. |
| What the unit needs from the building | Refrigerant piping and a heat-rejection path — air-cooled condenser, or a condenser-water loop. | Chilled-water supply and return piping, and a plant with capacity to serve it. |
| What a single unit failure costs | That unit’s capacity. | That unit’s capacity — but a plant or loop fault can affect every CRAH it serves. |
| How capacity is controlled | Compressor staging, or variable-speed and digital-scroll arrangements depending on the unit. | Fan speed plus coil valve modulation against the supply-water temperature the plant delivers. |
| Route to reduced compressor work | Unit-level arrangements — air-side economizer sections, pumped-refrigerant economizers. | Water-side economizer at the plant, which benefits every unit on the loop at once. |
| Where maintenance happens | Refrigeration work takes place inside the data hall. | Most refrigeration work takes place at the plant, away from live equipment. |
| What redundancy has to cover | The population of units, plus their heat-rejection path. | The population of units, plus chillers, pumps, piping and valves. |
| Where it commonly fits | Smaller rooms, edge sites and retrofits with no plant and no plan for one. | Halls where a central plant is already justified, or a campus already has one. |
The acronym describes the heat path, not the airflow
Both unit families do the same thing to air. Fans draw return air through filters, the air gives up heat to a coil, and the cooled air is delivered to the room — under a raised floor, through an overhead plenum, or straight out the front, depending on the arrangement. Nothing about “CRAC” or “CRAH” tells you the airflow direction, the fan technology or whether the room is contained.
What the acronym does tell you is what happens on the other side of that coil. In a CRAC, refrigerant boils in the coil, a compressor raises its pressure, and the heat is rejected through a condenser — to outdoor air, or to a condenser-water loop and a cooling tower. In a CRAH, chilled water warms as it passes through the coil and returns to a chiller, which does the refrigeration for the whole loop.
What a central plant actually commits you to
Choosing chilled water is not only a choice about the units in the hall. It commits the site to chillers, pumps, piping, expansion and make-up arrangements, water treatment, and a redundancy concept for all of it. The loop becomes a shared dependency: it is what allows plant-level efficiency strategy, and it is also what can take out more than one unit at a time.
That is why a CRAH population and a CRAC population need different redundancy conversations. Counting spare units answers a question about the room. It does not answer whether the chilled-water path itself can be maintained or lost without interrupting the load — which is the question the power chain lesson asks about electrical paths, applied to a mechanical one.
Efficiency is a plant question, not a label question
A CRAH is not inherently more efficient than a CRAC. It inherits the efficiency of the plant that feeds it. A large central plant with good part-load chiller performance and a meaningful number of water-side economizer hours can be very efficient, and every unit on the loop benefits from that at once. A small site with no plant cannot reach that advantage by renaming its units.
Both published efficiency guidance and the measured behaviour of real sites point the same way: the large wins come from air management, supply-temperature strategy and economizer hours, not from the unit family in isolation. A room full of efficient units still wastes energy if conditioned air bypasses the equipment and returns without doing work.
The failure domains have different shapes
With DX units, a failure is usually bounded by the unit and its own heat-rejection path. Redundancy is mostly a question of how many units the room has and whether the remaining ones can cover the load where it actually sits — a spare unit at the wrong end of a long hall does not cool the hot end of it.
With chilled water, unit-level redundancy is necessary but not sufficient. A pump, a valve, a control sequence or a section of pipe can be common to many units. Naming that common-mode dependency out loud is the same discipline the Academy applies to A/B power feeds: duplicated labels downstream do not create independence if the path upstream is shared.
At high density, neither one is the whole answer
Room air has a practical ceiling. As rack density rises — and AI and HPC deployments push it hard — the volume of air needed to carry the heat away becomes difficult to deliver and contain, whatever is producing the cold air. Rear-door heat exchangers, direct-to-chip cold plates and immersion move part or all of the heat into a liquid much closer to the source.
Those approaches do not delete the plant question; they change it. A liquid loop still needs heat rejection, still has a supply temperature, and still creates dependencies worth drawing on a diagram before anyone argues about equipment brands.
Where each one tends to fit
These are conditions commonly associated with each option, not a decision procedure. A real choice is made against site data, the adopted standards and the constraints of the specific building.
Conditions that tend to point toward DX room units
- No central chilled-water plant exists and none is planned or funded.
- The space is small, distributed or at the edge, where plant capital is hard to justify.
- A retrofit sits in a building with no loop infrastructure and no practical route for one.
- Independent per-unit operation is worth more than plant-level efficiency strategy.
Conditions that tend to point toward chilled-water room units
- A central plant already serves the site or campus, or is justified by the total load.
- The hall is large enough that plant-level efficiency and economizer hours dominate the energy picture.
- Keeping compressors and refrigeration maintenance out of the white space matters.
- The design already needs a water loop for other reasons, including liquid-cooled zones.
Where this comparison stops
Vendor naming is not reliable. Products marketed as “CRAC” units sometimes use chilled water, and the same catalogue may use both terms loosely. Read the heat-rejection path on the datasheet rather than trusting the acronym on the front panel.
This page compares two families of room-air unit. It does not select equipment. Real selection depends on available heat rejection, water strategy and availability, acoustic limits, structural and space constraints, maintenance access, the redundancy concept, local climate and the adopted standards for the site — and it is engineering work, performed by qualified people with verified site data, not a choice between two acronyms.
Frequently asked questions
What is the difference between CRAC and CRAH units?
A CRAC unit removes heat using its own direct-expansion refrigerant circuit, with a compressor inside the unit. A CRAH unit removes heat using a chilled-water coil supplied by a separate chiller plant, and contains no refrigeration of its own. The air-handling role in the room is similar; the heat path behind the coil is not.
What is a CRAH?
A computer room air handler: a room-air unit that moves data hall air across a chilled-water coil. The water warms, returns to a central plant, and the plant does the refrigeration for every unit on the loop.
Is a CRAH more efficient than a CRAC?
Not inherently. A CRAH inherits the efficiency of the plant that feeds it, so a large plant with strong part-load performance and many water-side economizer hours can be very efficient. A small site with no plant does not gain that advantage. Air management and supply-temperature strategy usually move facility energy more than the unit family does.
Can CRAC and CRAH units be used in the same facility?
Yes, and mixed estates are common. A legacy room may keep DX units while a newer hall is built around a chilled-water plant. The mixture is worth documenting, because the two populations fail and are maintained in different ways.
Do CRAC and CRAH units need a raised floor?
Neither requires one. Raised floor, overhead supply and in-row arrangements are all used with both families. The air distribution method is a separate decision from how the coil is cooled.
Which one is used in large data centers?
Large halls more often sit on chilled water, because a central plant is already justified at that scale and plant-level efficiency strategy applies to the whole loop at once. That is a tendency, not a rule — the site’s heat-rejection options and water strategy decide it.
Sources
These sources support the summary above. Use the current adopted edition, verified site information and qualified professional review for real work.