What is a CRAC unit?
A CRAC unit (Computer Room Air Conditioning unit) is a precision air conditioning system that controls temperature, humidity and airflow in server rooms, data centres and other critical rooms. Unlike comfort air conditioning built for people, a CRAC unit is built for heat-producing IT equipment that runs 24 hours a day, every day.
In HVAC terms, a CRAC unit is a high-sensible-cooling fan coil with tight control. Sensible cooling means it removes heat without removing much moisture, because IT rooms need stable humidity rather than heavy dehumidification. People search for this equipment as a CRAC unit, CRAC system, CRAC AC unit, CRAC cooling unit or CRAC air conditioning; they all describe the same class of precision cooling.
The core CRAC unit parts are similar across brands. The diagram and tables below show how those parts sit in the heat path and what each one does during a service visit.
- Air filters: clean the return air before it reaches the coil.
- Cooling coil: transfers heat from room air into refrigerant or chilled water.
- Supply fans: push cooled air to the racks (EC fans on modern units).
- Compressor (DX) or chilled water valve (CHW): drives the cooling stage.
- Condensate tray and drain: remove water that forms on the coil.
- Humidifier: holds room humidity inside the target band.
- Controller, alarms and BMS points: run setpoints and report faults.
1. What a CRAC unit diagram should show
A useful CRAC diagram shows the heat path, not only the box shape. The heat starts at servers and network gear, passes into the room air, enters the cooling unit, crosses the cooling coil, and is rejected to a condenser, cooling tower, dry cooler or chiller plant.
The diagram should separate three ideas: the air path inside the room, the refrigeration or chilled water circuit inside the cooling plant, and the controls layer that links alarms, sensors, BMS points and unit rotation.
The most useful diagrams label the parts that technicians inspect during CRAC maintenance: filter bank, return air sensor, supply fan, cooling coil, compressor or chilled water valve, condensate tray, humidifier, controller, alarm relay and BMS point set.
Practical tip
If a diagram does not show hot air returning to the unit and cold air reaching the rack face, it is not enough for maintenance planning.
2. DX CRAC unit diagram
A direct expansion CRAC unit has an indoor unit with a cooling coil, fans, controls and usually a compressor. Refrigerant carries heat from the indoor coil to an outdoor condenser. This is the common pattern for small and mid-size server rooms because it does not need central chilled water plant.
| Part | What it does | Service check |
|---|---|---|
| Return air path | Brings warm room air back to the unit | Check blocked grilles, bypass air and hot spots |
| Cooling coil | Transfers heat from air into refrigerant | Clean coil face and check pressure drop |
| Compressor | Moves refrigerant through the circuit | Check current draw, superheat and fault history |
| Outdoor condenser | Rejects heat to outside air | Clean coil, test fans and confirm clear airflow |
| Controls and alarms | Runs setpoints, staging and alerts | Test alarm outputs and BMS point mapping |
3. CHW CRAH diagram
A chilled water CRAH has no compressor inside the room unit. The indoor unit is a fan coil: warm return air passes across a chilled water coil, and heat is carried back to the chiller plant through the water loop. Many Australian sites still call this a CRAC because the service category is familiar.
The maintenance focus changes with CHW systems. Refrigerant work moves to the chiller plant, while the indoor unit service focuses on valve control, actuator response, coil condition, condensate, fan performance and temperature delta across the chilled water loop.
4. Hot-aisle and cold-aisle airflow diagram
The room airflow path matters as much as the cooling unit. Cold air must reach the rack intake face without mixing with hot exhaust air. Hot aisle and cold aisle layouts, blanking panels and containment all reduce bypass and recirculation.
Caution
A CRAC unit can be correctly serviced and still fail to cool the room if rack airflow is wrong. Missing blanking panels, open cable cut-outs and poor containment often cause the fault.
5. CRAC vs CRAH in one table
| Topic | DX CRAC | CHW CRAH |
|---|---|---|
| Heat carrier | Refrigerant between indoor unit and outdoor condenser | Chilled water between room unit and chiller plant |
| Compressor location | Usually inside the CRAC or paired outdoor system | At the chiller plant, not in the room unit |
| Best fit | Small to mid-size server rooms and edge sites | Large data halls and high-density rooms with central plant |
| Service focus | Refrigerant circuit, compressor, condenser and airflow | Valves, coil, water delta, fans and controls |
| Compliance focus | AS/NZS 5149, ARC licensing, AS/NZS 1668.2 | AS/NZS 1668.2, AS/NZS 3666 where water systems apply |
6. What to check during a site walk
When reviewing a CRAC or CRAH room, follow the heat path. Start at the rack intake temperature, then inspect the hot aisle, return path, filter condition, coil face, condensate, fans, control panel, alarms and heat rejection plant.
- Confirm the unit type: DX CRAC, CDW CRAC, CHW CRAH, in-row or rear-door heat exchanger.
- Record supply air and return air temperatures at each unit.
- Check rack intake temperature at the top, middle and bottom of each rack row.
- Look for bypass air through open floor tiles, missing blanking panels and cable cut-outs.
- Check filters, coil cleanliness, fan speed, condensate and alarm history.
- Confirm the heat rejection plant is working: condenser, cooling tower, dry cooler or chiller.