Computer rooms, server rooms and telecom spaces need cooling every hour of every day. The heat they generate is almost entirely sensible, from electronic equipment, with very little moisture added by people. Comfort air conditioning, designed for offices and homes, is built around a different load: a mix of sensible and latent heat, occupied hours, and comfort for people rather than protection of equipment.
A computer room air conditioner (CRAC) is designed for the IT environment. Understanding the differences helps owners decide when a comfort unit is acceptable, typically in a small, low-risk closet, and when precision cooling is worth the extra cost.
The key differences
| Characteristic | Comfort cooling | Computer room air conditioner |
|---|---|---|
| Sensible heat ratio | Typically about 0.65 to 0.80 | Typically about 0.85 to 1.0 |
| Design run hours | Occupied hours, seasonal | 8,760 hours per year |
| Airflow per kW of cooling | Lower | Higher, to match IT airflow needs |
| Temperature control | Thermostat with wide deadband | Tighter control, often on supply or rack inlet temperature |
| Humidity control | Incidental dehumidification only | Optional humidification and dehumidification control |
| Low ambient operation | Often limited | Designed for year-round cooling, including winter |
| Redundancy and alarms | Rarely included | Built-in alarms, communication and redundancy features |
| Rating standard | CSA C746 / AHRI 210/240 or 340/360 | ASHRAE Standard 127 for computer room units |
Sensible heat ratio
Sensible heat ratio (SHR) is the share of a cooling unit's capacity that lowers air temperature, rather than removing moisture. Comfort units are designed to remove a significant amount of moisture, because offices and homes have people, outdoor air and latent loads. In an IT room, that dehumidification is mostly wasted. Worse, continuous dehumidification can drive humidity well below recommended levels, and the unit's sensible capacity may be lower than its nominal rating suggests.
A CRAC is designed with higher airflow and coil characteristics that give an SHR close to 1.0 at typical IT room conditions, so nearly all of its capacity goes into removing equipment heat.
Run hours and durability
Comfort equipment is designed for seasonal operation and moderate duty cycles. Running 24/7, including through Canadian winters, puts stress on compressors, fans and controls. Many comfort units are not designed to cool reliably when outdoor temperatures are well below freezing unless fitted with low ambient controls. CRAC units are built for continuous duty, with components selected for long run hours and year-round operation.
Controls and monitoring
CRACs usually include microprocessor controls that maintain supply or return air temperature within a narrow band, manage multiple units with lead-lag rotation, and provide alarms for high temperature, water leaks, filter status and component failure. They commonly integrate with building automation or data centre infrastructure management systems. Comfort thermostats usually offer none of this.
Temperature and humidity targets
The ASHRAE TC 9.9 thermal guidelines are the most widely used reference for IT environments. For most current IT equipment, the recommended inlet air temperature range is 18 to 27°C (64.4 to 80.6°F), with wider allowable ranges depending on equipment class. Humidity recommendations are expressed in terms of dew point and relative humidity. Our guide to ASHRAE TC 9.9 thermal guidelines explains the classes and ranges.
Two points often surprise owners:
- IT rooms do not need to be cold. Cooling to 18°C or lower wastes energy and does not improve reliability within the recommended range.
- Rack inlet temperature is what matters, not room return temperature. A thermostat on a wall far from the equipment can mislead.
When is comfort cooling acceptable?
Comfort equipment, such as a ductless mini-split or a branch from the building system, can be reasonable for:
- Small telecom closets and network rooms with low heat loads
- Rooms where brief temperature excursions would not cause serious business impact
- Spaces with a backup cooling method or where equipment can tolerate a short outage
Even then, select units capable of year-round cooling and low ambient operation, size them on sensible load, set them to a reasonable setpoint, and add a temperature alarm. Our guide on server room and telecom closet cooling covers small rooms in detail.
Comfort cooling is a poor choice when:
- The room relies on the building's central system, which shuts down at night, on weekends or for maintenance
- Heat loads are significant, typically more than a few kilowatts
- The equipment is business-critical
- Redundancy is required
Types of precision cooling
| Type | Description | Typical use |
|---|---|---|
| DX CRAC (air-cooled) | Self-contained compressor unit with remote outdoor condenser | Server rooms and small data centres |
| DX CRAC (water or glycol-cooled) | Condenser cooled by a water or glycol loop, often with a dry cooler outdoors | Where long refrigerant runs are impractical; glycol allows free cooling coils |
| CRAH (chilled water) | Air handler with chilled water coil, no compressor | Larger facilities with a chiller plant |
| In-row units | Cooling units placed between racks | Higher density rows |
| Rear door heat exchangers | Coils on rack doors | High density racks |
The distinction between CRAC and CRAH is explained in CRAC vs CRAH units.
Free cooling in Canadian climates
Canada's cold climate is a major opportunity for IT cooling. Glycol-cooled CRACs with an economizer coil, or CRAHs on chilled water plants with waterside economizers, can provide cooling with compressors off for a large share of the year in most Canadian cities. Air-side economizers are another option, with filtration and humidity control considerations. Comfort units rarely offer these features.
Redundancy
Critical rooms need redundant cooling so that a single unit failure or maintenance shutdown does not cause overheating. Common practice is N+1, with lead-lag rotation and automatic failover. Power supply to cooling equipment, including emergency power and restart behaviour after outages, is just as important.
Frequently asked questions
Why does my server room comfort unit freeze up in winter?
Many comfort units lack low ambient controls. At low outdoor temperatures, head pressure drops, the evaporator gets too cold, and the coil frosts. Low ambient kits or precision units designed for winter operation solve this.
Do server rooms need humidity control?
Modern IT equipment tolerates a wide humidity range. Many facilities no longer humidify actively but monitor dew point and use static control practices. Very dry conditions in Canadian winters deserve attention, especially where equipment handling or static-sensitive work occurs.
Can I use the building's central air handler?
Only if it runs 24/7, has enough sensible capacity, and its maintenance and shutdowns are coordinated with IT needs. In most buildings, dedicated cooling is more reliable.
Key takeaways
- IT loads are almost entirely sensible and continuous; comfort cooling is designed for mixed loads and occupied hours.
- CRAC units have high sensible heat ratios, continuous duty components, tight controls and alarms.
- Comfort units can be acceptable for small, low-risk rooms if selected for year-round operation and monitored.
- Follow ASHRAE TC 9.9 recommendations and control based on rack inlet temperature.
- Use free cooling and redundancy in critical rooms; data centre cooling design can help match the solution to the risk.
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