When the air conditioner stops in a server room, the temperature rises within minutes. The servers first slow down, then protect themselves and shut off. That is why data center cooling is a different engineering problem from cooling an office.
The situation we see most often in the field is a wall-mounted split air conditioner installed in the server room. In the first years it seems to get by; but as the load grows, problems begin in the summer months and at hours when night maintenance cannot be done. Below we explain why precision air conditioning is a separate class of equipment and what to look at when selecting it.
What is precision air conditioning?
Precision air conditioning (CRAC or CRAH) is an industrial air conditioning unit built to keep temperature and humidity within a narrow range 24/7 in critical areas such as data centers, server rooms and telecom rooms. In the CRAC type, the compressor and refrigerant circuit are inside the unit; in the CRAH type, cooling is done with chilled water from a chiller.
These units are designed to run 8,760 hours a year. The fans, compressors, control board and sensors are suited to continuous operation; the unit runs self-diagnostics and reports problems as alarms.
How is it different from comfort air conditioning?
The most fundamental difference is the sensible heat ratio (SHR). Comfort air conditioners are designed for people and spend a significant part of their capacity condensing moisture in the air; their sensible heat ratios are usually around 0.65–0.75. The heat produced by servers, on the other hand, is almost entirely sensible. Precision air conditioners are therefore selected to operate at a sensible heat ratio of 0.90 and above.
In practice this means: a comfort air conditioner and a precision air conditioner with the same capacity on the label do not remove the same heat in a server room. The comfort unit removes unnecessary moisture, the room's humidity drops and the risk of static electricity increases.
| Feature | Comfort air conditioner | Precision air conditioning |
|---|---|---|
| Sensible heat ratio | Usually 0.65–0.75 | 0.90 and above |
| Working time | Seasonal, a few hours a day | 8,760 hours a year |
| Humidity control | None or limited | Humidification and dehumidification |
| Airflow rate | Low | High |
| Alarms and monitoring | Generally none | BMS, SNMP, Modbus, dry contact |
| Winter operation | Limited at low outdoor temperatures | Low ambient kit or free cooling |
Why does a split air conditioner fall short in a server room?
A split air conditioner is not suited to continuous operation or to cooling in winter. When the outdoor air drops below zero, many split units run unstably in cooling mode or go into protection; yet servers produce heat in winter too. In addition, split units mostly have no interface that reports faults, so when the unit stops, it takes a temperature alarm for anyone to notice.
Another problem is air distribution. A split air conditioner blows air into the upper part of the room; not enough cold air reaches the front of the cabinets while hot air at the back collects at the ceiling. The result is that some servers overheat while the average room temperature looks normal.
If a split air conditioner is used as a temporary solution in a small server room, set up at least two units, a rotation (alternating operation) scheme and an independent temperature alarm. This is not a permanent solution, but it reduces dependence on a single unit.
How is the right capacity determined?
Precision air conditioning capacity is calculated starting from the electrical power of the IT load in the room. Practically all the electrical energy going to the servers turns into heat; UPS losses, lighting, heat through the walls and, if any, the human load are added to this. Then growth allowance and redundancy are considered.
For example, in a room with a 20 kW IT load, the total heat load with the additional items usually comes to the 22–25 kW range. Splitting this load across two or three units instead of one lets cooling continue during maintenance and lets the standby unit take over in case of a fault. We explained redundant installation in detail in our N+1 redundancy article.
How should airflow be arranged?
Is your server room really safe?
We measure cabinet inlet temperatures, capacity and redundancy on site and give a written report.
The basic rule of airflow in a data center is that cold air goes to the front of the cabinets and hot air goes directly to the air conditioner's intake. To do this, cabinets are arranged in a hot aisle and cold aisle layout, with their fronts facing each other.
- Raised floor: The precision air conditioner pushes cold air under the floor, and perforated floor tiles deliver the air to the cold aisle.
- Aisle containment: Enclosing the cold or hot aisle with doors and ceiling panels largely prevents hot and cold air from mixing.
- Blanking empty slots: If blanking panels are not installed in empty U slots in the cabinets, hot air returns to the front of the cabinet.
- Cable penetrations: Open cable holes in the floor let cold air escape to the wrong place; they should be sealed with brush grommets.
These arrangements often eliminate hot spots without buying new equipment, and let the air conditioners operate at a higher return temperature and therefore more efficiently.
What temperature and humidity range is targeted?
The guidelines ASHRAE publishes for data processing environments generally give the recommended temperature range for server inlet air as 18–27 °C. For humidity, a range defined by dew point and an upper relative humidity limit is recommended. Measurement should be taken not in the middle of the room, but at the front of the cabinets, at the point where the server takes in air.
In practice, many facilities keep inlet air at around 22–24 °C to stay on the safe side. You can find the detailed values and where to place sensors in our server room temperature article.
Why are maintenance and monitoring critical?
Precision air conditioning is a unit that silently loses capacity when maintenance lapses. A dirty filter reduces airflow, a dirty condenser causes high-pressure alarms in summer, and a scaled humidifier cylinder disrupts humidity control. That is why maintenance in critical areas is usually done every three months, and filter condition is checked more often.
As important as maintenance is that alarms reach the right person. The unit's high temperature, high pressure, water leak and fan failure alarms should be connected to the building management system or to SMS/email notification, and who responds at night and on weekends should be determined in writing.
Where should you start with data center cooling?
The first step is to measure the current situation: IT load, cabinet inlet temperatures, the capacity and age of existing units, and the alarm infrastructure. Replacing equipment without this data often ends up with a system that is either oversized or has no redundancy.
At YOİ Mühendislik, we take these measurements during the site survey in server rooms and data centers and present a capacity and redundancy recommendation in writing. For existing units, our precision air conditioning maintenance and service also covers breakdown response; pricing is determined during the survey according to the number of units and the scope.
Frequently asked questions
Can a normal split air conditioner be used in a server room?
It can be used temporarily in very small, non-critical rooms, but it is not a permanent solution because of continuous operation, winter cooling, humidity control and the lack of alarms. If it is used, at least two units and an independent temperature alarm should be installed.
What is the difference between CRAC and CRAH?
A CRAC is a precision air conditioner with a compressor and a refrigerant cooling circuit inside. A CRAH has no compressor and cools with chilled water from a chiller; it is generally preferred in large data centers.
How often should precision air conditioning be maintained?
In critical areas, a comprehensive maintenance is usually done every three months, and filters are checked more often. The exact interval is set by the manufacturer's recommendation and the dust level of the environment.
Does precision air conditioning also work in winter?
Yes. Precision air conditioners are equipped with kits that work at low outdoor temperatures; some models have a free cooling feature that takes advantage of cold outdoor air in winter, significantly reducing energy consumption.