Almost every watt of power that goes into IT equipment comes out as heat, and that heat has to be removed continuously. Good data centre cooling delivers cool air to the front of the equipment, collects hot exhaust from the back without letting the two mix, and does it with redundancy so a single failure does not cause an outage. Whether you run a small server room or a full data centre, those principles are the same.
How much heat are we talking about?
The cooling load is close to the electrical load of the IT equipment, plus heat from UPS systems, lighting, people and the building envelope. A rack of servers drawing several kilowatts produces several kilowatts of heat, around the clock. As equipment gets denser, the heat per rack rises. Cooling design starts with a realistic estimate of current and future IT load, by rack and by room.
Temperature and humidity targets
ASHRAE Technical Committee 9.9 publishes thermal guidelines for data processing environments, including a recommended range for air entering IT equipment. The recommended inlet range is broader and warmer than many older server rooms were run, which is good news for energy use. What matters is the temperature of air entering the equipment, not the temperature at the room thermostat. Check your equipment manufacturer's limits and the current ASHRAE guidance when setting targets.
Humidity needs attention too: very dry air increases static discharge risk, and very humid air risks condensation. Modern guidance focuses on dew point and allows a wider range than older practice.
Cooling options
Room-based cooling
CRAC (computer room air conditioner) units have their own refrigeration compressors. CRAH (computer room air handler) units use chilled water from a central plant. Both are placed around the room and often supply cool air under a raised floor or through ducts. They work well with good airflow management, but can struggle with high-density racks.
Row-based and rack-based cooling
In-row coolers sit between racks and cool the air close to where it is used. Rear-door heat exchangers mount on the back of a rack and remove heat from the exhaust as it leaves. Both suit higher densities and rooms where raised floors are not practical.
Economizers
In much of Canada, outdoor air is cool for a large part of the year. Air-side or water-side economizers use that cold air or water to cool the room with little or no compressor use, which can cut energy use considerably. Filtration, humidity control and freeze protection need careful design in cold climates.
Liquid cooling
For very dense equipment, cooling liquid is brought directly to the chips or the equipment is immersed in a dielectric fluid. Liquid cooling is becoming more common with high-performance and AI computing, and brings its own plumbing, leak detection and maintenance requirements.
Airflow management
Most server room overheating is caused by poor airflow rather than lack of cooling capacity. The basics:
- Arrange racks in hot and cold aisles, with equipment fronts facing the cold aisle
- Install blanking panels in every empty rack space
- Seal cable openings in raised floors and rack tops with brush grommets
- Use aisle containment to stop hot and cold air mixing
- Keep cable bundles from blocking airflow under floors and at rack rears
- Place perforated tiles only in cold aisles, sized to the load
Layout is covered in more detail in rack layout and hot and cold aisles.
Small server rooms need dedicated cooling. A server closet that relies on the building's comfort air conditioning will overheat when that system sets back at night, on weekends or in winter. Provide cooling that runs all the time, and a temperature alarm that tells someone when it fails.
A typical case: one rack runs hot
A server room has plenty of cooling capacity on paper, yet one rack keeps triggering high temperature alerts. A methodical check usually finds the cause without adding cooling:
- Measure inlet temperatures. Take readings at the top, middle and bottom of the rack front, and at the racks next to it. Hot air at the top of the front usually means exhaust is curling over the rack and being drawn back in.
- Look for gaps. Missing blanking panels, open rack sides, or an empty rack position next door let hot exhaust flow back to the front.
- Check equipment orientation. A switch or appliance mounted backwards, or one with side-to-side airflow, can blow hot air into the cold aisle.
- Check the supply. On a raised floor, confirm a perforated tile is in front of the rack, that it is not blocked by cable under the floor, and that tiles have not been moved to the hot aisle.
- Compare the load. If the rack has been filled with denser equipment than the room was planned for, it may need in-row or rear-door cooling, or the load spread across more racks.
- Retest and record. Recheck inlet temperatures after each change, so you know which fix worked.
Planning the cooling for a new room
For a new server room or data centre, the cooling design follows from a few early decisions. Settle these before choosing equipment:
- IT load today and at the end of the plan, by rack, including how many racks may be high density
- Redundancy: how many cooling units can fail or be serviced while the room stays within temperature
- Heat rejection: where the heat goes, whether to a chilled water plant, roof condensers or dry coolers, and whether economizer hours are worth designing for
- Power: whether cooling runs on emergency power, and how long the room can ride through while it restarts
- Monitoring: inlet sensors per rack, leak detection under units and pipes, and where alarms go
- Maintenance access: space to service units, change filters and clean coils without shutting down
These decisions also affect the building's mechanical systems. For background on how building HVAC fits together, see how HVAC systems work.
Redundancy and monitoring
Cooling is as critical as power: without it, equipment shuts itself down within minutes to hours, depending on load and room size. Common practices include N+1 cooling units, so one can fail or be serviced without overheating, cooling units on emergency power, and temperature sensors at rack inlets connected to an alarm or building automation system. See data centre power and UPS for how cooling fits into the power plan.
How ehZee can help
Our team brings a mechanical systems background to cooling design as well as the network side. Through our data centre design and build service we start with what the room must support, then design and build the cooling, power and layout together and verify the result.
Common questions
Can we run the server room warmer to save energy?
Often yes, within the ASHRAE recommended range and your equipment's limits, provided airflow is well managed. Raise setpoints gradually and monitor rack inlet temperatures.
Is a raised floor necessary?
No. Many modern rooms use overhead or in-row cooling on a slab floor. The right choice depends on density, room height and cooling type.