In a hot aisle and cold aisle layout, rows of racks face each other front to front and back to back. Equipment draws cool air from the shared cold aisle and exhausts hot air into the shared hot aisle, so the two streams do not mix. Combined with blanking panels and some form of containment, this simple arrangement does more for server room temperatures than adding cooling capacity.
Why layout matters
Almost all IT equipment pulls air in at the front and pushes it out the back. If racks all face the same way, each row breathes the exhaust of the row in front. If gaps in racks let hot air circle back to the front, servers at the top of the rack run hot even when the room thermostat looks fine. Most hot spots come from recirculation like this.
Hot and cold aisles
- Cold aisles are where rack fronts face each other. Cool supply air is delivered here, through perforated floor tiles, overhead ducts or in-row coolers.
- Hot aisles are where rack backs face each other. Exhaust is collected here and returned to the cooling units.
Cooling units should be placed so their returns draw from hot aisles. In small rooms with only one row, the same principle applies: keep a defined intake side and exhaust side, and make sure the cooling unit's return pulls from the exhaust side.
Containment
Containment physically separates the two air streams:
- Cold aisle containment encloses the cold aisle with doors at the ends and a roof or panels above. The rest of the room becomes the hot return.
- Hot aisle containment encloses the hot aisle and ducts exhaust back to the cooling units. The rest of the room stays cool and comfortable.
- Chimney cabinets duct the exhaust from each rack straight into a ceiling return.
Each works well when designed with the cooling system. Containment must be coordinated with sprinklers and fire detection, so that panels and roofs do not block sprinkler discharge or smoke reaching detectors. Check the requirements with your fire protection designer and the authority having jurisdiction.
Inside the rack
- Blanking panels in every empty rack unit
- All equipment facing the same direction, with intakes on the cold side
- Side-breathing switches fitted with airflow kits or ducts so they do not draw hot air
- Brush grommets or seals at cable openings in rack tops and floors
- Heaviest equipment, such as UPS units and storage, at the bottom
- Rack PDUs mounted so they do not block exhaust
- Patch cords managed with horizontal and vertical cable managers
Blanking panels are the cheapest fix. A rack full of gaps lets hot exhaust return straight to equipment intakes. Filling empty spaces with blanking panels is low-cost and often lowers inlet temperatures noticeably.
Planning the room
Clearances
Leave enough space in front of and behind racks to install and remove equipment and to work safely. Electrical equipment such as panels, UPS units and PDUs has working-space requirements under the electrical code that must be respected. Allow space for rack doors to swing and for equipment to be moved in on carts.
Cabling
Overhead cable tray or ladder rack keeps cabling out of underfloor airflow and is easier to maintain. Separate power and data pathways. Plan fibre and copper routes between rows, and leave room to grow. See structured cabling standards.
Weight and seismic restraint
Loaded racks are heavy. Check floor loading with a structural engineer, especially on raised floors and upper storeys. In seismic regions such as coastal British Columbia, racks and equipment may need seismic bracing and anchorage designed to the applicable building code.
Growth
Reserve rack space, power and cooling for expansion, and place higher-density racks where cooling is strongest. A room planned for a few racks often ends up with twice as many.
Checking the result
Measure air temperature at the front of racks at the bottom, middle and top. Inlet temperatures that vary widely, or are much warmer at the top, point to recirculation or poor air delivery. Temperature sensors at rack inlets, connected to monitoring, keep watch over time. Cooling principles are covered in data centre cooling.
How ehZee can help
Through our data centre design and build service we lay out racks, airflow, cabling and power together, coordinate with the other trades, and verify temperatures and installation once the room is built.
Common questions
Is containment worth it for a small server room?
For one or two racks, blanking panels, sealed openings and a clear intake and exhaust side usually do most of the job. Containment pays off as density and rack count rise.
Which is better, hot or cold aisle containment?
Both work. Hot aisle containment keeps the rest of the room cool; cold aisle containment is often easier to retrofit. The choice depends on the room and the cooling system.