Hot aisle / cold aisle layout: airflow basics for small data centers

1. How the layout works, and why mixing hurts

Most IT equipment breathes front to back: fans pull room air in at the front, push it across the boards and exhaust it at the rear, warmer. Face two rows front to front and their intakes share one aisle; turn the next row back to back and the exhausts share another. The cooling supplies the first and takes its air back from the second.

Two things go wrong when the streams mix:

• Recirculation: hot exhaust finds its way back into an inlet: over the top of the rack, around the row ends, through an empty U. The top of a rack and the racks at the end of a row see it first.
• Bypass: cold supply air returns to the cooling unit without passing through any IT, through a perforated tile in the hot aisle, a cable cut-out, or an oversized supply.

Both lead to the same bad trade: the cooling runs colder supply air, or more fans, to keep the worst inlet in range, while its return air comes back cooler and the unit delivers less of its capacity.

The basics in one place: cold aisles about 1.2 m wide (two 600 mm floor tiles); supply through perforated tiles in the cold aisles of a raised floor, or from overhead or in-row units, and return from the hot aisles; about 300 m³/h (175 CFM) of air per kW at a 10 K rise; 18–27 °C at the inlet (ASHRAE); fixes in cost order, from blanking panels and seals to containment to more cooling.

2. Lay out the rows

• Rows front to front, then back to back. With four rows: rows 1 and 2 face each other (cold aisle), rows 2 and 3 turn their backs (hot aisle), rows 3 and 4 face each other again.
• Aisle width. A common choice is a cold aisle of two 600 mm floor tiles, about 1.2 m, so a full row of perforated tiles fits; hot aisles are often narrower. TIA-942 covers data center spaces and pathways, but take your aisle widths from your design and local codes. Check the clearances your rack and server vendors need to open doors and slide out the deepest device.
• Orientation of odd devices. Switches often breathe side to side or front to back depending on the fan module. Order the fan option that matches how the switch is mounted (port side in the hot aisle or the cold aisle), or fit a duct. Mounted backwards, a switch breathes hot aisle air.
• No holes in a row. A missing rack, an open side or an empty row end lets the hot aisle into the cold aisle. Fill gaps with blank panels or an empty rack and close the row ends.

Cooling unit placement:

Cooling	Supply	Return	Place it
Perimeter units with a raised floor	Under the floor, up through perforated tiles in the cold aisles	From the room, ideally from the hot aisles	At the ends of the hot aisles, perpendicular to the rows; perforated tiles only in cold aisles
Perimeter units, no raised floor	Into the room or overhead ducts toward the cold aisles	From the room or a ceiling plenum above the hot aisles	Facing the cold aisles, returns over the hot aisles
In-row units	Into the cold aisle from inside the row	From the hot aisle behind them	Between racks, close to the heaviest loads
Rear-door heat exchangers	The rack’s own exhaust, cooled at its rear door before it reaches the room	The heat leaves through the door’s water or coolant loop	On the racks too dense for room air

Small rooms and single rows. With one row, face the racks toward the cooling supply and keep the return path at the back of the row. With a wall-mounted split unit, put its supply at the rack fronts and its intake behind them; a unit blowing across the backs of the racks feeds them their own exhaust.

3. Blanking panels, gaps and containment

Close gaps before you buy anything, cheapest first:

1. Blanking panels in every empty U. An empty U is an open window from the hot aisle to the cold aisle.
2. Brush grommets or seals on every cable cut-out in the floor and the rack.
3. Side gaps and row ends closed; no perforated tiles in hot aisles or under racks.
4. Containment, once the gaps are closed:

	Cold aisle containment	Hot aisle containment
What is enclosed	The cold aisle: doors at the ends, a roof or panels above the racks	The hot aisle, returned to the cooling through a ceiling plenum or in-row units
The rest of the room	Runs at hot aisle temperature	Stays at cold aisle temperature
Retrofit	Usually easier: doors and roof panels on existing rows	Needs a return path: ducts, plenum or in-row units
Watch out for	Equipment outside the aisle (tape, wall-mounted gear) breathing hot room air	Working conditions in the hot aisle during maintenance

The order is also the cost order: blanking panels and grommets are consumables, containment is a project, and moving or adding cooling units is the largest step.

4. Inlet temperature: the numbers to aim for

Measure at the IT equipment inlet, not at the cooling unit’s return. ASHRAE TC 9.9, Thermal Guidelines for Data Processing Environments (2015 table) gives:

Range	Dry-bulb inlet temperature
Recommended, classes A1 to A4	18–27 °C
Allowable, class A1 (enterprise servers and storage)	15–32 °C
Allowable, class A2	10–35 °C
Allowable, class A3 / A4	5–40 °C / 5–45 °C

The fifth edition (2021) adds class H1 for high-density air-cooled equipment, with a narrower recommended 18–22 °C. Your hardware’s class is in its specification.

The three thresholds DC Sim reports when cooling fails, 27, 32 and 35 °C, are the top of the recommended range, the top of A1 allowable and the top of A2 allowable.

5. How much air a rack needs

From the sensible-heat equation for air (standard air at sea level, about 1.2 kg/m³ and 1.0 kJ per kg per kelvin), every kilowatt of IT load at a 10 K rise from front to back needs about 300 m³/h, roughly 175 CFM. The servers’ fans set the rise, so the cooling must deliver at least what the racks pull, or the shortfall comes from recirculation.

Rack load	Airflow at a 10 K rise	Airflow at a 15 K rise
5 kW	1,500 m³/h (880 CFM)	1,000 m³/h (590 CFM)
10 kW	3,000 m³/h (1,760 CFM)	2,000 m³/h (1,170 CFM)
15 kW	4,500 m³/h (2,640 CFM)	3,000 m³/h (1,760 CFM)
20 kW	6,000 m³/h (3,510 CFM)	4,000 m³/h (2,340 CFM)

Every watt a rack draws is a watt of heat in its hot aisle, so the rack power calculator at the top of this page is also a heat calculator. When a rack needs more air than its tile or its door can pass, rear-door heat exchangers or direct-to-chip cooling are the next step: see liquid cooling and the CDU manifold leak.

6. Hot spots: find the cause before adding cooling

Behind a warm cold aisle or a hot rack, check these causes, cheapest fix first:

1. Missing blanking panels or open cable cut-outs in or near the hot rack.
2. A rack at the end of a row, or a gap in the row, with hot air wrapping around.
3. A device mounted backwards, or a switch with the wrong fan option.
4. Perforated tiles in the wrong place: in the hot aisle, under a rack, or too few in front of the hottest rack.
5. The rack is simply denser than the cooling near it, or the cooling unit that serves it is undersized, failed or set too warm.

Fix 1 to 4 before buying cooling. For 5, the answer is a load move, a cooling change, or liquid.

7. What DC Sim’s Air layer estimates

DC Sim builds the room from the template’s sheets and estimates the air with a zonal model, not CFD:

• Aisles from the Racks sheet: rack, row and position place the racks; aisles alternate by row unless front_faces (A or B) says otherwise, and the import report marks the alternating default as assumed. Change front_faces and the aisle flips.
• Cooling from the Cooling sheet: name, type, capacity_kw, serves, model, and chilled_from (the chiller that feeds a CRAH; empty means a DX unit). Heat above cooling capacity is flagged at import.
• The Air view: rack inlet and exhaust temperatures, a floor heat map and airflow particles. Each rack front is coloured per U band from the model’s inlet profile, which adds a recirculation term that grows toward the top of the rack. Monitor shows the hottest inlet as a KPI, and temperature history from your monitoring goes into the Readings sheet so the model fits your room.
• Placement checks: adding a device in Design mode warns about side or rear intake and about heat against the cooling at the busy hour.
• Stop a cooling unit: fail a CRAH, chiller or AC unit and read the minutes until each rack inlet passes 27, 32 and 35 °C, estimated. In the demo hall, AC-1 stopping at peak load takes the hottest inlet past 27 °C after 10:35, 32 °C after 21:15 and 35 °C at 28:00, peaking at 35.9 °C with no devices off; the remaining units AC-2 and AC-3 run at 95 % of capacity. The clip below is that run.

What it does not do: containment is not a template column, so an imported room uses the model’s default for it. Use DC Sim to find the hot racks, the missing capacity and the minutes you have; have a mechanical engineer’s airflow study done before you build containment or a new hall. The facility layers page describes the Air, Water and Power layers, and Simulate lists the whole scenario library.