In many data centers, the cooling units produce far more cold air than the IT equipment actually draws, yet some servers still run hot. The reason is poor airflow management. Cold air escapes through gaps and openings before it reaches server inlets, and hot exhaust finds its way back to the front of the racks. Operators respond by lowering setpoints and running more units, which wastes energy without fixing the cause.
Airflow management is the set of physical measures that make cold air go where it is needed and keep hot air away from inlets. It is usually the cheapest efficiency improvement in a data center and a prerequisite for raising temperatures, using economizers or adding density.
Two problems: bypass and recirculation
Bypass airflow is conditioned air that returns to the cooling units without passing through IT equipment. It leaks through cable cutouts, gaps around racks, misplaced perforated tiles and open spaces in the floor or ceiling. Bypass air wastes fan energy and lowers return temperature, which reduces cooling unit capacity and efficiency.
Recirculation is hot exhaust air that flows back to server inlets, through empty rack spaces, over the tops of racks, around row ends or under racks. Recirculation causes hot spots, which lead operators to lower supply temperature for the whole room.
The two problems often coexist. A room can have both excess supply airflow and hot spots, because the cold air is going to the wrong places.
The main fixes
Blanking panels
Empty rack units let hot exhaust flow from the back of a rack to the front. Blanking panels close these openings. They should be installed in every unused rack position, including at the top and bottom of racks and in partially filled positions.
Tool-less snap-in panels make it more likely that panels go back in after equipment moves. Some organisations make blanking part of the change management procedure so it is not forgotten.
Rack-level sealing
Gaps between the equipment rails and the rack sides, and under the rack, allow air to bypass the equipment. Side panels or seals between the mounting rails and the cabinet frame, and bottom seals or brush strips under racks, address these paths. Gaps between adjacent racks in a row also leak air and should be closed.
Cable cutouts and floor openings
In raised-floor rooms, unsealed cable cutouts under racks are a major source of bypass. Brush grommets or sealed cutout collars let cables pass while blocking most of the air. Openings under PDUs, cooling units and electrical equipment, and gaps at the perimeter of the floor, should also be sealed. Unsealed cutouts in the hot aisle waste cold air directly into the return path.
Perforated tile placement
Perforated tiles and grates should only be in the cold aisle, in front of racks that need air. Tiles in hot aisles, open areas or near cooling units waste air and reduce underfloor pressure elsewhere. The number and open area of tiles should match the airflow demand of the racks they serve. Standard perforated tiles are commonly about 25 percent open; high-flow grates may be around 50 percent or more open and pass much more air at the same pressure.
Row ends and gaps in rows
Missing racks in a row and open row ends let hot air wrap around into the cold aisle. Filling gaps with blanking cabinets or panels, and adding end-of-row doors, closes these paths. Full aisle containment takes this to its conclusion.
Ceiling and return paths
In rooms with ceiling return plenums, return grilles should be over hot aisles, not cold aisles. Unsealed ceiling tiles and openings over cold aisles let cold air escape.
Typical findings and fixes
| Finding | Effect | Fix |
|---|---|---|
| Empty rack units without blanking | Recirculation, hot spots | Install blanking panels |
| Unsealed cable cutouts | Bypass, low floor pressure | Brush grommets or sealed collars |
| Perforated tiles in hot aisle | Bypass, low return temperature | Relocate or replace with solid tiles |
| Gaps between racks or missing racks | Recirculation around rows | Fill gaps, add panels or doors |
| Too many or too few perforated tiles | Uneven delivery | Match tile count and type to rack loads |
| Cable bundles blocking underfloor air | Uneven floor pressure | Remove abandoned cable, reroute |
| Return grilles over cold aisles | Bypass | Relocate returns over hot aisles |
A practical procedure
- Baseline: record rack inlet temperatures at top, middle and bottom of racks, cooling unit supply and return temperatures, fan speeds and IT load. Measure underfloor pressure at several points if there is a raised floor.
- Survey: walk every row and record open rack units, cutouts, tile locations, gaps and obstructions.
- Seal and blank: install panels, grommets and seals. These are low-risk changes that can be done during normal operation.
- Adjust tiles: relocate or swap perforated tiles to match demand.
- Measure again: repeat the baseline readings. Inlet temperatures should be more uniform and return temperatures higher.
- Optimize: once inlets are uniform, reduce cooling unit fan speeds, turn off unneeded units, and raise supply temperature while keeping all inlets within the ASHRAE TC 9.9 recommended range.
- Sustain: add airflow checks to change management and periodic audits.
Measuring success
Useful indicators include:
- Return temperature at cooling units: should rise after improvements.
- Inlet temperature spread: the difference between the hottest and coolest server inlets should shrink.
- Ratio of cooling airflow to IT airflow: values well above 1 indicate bypass. Well-managed rooms often operate at a modest surplus.
- Rack cooling index (RCI) and return temperature index (RTI): metrics often reported in CFD studies and assessments to describe how well inlets stay within range and how much bypass or recirculation exists.
Infrared imaging can quickly reveal leaks and hot spots across a row. See infrared thermography for how thermal imaging is used in building diagnostics.
Energy impact
Fixing airflow alone does not save energy. Savings come when the cooling system is then adjusted: fewer units running, lower fan speeds, warmer supply air and chilled water, and more economizer hours. Because fan power varies roughly with the cube of speed, even modest airflow reductions cut fan energy substantially. See power usage effectiveness.
Frequently asked questions
Do blanking panels really matter if I have containment?
Yes. Containment controls air at the aisle level, but open rack units still let air leak through the racks from one side of the containment to the other. Blanking is part of making containment work.
Can I make these changes in a live data center?
Most measures, such as blanking panels, grommets and tile changes, are low risk and done routinely in live rooms. Make changes incrementally and monitor inlet temperatures as you go.
Key takeaways
- Bypass air wastes cooling; recirculation causes hot spots. Most rooms have both.
- Blanking panels, sealed cutouts, correct tile placement and closed row gaps are low-cost fixes.
- Measure before and after, using rack inlet temperatures and cooling unit return temperatures.
- Savings only appear after setpoints, fan speeds and running units are adjusted.
- Sustain results through change management and periodic audits.
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