Do All Perimeter AC Units Require Raised Floors?

Spoiler: no, not always. The idea that perimeter AC requires a raised floor persists in many projects, but modern design offers real alternatives.

When planning precision cooling for a technical room, one of the first decisions is how to distribute cold air. There are two main routes: the raised floor with bottom discharge, or configurations that send air from the top or front without needing a technical floor.

What is a perimeter AC unit

A perimeter AC unit is a precision cooling system installed at the edge or perimeter of the room, outside the rack area. Its function is to capture hot air from the hot aisle, cool it, and return it to the cold aisle.

In traditional configurations, this return happens below the raised floor through ducts and diffusers. Cold air exits through the floor, rises through the front of the racks, and creates the hot/cold aisle containment pattern that the TIA-942 standard describes as efficient.

When raised floor is still the best option

A raised floor, typically between 30 and 60 cm per TIA-942, makes three things easier: distributing cold air uniformly across the room, hiding electrical cabling, data, and chilled water or condensate piping, and allowing future reconfigurations without opening walls or ceilings.

In new data centers with sufficient ceiling height, it remains the most flexible option and the one that best adapts to hot or cold aisle containment designs.

When you cannot or should not use a raised floor

There are three scenarios where raised floor becomes unviable: older buildings with low ceilings or limited structural loads, repurposed spaces (converted offices, industrial warehouses), and projects where the cost of raising the floor outweighs the benefit.

In these cases, manufacturers offer configurations that discharge air from the top or front of the unit, without needing a lower plenum.

Alternatives available in 2026

Perimeter units with top or front discharge: brands like Vertiv (Liebert CRV), Stulz (CyberAir), and Schneider Electric (InRow) offer units that connect to overhead ducts or cool the cold aisle air directly.

In-row solutions: installed between racks, in row, they cool the air at the point where heat is generated. They are an alternative to traditional perimeter units when installing a raised floor is not desired or feasible.

Rear-door heat exchangers: mounted on the back of the rack, they extract heat from the equipment before it reaches the room. They work with chilled water or refrigerant and allow operation without precision air in the aisle.

Conclusion

The raised floor remains useful and remains the default option in most data centers above 50 kW. But it is no longer mandatory. What matters is designing based on the real conditions of the site: ceiling height, structural load, rack type, and density per cabinet.

Sources

[1] TIA — ANSI/TIA-942-B Telecommunications Infrastructure Standard for Data Centers — https://tiaonline.org/product/ansi-tia-942/

[2] Vertiv — Liebert CRV in-row cooling — https://www.vertiv.com/en-us/products/thermal-management/liebert-crv/

[3] Stulz — CyberAir precision cooling — https://www.stulz.com/en/products/precision-air-conditioning/cyberair/

[4] Schneider Electric — InRow cooling — https://www.se.com/us/en/product-category/4700-inrow-cooling/

[5] Wikipedia — Raised floor — https://en.wikipedia.org/wiki/Raised_floor