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Residual Heat in Liquid-Cooled Racks: Why Air Still Matters

A liquid-cooled GPU rack still sends part of its heat to the room air. In the NVIDIA GB300 NVL72 reference design, about 10% of the rack’s heat goes to air, which is roughly 14 kW per rack. That is more than the most common rack in today’s data centres draws in total, and it has to be cooled with much colder water than the chip loop needs.

How much heat does a liquid-cooled rack still send to air?

Direct-to-chip cooling only takes the heat that reaches the cold plates. In Lenovo’s product guide for the GB300 NVL72, the CPUs, GPUs and NVLink switches are liquid-cooled, while network modules, drives and power distribution are cooled by fans. At rack level, the split is about 90% to liquid and 10% to air.

NVIDIA’s reference architecture allows up to 142 kW per GB300 NVL72 rack. Ten per cent of that is about 14 kW. For comparison, Uptime Institute’s 2026 Global Data Center Survey puts the most common rack density at 11 kW, and the average at 7.8 kW when facilities built for more than 30 kW are left out.

In our own designs, chip-cooled racks of 100 kW and below typically leave around 25 kW in the air. Newer, more powerful racks leave less, down to about 15 kW. Earlier GPU generations sent up to 20 to 30% of their heat to air.

Why does the air side need colder water than the chip loop?

A cold plate sits directly on the chip, so the water can be warm and still carry the heat away. At CES in January 2026, NVIDIA said its Vera Rubin racks can be cooled with 45 °C supply water, with no water chillers needed.

Air is different. The heat first has to pass from the components into the air, and then from the air into a water coil. Every step costs temperature. That is why the air side runs on much colder water. In Danish data centres, typical supply temperatures are 6 to 7 °C for CRAH and CRAC units and 12 to 21 °C for rear-door heat exchangers. Our rear-door designs typically run on 18 to 22 °C supply water.

When does a dry cooler stop being enough?

A dry cooler can cool water to around 4 °C above the outdoor air. That rule decides when the chillers start.

  • 45 °C supply water works whenever it is below about 41 °C outside. In Denmark, that is practically all year.
  • 20 °C supply water only works when it is below about 16 °C outside.

Above that, a chiller has to take over, and a chiller can use up to 10 times as much energy per kW of cooling as a dry cooler. So a hall can have a chip loop that never needs a chiller and an air side that still does. Several liquid cooling experts made the same point after NVIDIA’s announcement: warmer chip water reduces the need for chillers, but it does not remove it.

What does the air-side temperature mean for the energy bill?

The chiller is the expensive part, so the goal is to let the dry cooler take as much of the heat as possible and leave only the rest to the chiller. A chiller also works more efficiently the warmer the water it receives, because the temperature lift to the outdoor air is smaller.

Every degree warmer on the air side adds hours of free cooling. In one optimised case of ours, free cooling covered 92 to 96% of the year. In a calculation for a typical year at Tirstrup near Aarhus, running 24/7 at 22/32 °C, the dry cooler delivered 97.7% of the cooling and the chiller 2.3%. The chiller still used 42% of the cooling power. That is a calculation, not a measurement, but it shows where the energy goes: into the few hours the air side cannot be cooled for free.

For a CFO, the air-side supply temperature decides whether the chiller line leaves the budget, or only gets smaller. More on this on our Energy Efficiency page.

How do rear doors handle residual heat from chip-cooled racks?

A rear-door heat exchanger removes the heat at the back of the rack, before it reaches the room. That makes it a good match for the residual heat from chip-cooled servers, including NVIDIA GPU racks from Dell, Lenovo and Supermicro. Our RDHx Active Flow mounts directly on existing racks, so it needs no extra white space, and it can be retrofitted without rebuilding the building. The water comes from our modular CDU, with a capacity of up to 1200 kW. You can read more about the temperature side in The Value of Hot Water.

FAQ

Does direct-to-chip cooling remove the need for room cooling?

No. In the GB300 NVL72 reference design, about 10% of the heat still goes to air, roughly 14 kW per rack at 142 kW.

What supply temperature do rear-door heat exchangers use?

In Danish data centres, typically 12 to 21 °C. Our designs typically run on 18 to 22 °C.

Can free cooling handle the air side in Denmark?

Most of the year, depending on supply temperature, climate and design. In one optimised case, free cooling covered 92 to 96% of the year.

Sources

This article builds on a LinkedIn post from 19 October 2026.

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