Warm-Water Liquid Cooling For AI Data Centers

Warm-water liquid cooling is a design strategy for AI data centers because it can reduce and eliminate mechanical chiller operation. The idea sounds counterintuitive: rather than supplying very cold coolant to servers, operators allow liquid temperatures to rise so heat can be rejected more easily to the outdoor environment.

This matters as AI accelerators, CPUs, memory and networking equipment concentrate far more heat inside each rack. Direct-to-chip systems can capture that heat efficiently, but the facility must still move it outdoors. When return water is warm enough, dry coolers or economizers may reject the heat without energy-intensive compressor refrigeration for much of the year.

The potential benefits include lower cooling energy, reduced water consumption and simpler mechanical plants. However, “chiller-free” requires qualification. Climate, coolant temperatures, heat-exchanger performance, hardware limits, residual air loads, redundancy and extreme weather determine whether chillers can disappear or simply operate less often.

How Warm-Water Liquid Cooling Works

Mechanical chillers use compressors to move heat from a relatively cold water loop to a warmer environment. Producing colder water generally requires more work. If IT equipment safely accepts warmer coolant, the facility avoids creating such a low supply temperature. During suitable weather, pumps and fans can transfer heat through dry coolers, cooling towers or water-side economizers.

This process is called free cooling or economization, although it is not literally free. Pumps, fans, controls and heat exchangers still consume electricity. The major efficiency gain comes from avoiding or minimizing compressor operation, often one of the largest cooling loads in a conventional data center.

ASHRAE Liquid-Cooling Classes

ASHRAE’s liquid-cooling classes help explain the temperature opportunity. They include W17, W27, W32, W40, W45 and W+. The number indicates the maximum facility supply liquid temperature supported by compliant equipment.

W17 and W27 systems are commonly associated with chilled-water infrastructure, potentially supported by economization. W32 and W40 facilities may operate without chillers in many locations, although climate remains decisive. W45 and W+ equipment provides a wider envelope intended to support chiller-free design and reduce facility energy and capital requirements.

A W45 rating does not automatically guarantee chiller-free operation. It means the IT equipment can accept warmer facility water, giving engineers more options. Every component between the chip and outdoor air introduces a temperature difference that must be included in the design.

Why CDU Approach Temperature Matters

A coolant distribution unit, or CDU, usually separates facility water from the technology cooling loop circulating through racks and cold plates. Heat crosses a heat exchanger between these circuits. That transfer requires a temperature difference known as the approach temperature.

If servers require 45°C coolant, facility water may need to be cooler so the CDU can transfer heat at full load. The exact margin depends on heat-exchanger design, flow, pressure and operating conditions. Consequently, a server advertised for 45°C coolant does not mean the external heat-rejection system can supply water at precisely 45°C.

NVIDIA Rubin Raises The Temperature

NVIDIA’s Rubin-generation AI infrastructure illustrates the direction of travel. NVIDIA says the platform supports cooling liquid up to 45°C, or 113°F, and is designed for 100% liquid cooling across compute and networking components without server fans.

Higher coolant temperature expands opportunities for dry cooling. A dry cooler works like a large radiator: water moves through coils while fans drive outdoor air across them. When ambient air is sufficiently cooler than required facility water, heat can be rejected without compressor-based chilling.

Climate Determines Chiller-Free Hours

A cool northern location may provide enough low-temperature hours for nearly continuous dry-cooler operation. Hot desert and tropical markets face a harder challenge. If outdoor air approaches or exceeds the required supply temperature, dry cooling alone cannot reject the full load. Mechanical assistance, evaporative cooling, thermal storage or another method may be necessary.

Dry coolers depend mainly on dry-bulb temperature. Cooling towers and evaporative systems respond more strongly to wet-bulb temperature, which measures evaporation’s cooling potential. Hot, dry air can support effective evaporative cooling; hot, humid air provides less benefit.

Designers therefore need hourly weather records, not only a city’s headline maximum temperature. Extreme events, equipment approach temperatures, redundancy requirements and future climate assumptions must be modeled against the full cooling chain.

Chiller-Free Does Not Mean Cooling-Free

An AI facility without chillers still needs pumps, CDUs, heat exchangers, valves, controls, expansion systems, water treatment, pipework and outdoor heat-rejection equipment. Fan and pumping energy can remain material at large loads.

The architecture removes or reduces a particularly energy-intensive process; it does not eliminate thermal infrastructure. Controls also become important because flow, temperature and outdoor conditions must be coordinated without reducing the temperature margin available to processors.

Warm-Water Cooling And Water Consumption

Liquid cooling is often presented as a water-saving technology, but definitions matter. The technology loop in a direct-to-chip system is typically closed, repeatedly circulating coolant between cold plates and the CDU rather than consuming it continuously.

Site water use depends primarily on heat rejection. Cooling towers lose water through evaporation, whereas dry coolers can operate with very little ongoing water consumption. A high-temperature, dry-cooler design may approach zero operational cooling-water use in suitable conditions, but that outcome cannot be generalized to every liquid-cooled site.

The Server-Level Engineering Trade-Off

Warmer facility water creates a tougher thermal problem inside the server. Processors still have maximum junction temperatures. When coolant enters the cold plate warmer, the thermal resistance between silicon and liquid must remain low enough to protect the chip.

That increases demands on cold plates, flow distribution, thermal-interface materials, pumps and component packaging. The core trade-off is straightforward: warmer water can simplify facility cooling while requiring more sophisticated heat transfer close to the electronics.

Rising AI chip heat flux could also push some future equipment toward colder liquid despite improvements in packaging. Infrastructure teams should avoid designing around a single hardware generation. Future refreshes may have different temperature, flow and water-quality requirements.

Residual Air Loads And Resilience

Many direct-to-chip platforms do not capture all server heat in liquid. Power supplies, storage, memory or networking components may still rely partly on airflow, leaving a residual load for data-hall air systems. The percentage of heat captured by liquid directly affects plant size and achievable efficiency.

Operators may also retain chillers for resilience even when normal weather supports dry cooling. Extreme heat, maintenance, equipment failures, unusual workloads or future servers can reduce available margin. A smaller supplemental chiller may provide insurance while operating only during exceptional periods.

This means the decision is not binary. Reducing compressor use from thousands of hours annually to a limited number of peak periods can still deliver meaningful energy and water savings.

Planning Heat Reuse And Retrofits Carefully

Retrofitting existing data centers is more complicated. Legacy chillers, towers, pumps and air handlers may be optimized for cold water. Raising set points can improve efficiency, but pipe capacity, heat exchangers, pump curves, dry-cooler space, controls and redundancy all require assessment. Keeping existing chillers as backup while adding dry coolers and liquid loops can provide a practical transition.

Design Checklist For Data Center Leaders

• Confirm the facility supply temperature supported by the selected IT equipment.
• Calculate the CDU approach temperature at full design load.
• Model hourly dry-bulb and wet-bulb weather, including extreme conditions.
• Measure the percentage of rack heat captured by liquid.
• Size dry coolers or economizers for realistic peak performance.
• Define the role of chilling during failures, maintenance and heat waves.
• Plan coolant chemistry, filtration, monitoring and leak detection.
• Test compatibility with likely future AI hardware generations.
• Evaluate water use and heat-reuse opportunities using site-specific data.

Frequently Asked Questions

What Is Warm-Water Liquid Cooling?

It is a data center strategy in which IT equipment accepts relatively high coolant supply temperatures. The warmer thermal loop makes outdoor heat rejection easier and increases the hours when mechanical chillers can remain off.

Can An AI Data Center Operate Without Chillers?

Yes, when equipment supports sufficiently warm coolant and the local climate allows reliable heat rejection. CDU approach temperature, peak weather, residual air loads and resilience requirements may still justify backup chilling.

What Does ASHRAE W45 Mean?

W45 identifies equipment designed to support facility supply liquid temperatures up to 45°C. That higher limit expands the range of conditions in which dry cooling or economization can replace refrigeration.

Does Direct-To-Chip Cooling Consume Water?

The internal technology loop normally recirculates coolant. Total consumption depends on the outdoor system: evaporative towers use water, while dry coolers can operate with minimal water.

Conclusion

Operators should validate these assumptions through full-load testing, seasonal commissioning and continuous monitoring so the cooling system remains efficient, resilient and compatible with changing AI equipment over time.

Warm-water liquid cooling can change AI data center economics, but its value comes from the complete thermal path rather than simply replacing air with liquid. Equipment that accepts 40°C or 45°C supply temperatures broadens the opportunity to minimize refrigeration, reduce water use and simplify mechanical infrastructure.

Climate and engineering still determine the result. Operators should ask how many chiller hours can be removed without compromising component temperatures, resilience or future compatibility. In favorable designs, the answer may be almost all of them. Elsewhere, chillers will remain valuable backup. Either outcome can improve efficiency when server and facility cooling are designed as one coordinated system.