PUE on lo0 — Tech Blog

Cooling the AI datacenter (1/4): the thermal challenge

Sat, 27 Jun 2026 20:00:00 +0200

The power series (in Spanish) followed the watt from the utility feed all the way to the chip. But there is an implacable physical law that closes the loop: almost every watt that goes into a server comes back out as heat. An inference factory does not only have to deliver megawatts to its GPUs; it has to remove those same megawatts as heat, and remove them fast enough that the silicon does not cook itself. That is the other half of physical infrastructure, and it is the half the AI era has turned upside down. This article opens the second series in the vertical: cooling.

The approach is the same one used in the power series: universal concepts, valid for any datacenter, read throughout against the question of why AI, with its 130 kW racks and its roadmap toward the megawatt, is forcing a historic transition from air to liquid.

The law that governs everything: power is heat

Start with the physical principle, because everything else follows from it. The electrical energy a server consumes does not disappear: by conservation of energy, practically all of it turns into heat that has to be evacuated. For practical purposes, the thermal power to dissipate equals the electrical power consumed. A rack that swallows 130 kW is a 130 kW radiator, the equivalent of several dozen domestic heaters switched on at once inside a single cabinet. Cooling means moving that heat from the chip to, eventually, the atmosphere, crossing a chain of media (air, water, refrigerant) as real as the electrical chain that fed it.

The ability of a fluid to carry heat obeys a simple but revealing relation: the heat a flow can remove is proportional to its mass flow rate, its heat capacity and the temperature rise it accepts,

$$\dot{Q} = \dot{m} \cdot c_p \cdot \Delta T$$

That formula alone explains why air is in trouble. Air has a low heat capacity and a minimal density, so removing a lot of heat means moving enormous quantities of it. Water, for the same volume, carries on the order of a thousand times more heat than air. When rack density spikes, sticking with air is like emptying a swimming pool with a hairdryer: technically conceivable, practically absurd.

Put numbers on it. Solving the formula above for flow rate, removing a given power with a fixed temperature rise requires $ \dot{m} = \dot{Q} / (c_p \Delta T) $. For a 50 kW rack cooled by air with a reasonable rise of about 11 °C (the usual 20 °F), the required airflow is around 7,800 cubic feet per minute: literally a gale blowing through the cabinet. Doubling density to 100 kW doubles the gale, and there is no way to contain that much moving air without the turbulence, noise and recirculation that ruin efficiency. Water does the same job with a ridiculously smaller flow and volume, because its $ \rho \cdot c_p $ product, the capacity to carry heat per unit of volume, is overwhelmingly higher. Physics leaves no room for nostalgia: above a certain density, air simply cannot.

AI density breaks the model

For decades, air was more than enough. A traditional rack dissipated between 5 and 15 kW, and a well-designed room cooled it without breaking a sweat. Air is comfortable: cheap, safe, no pipes to drip onto the electronics. The problem is that AI has multiplied density by ten.

The root of it is the chip. An H100 (Hopper) GPU dissipated about 700 W; a B200 (Blackwell) goes up to 1,000 W, and as far as 1,200 W in its full-specification version. That increase in power per chip is the first cause of the whole problem: it is not only that there are more chips, it is that each one dissipates almost twice what the previous generation did, on a silicon area that barely grows, so heat density per square centimetre spikes. An air heatsink has a physical limit to how much heat it can pull off a small surface before the surrounding air saturates; past that point, no matter how many fans are added, the chip throttles thermally and drops its clock so as not to burn, which means wasting very expensive silicon. Liquid, with direct contact and far greater capacity, pulls that heat away without the chip having to back off.

And those chips do not travel alone: the NVIDIA GB200 NVL72 packs 72 GPUs and 36 CPUs into a single rack rated at 120 kW, with real deployments measuring 130-132 kW at full load. The roadmap does not let up: the Vera Rubin generation points at 190-230 kW per rack, Rubin Ultra “Kyber” at around 600 kW in 2027, and there is already talk of the 1 MW rack toward 2028. The distant figures should be taken for what they are, vendor roadmap rather than deployed installations, but the slope is unmistakable.

Where is the limit of air? Industry figures place it with fair consensus: air is efficient up to about 10-20 kW per rack, room-level cooling reaches about 27 kW with effort, and row-level cooling about 30 kW. With advanced techniques such as rear-door heat exchangers it can be stretched to 40-60 kW. Above that, thermodynamics stops cooperating: cooling a 50 kW rack with air would take on the order of 7,800 cubic feet per minute, a gale impossible to contain inside a cabinet. Between 50 and 100 kW, liquid stops being an option and becomes the only viable physics. AI racks sit well above that boundary. That is why 2025-2026 is the moment of the great transition.

The spectrum of solutions

It helps to see cooling not as one technology but as a spectrum ordered by density. As heat per rack rises, you climb from one rung to the next:

Method	Typical density supported	Medium that touches the heat
Room air (CRAC/CRAH) + containment	up to ~30 kW/rack	Air
Rear-door heat exchanger (RDHx)	up to ~40-60 kW/rack	Air → water
Direct liquid to chip (DLC)	~50 kW to >130 kW/rack	Water/refrigerant in a cold plate
Immersion	100-250+ kW/rack	Dielectric fluid

Each rung brings the coolant closer to the hot spot. Air cools the whole room; the rear door intercepts hot air at the rack outlet; direct liquid to chip brings water to a cold plate pressed against the processor itself; and immersion submerges the entire electronics in a fluid that does not conduct electricity. The closer the liquid gets to the chip, the more heat is captured and the higher the density that can be operated. The next three articles in this series walk that spectrum in detail: air and its limits, direct liquid to chip, and immersion.

The reality of 2026, though, is not a clean replacement but coexistence. Even a direct-liquid rack still evacuates a fraction of its heat through air, between 2 % and 20 % depending on how close the liquid gets to each component, so the hybrid room, liquid for the GPUs and air for the rest, is the norm during the transition. Vendors have understood this: the most widely deployed solutions combine both worlds.

The journey of heat, end to end

Just as the power series followed the watt from the utility feed to the chip, it is worth following the heat in the opposite direction, from chip to atmosphere, because cooling is a chain with as many links as the electrical one. Heat is born in the transistor and conducts to the chip package; from there it passes to a heatsink or a cold plate; that first medium, air or liquid, carries it to a heat exchanger; a second loop, the facility water loop, takes it out of the room; and a final rejection stage, a cooling tower, a dry cooler or a chiller, hands it to the atmosphere. Every hop has a temperature difference and a loss, and the art of cooling consists of minimising the number of hops and maximising the temperature at which you work, so that less energy (and less water) is needed for the final rejection.

Here a concept appears that runs through the whole series: the coolant distribution unit (CDU), the thermal equivalent of a transformer. The CDU separates the loop that touches the equipment, clean and controlled at precise flow and temperature, from the facility loop, and exchanges heat between the two without mixing them. It is the boundary between the server vendor’s world and the building operator’s, and where a good part of the efficiency is decided. We will look at it in detail when we get to direct liquid to chip.

The metrics of heat

To govern cooling you have to measure it, and several metrics belong in an architect’s toolkit. The first is already familiar from the power series: PUE (Power Usage Effectiveness), the ratio between total facility energy and the energy that reaches IT equipment. What matters for cooling is that cooling and air movement consume between 30 % and 45 % of a traditional datacenter’s energy: by a wide margin, the largest energy overhead on top of compute. The industry average has been stuck around 1.54 for years, according to the Uptime Institute, precisely because air cooling is inefficient and hard to improve on.

The second metric, which AI has pushed to the foreground, is WUE (Water Usage Effectiveness), the litres of water consumed per kWh of IT:

$$\text{WUE} = \frac{\text{litres of water}}{\text{kWh of IT}}$$

The average sits around 1.8-1.9 litres per kWh, although the most efficient facilities aim to go below 0.2. Water is the hidden face of cooling, and we will come back to it.

The third is the ASHRAE TC9.9 framework, the reference that defines the temperature at which equipment may operate. For air, its classes run from A1 (15-32 °C inlet) to A4 (up to 45 °C); for liquid, a recent update defines water classes W17 to W45, where the number is the maximum supply temperature in degrees. The underlying trend is clear and counterintuitive: raise operating temperatures. The hotter the coolant can come in, the less it has to be chilled artificially, and the more ambient air or water can be used without compressors. Operating hot is operating efficiently.

That idea crystallises in the concept of warm-water cooling and the ambition to operate chillerless. The reasoning is elegant: if the chip is perfectly happy with 40 or 45 °C inlet water, there is no point spending energy producing 15 °C chilled water with compressors; a dry cooler rejecting heat to outside air, which is almost always below 45 °C, is enough. NVIDIA promotes exactly that, 45 °C inlet water (class W45) for its Vera Rubin generation, with a wide thermal rise (45 °C in, 65 °C return) that reduces pumping flow and, above all, eliminates chillers in most climates. The term deserves a caveat: “chillerless” usually means “almost chillerless”, because on the hottest days some mechanical cooling backup is still needed. But the direction is unmistakable, and it is the one that saves energy, saves water and, as we will see, allows heat reuse.

One measurement caveat to close on: PUE does not capture the consumption of the fans that live inside the server, which in an AI node is not negligible. Hence TUE (Total Usage Effectiveness), which combines PUE with the equipment’s own internal efficiency (ITUE) to give a more honest picture of how much energy actually reaches the transistors. As liquid removes server fans, the distinction gains importance: in an air-cooled AI node, internal fans can consume a far from negligible percentage of the server’s own power, energy that liquid recovers by making them unnecessary. It is one of the gains of liquid that PUE, on its own, cannot even see.

Water: the new constraint

If energy is the AI datacenter’s input constraint, water is becoming an equally hard output constraint. Many efficient cooling techniques, cooling towers and evaporative cooling among them, trade water for energy by evaporating water, and at AI scale that bill has become enormous. Estimates of AI datacenter water consumption already run into hundreds of billions of litres per year, with projections doubling toward 2030. The per-query figures in circulation, from a few millilitres to half a litre per request, should be handled with care, because they vary enormously with methodology, but the aggregate order of magnitude is real and concerning.

It is worth understanding the underlying technical dilemma, because it is a trade-off rather than a defect. A datacenter’s heat rejection equipment moves inside a triangle of energy, water and temperature. A cooling tower evaporates water to cool better: little energy, a lot of water (good PUE, bad WUE). A dry cooler evaporates nothing, so it barely uses water, but it can only cool to near ambient temperature, which demands operating with hotter water and, in warm climates, more backup energy. A mechanical chiller cools to any temperature but at a high energy cost. Raising operating temperatures, as noted above, is what allows the move from tower to dry cooler without an energy penalty: warm water and the waterless datacenter are two sides of the same coin. AI, by concentrating so much heat, forces a conscious choice inside that triangle where the choice used to be made by inertia.

And it is not only a technical problem but a social one. Competition for water with communities and agriculture has begun to stall projects: in the United States, “Project Blue” near Tucson, which expected to consume hundreds of millions of gallons a year, was cancelled after local opposition. The industry’s answer is twofold: on one side, waterless designs using dry coolers in exchange for operating with hotter water (Microsoft reports reaching a WUE of 0.30 and deploying zero-evaporation cooling); on the other, direct liquid to chip itself, which by capturing heat at a higher temperature makes it easier to reject to ambient without evaporating. The paradox of 2026 is that liquid, well designed, can use less water than the evaporative air it replaces.

Waste heat: from problem to resource

There is an optimistic twist. All that heat that costs so much to remove is, after all, energy, and reusing it rather than dumping it into the atmosphere is increasingly on the table. Waste heat reuse for district heating has gone from curiosity to regulatory obligation: the European energy efficiency directive pushes in that direction, and national laws already set quotas, with Germany requiring 10 % reuse in 2026 and up to 30 % in 2028.

Here liquid again has a decisive advantage over air. Modern district heating networks need water at 50-70 °C, a temperature that a liquid-cooled datacenter with its wide thermal rise (45 °C inlet and 65 °C return, for example) can deliver almost directly, whereas the hot air of a traditional room is too cool and too diffuse to be useful. It is no accident that the large reuse cases, heating tens of thousands of homes in northern Europe with Microsoft, Google and Meta datacenters wired into district heating networks, rest on liquid cooling. For an operator, waste heat can go from cost to revenue: heat delivered to a district network can be worth considerably less than the gas it displaces, with payback periods under a decade when the datacenter sits one or two kilometres from the network. At continental scale the potential is large: European datacenter waste heat is estimated to be able to cover around 10 % of EU space heating by 2030. What was vented to the atmosphere for decades is starting to be seen for what it is: wasted high-quality energy.

What is holding the transition back

If liquid is thermodynamically inevitable, why has it not swept air aside already? Because changing cooling paradigm touches everything an operator fears touching. The first brake is fear of leaks: bringing water within centimetres of the most expensive electronics on the planet makes anyone nervous, and although modern solutions use dripless quick connectors, leak detection and drip trays, the perception of risk weighs. The second is lack of standardisation: for years every vendor brought its own fluid, its own connectors and its own temperatures, which locked customers into a supplier and complicated maintenance; the sector is converging, but slowly. The third is cost and construction: liquid demands piping, CDUs, manifolds and often a rebuilt room, an investment that only high densities justify. And the fourth is operational and cultural: a team used to managing air conditioning has to learn to operate a hydraulic system, with its water chemistry, its pumps and its gaskets.

The result is that air does not disappear: it is reserved for loads that need nothing more, while liquid enters where density forces it. The question for an architect is rarely “air or liquid?” in the abstract, but “which part of my room needs liquid, when, and how does it coexist with air everywhere else?”. Retrofit, adapting existing air rooms to host liquid islands, is one of the big businesses of 2025-2026, precisely because almost nobody starts from scratch.

Where the market is in 2026

How far along is the transition? Uptime Institute data from 2025 gives a nuanced picture: direct liquid to chip is already present in around 22 % of operators, but perimeter air remains the majority at 75 %, and 61 % of those not yet using liquid are considering it. The barriers they cite are revealing, being lack of standards, cost and fear of a leak failing over the electronics, the same ones that hold back any paradigm change. The liquid cooling market, however, is growing at high double-digit rates, with estimates multiplying it by five or six over a decade, and the large players are repositioning fast: Vertiv leads with a good share of the first GB200 racks and has launched solutions combining direct liquid to chip and air in a single module; Schneider Electric bought Motivair to enter in earnest; and alongside them live cold-plate specialists (CoolIT, Boyd, JetCool, Asetek) and immersion specialists (Submer, GRC, LiquidStack). The dominant pattern is hybrid: rooms combining liquid for the GPUs and air for everything else, as a bridge to a mostly liquid future. That vendors package both worlds into a single product says a great deal about where the transition is heading: not to a clean cut, but to a managed coexistence lasting years.

For an inference factory

What should someone building an inference factory take from this? Three ideas that run through the whole series. First, that cooling is not an add-on but a co-design: the density of AI racks forces the cooling method to be decided at the same time as the power method, because a 130 kW rack cannot simply be dropped into a room designed for air. Second, that the move to liquid is no longer optional above a certain density; the question is not whether, but which, direct to chip or immersion, and how to manage coexistence with air for years. And third, that the metrics that matter have widened: watching PUE is no longer enough, water (WUE) has to be watched too and, increasingly, what is done with waste heat, because operating cost, the social licence to build and, in some geographies, legal compliance are all decided there.

It is also worth keeping in mind that cooling is not only efficiency: it is availability. As the power series showed, a thermal failure can bring a factory down as fast as a power cut, in fact faster, because a 130 kW rack without cooling reaches dangerous temperatures in seconds. That is why critical cooling is designed with the same philosophy as power: redundancy in pumps and CDUs, early detection, and testing. A cooling system nobody has exercised at full load is, once again, an unverified promise.

This article has drawn the map of the thermal challenge. The next three go down into the detail of each rung of the spectrum: starting with air cooling and its limits, the point everyone is coming from, moving on to the direct liquid to chip that is feeding GB200 racks, and finishing with immersion and the horizon of the megawatt rack. Feeding the GPUs was half the battle; the other half is keeping them from melting.

Sources

Network World, Why AI rack densities make liquid cooling nonnegotiable — https://www.networkworld.com/article/4149069/why-ai-rack-densities-make-liquid-cooling-nonnegotiable.html
Introl, Liquid Cooling vs Air: The 50kW GPU Rack Guide (2025) — https://introl.com/blog/liquid-cooling-gpu-data-centers-50kw-thermal-limits-guide
ToneCooling, NVIDIA GB200 NVL72 Cooling Requirements — https://tonecooling.com/nvidia-gb200-nvl72-cooling-requirements/
TweakTown, NVIDIA full-spec Blackwell B200 uses 1200W — https://www.tweaktown.com/news/97059/nvidias-full-spec-blackwell-b200-ai-gpu-uses-1200w-of-power-up-from-700w-on-hopper-h100/index.html
DCD, Nvidia’s Rubin Ultra NVL576 rack expected to be 600kW — https://www.datacenterdynamics.com/en/news/nvidias-rubin-ultra-nvl576-rack-expected-to-be-600kw-coming-second-half-of-2027/
Computer Weekly, Huge grid and heat challenges ahead as Nvidia set for 1MW rack — https://www.computerweekly.com/news/366639658/Huge-grid-and-heat-challenges-ahead-as-Nvidia-set-for-1MW-rack
mgrid / Uptime Institute, Global Data Center PUE Stalls at 1.54 (2025) — https://mgrid.org/2025/10/01/uptime-institute-data-center-pue-stagnation-2025-liquid-cooling/
ClearComfort, 2026 PUE and WUE Checklist — https://clearcomfort.com/pue-wue-ai-data-centers/
Introl, Water Usage Effectiveness (WUE) AI Data Center Cooling Guide 2025 — https://introl.com/blog/water-usage-efficiency-wue-ai-data-center-cooling-guide-2025
CKY, ASHRAE TC 9.9 Thermal Guidelines (5th Ed.) — https://www.cky.com.tw/en/insights/ashrae-tc9-datacenter-thermal-guidelines
DCD, Hot water, cold water: the right temperature for water cooling — https://www.datacenterdynamics.com/en/analysis/hot-water-cold-water/
EESI, Data Centers and Water Consumption — https://www.eesi.org/articles/view/data-centers-and-water-consumption
Microsoft Datacenters, Measuring energy and water efficiency — https://datacenters.microsoft.com/sustainability/efficiency/
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Uptime Institute, Global Data Center Survey 2025 — https://uptimeinstitute.com/resources/research-and-reports/uptime-institute-global-data-center-survey-results-2025

Cooling the AI datacenter (2/4): air and its limits

Sat, 27 Jun 2026 19:00:00 +0200

The first article drew the map of the thermal challenge and laid out the spectrum of solutions. Now for the bottom rung, the one almost everybody already has installed: air cooling. There is a temptation to treat it as legacy technology, a leftover of the pre-AI datacenter that liquid will sweep away in a couple of years. The data says otherwise. The Uptime Institute’s 2025 cooling systems survey puts perimeter air, the classic room CRAC or CRAH, in 75 % of installations, against 22 % reporting direct liquid cooling. Air is the base case, and it will remain so across most of the installed base for years.

This article has two goals. The first is to understand air well enough to squeeze it: where the energy goes, which measures pay back, and how much headroom is left in a poorly tuned room. The second is to understand exactly why it breaks, because the limit of air is not set by a liquid-cooling vendor’s catalogue: it is set by room geometry and by an exponent.

Inside the air-cooled room

The equipment that cools a traditional room comes in two variants that are often confused. A CRAC (Computer Room Air Conditioner) is a direct-expansion machine: it carries its own compressor and its own refrigerant circuit, and runs autonomously without depending on anything external. A CRAH (Computer Room Air Handler) has no compressor: it receives chilled water from a central plant and passes it through a coil while the fan moves room air. The difference matters for two reasons. The CRAH is usually more efficient, because compression work is concentrated in large, well-optimised chillers instead of being spread across dozens of small machines. And above all, the CRAH integrates naturally with an economiser: if the water can be cooled with outside air, the whole room runs without compressors. The CRAC needs its compressor whatever the weather.

Behind those units sits the final heat rejection equipment, and there the picture of the installed base is revealing. According to the same Uptime survey, rejection splits between air-cooled chillers (35 %), evaporative cooling towers (22 %), direct expansion (11 %), dry coolers and fluid coolers (6 %), free cooling with outside air (4 %) and adiabatic misting (3 %). It is a base dominated by the mechanical chiller, which is exactly the piece modern design tries to keep switched off for as many hours as possible.

Between the room and the rack there is an intermediate step, close-coupled cooling: in-row units placed between cabinets, shortening the air path and raising the density that can be handled. A typical row unit today, such as the Vertiv CoolPhase Row range, moves 30 to 40 kW per unit. Some 32 % of surveyed installations already have some form of this cooling, almost always coexisting with perimeter air.

The geometry of airflow

The formula that governs any fluid cooling system already appeared in the previous article:

$$\dot{Q} = \dot{m} \cdot c_p \cdot \Delta T$$

Solved for air with a realistic server temperature rise, it yields a rule of thumb every room designer knows: roughly 140 to 160 CFM per kW dissipated. Schneider works through an example with an 18 kW rack needing 2,500 CFM (1,180 l/s) at the inlet; an air-cooled DGX B200 asks for 1,550 CFM on its own. Those figures are not large in the abstract. They become a problem when crossed with the other half of the system, which is where that air comes in.

A perforated raised-floor tile delivers around 300 CFM (142 l/s). A grate-type tile reaches 700 CFM. Going beyond 500 CFM per tile takes special pieces and a very carefully designed plenum, because pressure under the floor is not infinite and tiles compete with each other. That is where the most quoted and least disputed limit of classic air comes from: a conventional distribution with one vented tile per rack does not sustain much more than 6 kW per rack. With containment and serious airflow management you reach around 10 kW sustained, and highly optimised designs stretch into the twenties.

The arithmetic of an AI rack breaks that beyond argument. At 140 CFM/kW, a 50 kW rack asks for about 7,000 CFM. With standard 300 CFM tiles that would take twenty-three of them under a cabinet whose footprint is worth two. With 700 CFM grates, ten. There is no floor area through which to push that flow, nor plenum pressure to distribute it. Air fails on geometry before it fails on thermodynamics: the room runs out of places to deliver the fluid long before the fluid runs out of capacity to absorb heat.

And there is a second, subtler problem that shows up when someone tries to force the machine.

The cube law, friend and executioner

The power a fan draws grows with the cube of its rotational speed. The US Department of Energy illustrates it with a direct case: a fan drawing 6.5 kW at full speed drops to 3.3 kW at 80 % of revolutions, because $ 0.8^3 \approx 0.51 $. The same relation seen from the air circuit says the required pressure grows with the square of the flow, $ P = Z \cdot Q^2 $, so doubling the flow quadruples the pressure and multiplies fan power by eight.

That exponent hands out very unequal prizes. At low density it is the greatest gift air has, and the measured numbers back it up. Variable-speed fan retrofits documented by the DOE at three facilities cut cooling system consumption by 22 % to 32 %, with payback under two years. The LBNL demonstration at a Digital Realty site in El Segundo, with 72 CRAHs and about 2.6 MW of IT load, is even more emphatic: swapping scroll fans for plug fans dropped CRAH power from 439 to 233 kW, a 47 % cut; adding intelligent speed control brought it to 147 kW. The combined effect was a 66 % reduction in CRAH-associated power, 2.9 million kWh a year, with payback again under two years.

At high density that same exponent presents the bill. Every attempt to compensate for missing airflow by raising revolutions is paid at the cube, and not only in room units. The same happens inside the server, and there Uptime has a clean data point from HPE: two single-socket servers of practically identical configuration, one in 1U and one in 2U, differ by 30 to 60 W of consumption, and at low utilisation the 1U burns around 30 % more power to do the same work. The cause is geometric: a 1U chassis forces small fans that have to spin fast. In an air-cooled AI node, where fans in high-performance systems can take 10 % to 20 % of system power, that penalty stops being anecdotal and becomes a cost line.

Containment: cold aisle or hot aisle

Before buying anything, almost every room has headroom in airflow management. The enemy goes by two names: recirculation, hot exhaust air finding its way back to server inlets, and bypass, cold air reaching the return without passing through any equipment. Without airflow management, the installed cooling capacity needed can reach two or three times what theory requires, because the designer compensates for poor mixing by lowering the setpoint of the whole room.

The metrics to diagnose it have existed for years and remain underused. The RCI (Rack Cooling Index) measures how effectively inlet temperatures stay within specification, with two separate indices for the high and low ends; a well-designed room should be at 90 % or above. The RTI (Return Temperature Index) compares the temperature rise of the cooling units with that of the IT equipment and separates the two ills: below 100 % there is bypass, above it there is recirculation. A couple of measurement sessions and a handful of blanking panels in the rack gaps recover more efficiency than any catalogue promises.

The next step is physical containment, and here the choice between containing the cold aisle (CACS) or the hot one (HACS) has a consequence that gets overlooked. Schneider’s model of a 700 kW, 100-rack facility in Chicago, comparing both under the realistic condition that the uncontained zone stays at 24 °C so staff can work, shows a large difference: hot-aisle containment consumes 43 % less cooling energy and drops annualised PUE from 1.98 to 1.69. The reason lies in the setpoint, not in the physics of the exchange. Contain the heat and the whole room lives at 24 °C, equipment draws air at 24 °C and the economiser can run 5,319 hours a year. Contain the cold and the room is the hot aisle, and keeping it habitable at 24 °C forces supply air at 10 °C, which leaves the economiser at zero hours.

Figures published by ENERGY STAR point the same way with similar orders of magnitude: hot and cold aisle layout without containment already saves 10 % to 35 %; cold-aisle containment reaches up to a 30 % reduction in cooling cost; hot-aisle containment adds around 40 % on top of that. Typical payback on containment is under two years.

The thermal envelope and class H1

All of the above rests on a prior question: at what temperature can the equipment work. ASHRAE TC 9.9 answers with its air classes, and the ranges are less demanding than many rooms actually practise.

Class	Recommended	Allowable	Maximum dew point
A1	18-27 °C	15-32 °C	17 °C
A2	18-27 °C	10-35 °C	21 °C
A3	18-27 °C	5-40 °C	24 °C
A4	18-27 °C	5-45 °C	24 °C
H1	18-22 °C	18-25 °C	17 °C

The recommended range is identical for every A class: 18 to 27 °C equipment inlet temperature. What changes between classes is how far you can stray from it without losing warranty. A decade of PUE improvement was built on that table, raising setpoints and widening the hours in which outside air is enough.

The fifth edition of the guidelines, published in 2021, added a class that deserves attention: H1, intended for high-density air-cooled equipment. Its recommended range is 18 to 22 °C and its allowable limit drops to 25 °C, against A1’s 32 °C. ASHRAE’s stated reason is physical and direct: in some dense systems there is no room for heatsinks or fans capable of keeping components within their limit if the incoming air is warm. Class H1 is the formal acknowledgement, by the body that wrote the thermal envelope, that high density on air no longer fits inside it.

The efficiency consequence is the part that interests an architect. Going from an allowable of 32 °C to one of 25 °C, and from a recommended of 27 °C to 22 °C, destroys much of the annual economiser hour budget. Facilities designed around free cooling and adiabatic cooling do not sit well in that envelope, and Uptime says so without hedging: the way out is to deploy liquid cooling for the dense part. Air does not run out of thermal capacity when density rises. It runs out of efficient thermal capacity, which is what made the model attractive.

Free cooling and a climate on the move

What the economiser is worth depends on where you are. Vertiv’s analysis of a 1 MW datacenter gives three figures per city that serve as a quick reference:

City	Air-cooled chiller	Free cooling	Adiabatic free cooling
London	pPUE 1.21	pPUE 1.09	pPUE 1.06
Madrid	pPUE 1.22	pPUE 1.12	pPUE 1.07
Dubai	pPUE 1.31	pPUE 1.24	pPUE 1.18

In annual operating modes, London spends 81 % of the year in free cooling or adiabatic free cooling, and Madrid 74 %, which is on the order of 7,100 and 6,480 hours without mechanical compression. Adiabatic savings against a standard air-cooled chiller at full load land between 60 % and 65 %.

That window is closing, and it has been measured. A paper published in Scientific Reports in June 2026 quantifies out-of-specification hours (inlet above 27 °C, or dew point above 15 °C, or relative humidity above 70 %) for the 1980-2024 period and projects to 2050. Northern Europe remains comfortable, with about two hours a day on average out of range. The Mediterranean and the Middle East add more than four hours a day per decade. Dallas-Fort Worth exceeds 20 % of the year; Singapore, 85 %. The fraction of sites in the optimal tier falls 6 % per decade while the most constrained tier grows 9.1 %. For a design with a fifteen-year life in southern Europe, the 2026 economiser hour budget is not the 2040 one.

What it costs in water

Air has a cheap route to better energy efficiency: evaporate water. And there lies one of the most uncomfortable trade-offs in physical infrastructure. Vertiv’s numbers settle it in a single sentence. Adding an adiabatic misting system on top of an air-cooled chiller improved pPUE by 0.01 points and raised WUE from 0 to 0.165 gallons per kWh (about 0.62 l/kWh). At a 3 MW facility that is 4.3 million gallons a year, on the order of 16,000 m³, the consumption of about forty homes. Indirect evaporative cooling sits around one litre per kWh. An air-cooled chiller without adiabatic assist, or a pumped-refrigerant direct-expansion system, has zero WUE and pPUE of 1.12 to 1.13.

One hundredth of a pPUE point in exchange for four million gallons a year was for years an obvious trade in water’s favour, because energy was paid for and water almost was not. In 2026, with moratoria and public opposition around several projects, that sum no longer always works out, and in some jurisdictions it is not even legal. The reference case for evaporative air done well is still Prineville, where Meta reported a WUE of 0.22 l/kWh with 100 % outside-air economisation, direct evaporative misting and evaporation efficiency around 85 %, with no chillers and no towers.

The ceiling of air, and who reaches it

Well-designed air goes a long way. Google reports a fleet PUE of 1.09, and its best site, in central Ohio, sustains 1.04. Yahoo’s Computing Coop in Lockport was designed for a PUE of 1.08 using ambient air 99 % of the time, with the roof acting as a chimney and cooling dropping from 25 % to 1 % of total power, although that figure is design rather than measurement. Kyoto-type thermal wheels, with 4.6 metre rotors turning at 1 to 6 rpm and around 300 kW per unit, have taken whole facilities in Chicago to pPUE 1.19 with up to ten months of free cooling a year.

Against that, the industry average measured by Uptime has been stuck around 1.54 for six years: 1.59 in 2020, 1.55 in 2022, 1.54 in 2025. The gap between 1.09 and 1.54 is not technological. It is about design, scale and the ability to build from scratch in the right place. Air has a very high efficiency ceiling that almost nobody reaches, and that is the best news for an operator with an existing room: there is nearly always headroom before touching infrastructure.

Rear doors: the last rung

Between room air and liquid on the chip there is a step that has become the big retrofit product of 2025 and 2026: the rear-door heat exchanger, or RDHx. The idea is simple. Instead of cooling the room, hot air is intercepted right at the rack outlet, its heat removed with water, and it is returned to the room at the temperature it came in at. From the rest of the facility’s point of view, that 60 kW rack does not exist thermally. This is what vendors call room neutrality, and it explains why a rear door forces neither a containment redesign nor a plenum recalculation.

There are two families. The passive kind consumes no electricity: it uses the push of the server’s own fans to cross the coil, and sits in the lower capacity band. The active kind adds variable-speed fans in the door, compensating the coil’s pressure drop and responding better to hot spots, at the cost of its own consumption. Capacities declared by serious vendors converge on a reasonable band: Vertiv announces up to 80 kW per rack in its CoolLoop, Motivair up to 75 kW in the ChilledDoor, nVent up to 78 kW with 14 °C water and a maximum draw of 1,500 W per door. Other catalogues publish figures of up to 200 kW per cabinet without specifying water temperature or flow, and those should be read for what they are. The defensible band today is 40 to 80 kW per rack.

The detail that makes the rear door attractive is water temperature. It works with warm water, on the order of 14 °C, well above the 7 °C of a classic chiller plant, which widens free cooling hours instead of reducing them. And it deploys rack by rack, with no general construction and no room shutdown. Adoption reflects this: the 2026 AFCOM report puts operators who have deployed or are evaluating rear doors at 37 %, and Schneider Electric’s purchase of Motivair confirms that the large vendors see them as a strategic product.

The real density of the installed base

Public conversation revolves around 130 kW racks. The real density distribution of the global installed base, measured by Uptime in 2025, tells another story:

Average rack density	Facilities
1-3 kW	16 %
4-5 kW	30 %
6-7 kW	18 %
8-9 kW	10 %
10-14 kW	14 %
15-19 kW	5 %
20-29 kW	4 %
30 kW or more	4 %

Some 74 % of facilities have an average density below 10 kW per rack, and 82 % do not exceed 30 kW even in their densest cabinet. Asked about the threshold beyond which air stops being enough, operators give a modal answer of 40-49 kW (25 % of responses), and 44 % put it at 40 kW or above, well past the “over 20 kW already needs liquid” repeated in presentations. Uptime marks the impracticable boundary at around 50 kW per rack.

From that comes the correct reading of the moment: air is not being replaced, it is being segmented. The training and dense inference frontier is moving to liquid, and the headline goes with it. The bulk of the installed base, still below 30 kW, is solved with optimised air, containment and, when a GPU island appears, a rear door. The useful question for an operator is not when air dies, but which density percentile their room sits in.

For an inference factory

Three concrete decisions follow from all of the above.

The first is to exhaust airflow management before buying hardware. Measuring RCI and RTI, closing rack gaps with blanking panels, matching tiles to the real load distribution and raising the setpoint to the top of ASHRAE’s recommended range costs little and frees capacity that is already installed. The ventilation retrofits measured by DOE and LBNL, with 22 % to 66 % reductions in cooling power and payback under two years, are the best return available in an existing room.

The second is to choose hot-aisle containment if the goal is the economiser, and to design the system around the highest water temperature the equipment tolerates. The decision chain runs in that order: the setpoint sets free cooling hours, free cooling hours set the PUE and, in many geographies, the absence of compression is what allows the evaporative tower and its water consumption to be dropped. Class H1 pushes the other way for dense equipment, and that tension is what decides when liquid becomes due.

The third is to size by percentiles rather than by the hottest rack in the catalogue. A room about to host two 40 kW GPU racks alongside forty 6 kW cabinets does not need rebuilding: it needs rear doors on those two and serious airflow management on the rest. Moving to liquid makes sense when sustained density forces it, and that moment arrives sooner through efficiency than through capacity.

One idea runs through the rest of the series. Even when a whole room moves to direct liquid to chip, 2 % to 20 % of the heat still leaves through air, and somebody has to remove it. Air does not disappear from the AI datacenter. It changes role: it stops being the cooling system and becomes the system that picks up what the liquid does not capture. The next article goes into that liquid, into the cold plate that carries it to the silicon, and into the CDU that governs the loop.

Sources

Uptime Intelligence, Data Center Cooling Systems Survey 2025 (UI Field Report 181) — https://intelligence.uptimeinstitute.com/sites/default/files/2025-07/UI%20Field%20181_Data%20center%20cooling.pdf
Uptime Institute, Global Data Center Survey 2025 — https://datacenter.uptimeinstitute.com/rs/711-RIA-145/images/2025.Annual.Survey.Report.pdf
Uptime Institute Journal, New ASHRAE guidelines challenge efficiency drive — https://journal.uptimeinstitute.com/new-ashrae-guidelines-challenge-efficiency-drive/
Uptime Institute Journal, Lower density brings server efficiency and cooling gains — https://journal.uptimeinstitute.com/lower-density-brings-server-efficiency-and-cooling-gains/
Upsite Technologies, Major Changes to ASHRAE’s Fifth Edition of Thermal Guidelines, Part 2: New Air-Cooled Class for High Density Compute Equipment — https://www.upsite.com/blog/major-changes-to-ashraes-fifth-edition-of-thermal-guidelines-part-2-new-air-cooled-class-for-high-density-compute-equipment/
LBNL / DOE Center of Expertise, Thermal Guidelines and Temperature Measurements in Data Centers — https://datacenters.lbl.gov/sites/default/files/FINAL%20Thermal%20Guidelines%20and%20Temp%20Measurements%209-15-2020.pdf
US DOE / FEMP, Variable-Speed Fan Retrofits for Computer-Room Air Conditioners — https://www.energy.gov/sites/prod/files/2013/10/f3/dc_fancasestudy.pdf
LBNL, Demonstration of Intelligent Control and Fan Improvements in Computer Room Air Handlers — https://eta-publications.lbl.gov/sites/default/files/control-and-fan-improvements-in-crahs.pdf
Schneider Electric / APC, Impact of Hot and Cold Aisle Containment on Data Center Temperature and Efficiency (WP 135) — https://docs.media.bitpipe.com/io_10x/io_105811/item_561497/DBOY-7EDLE8_R2_EN.pdf
Schneider Electric / APC, Cooling Strategies for Ultra-High Density Racks and Blade Servers — https://www.facilitiesnet.com/whitepapers/pdfs/20130227-SchneiderElectric.pdf
ENERGY STAR (US EPA), Utilize Containment / Enclosures — https://www.energystar.gov/products/data_center_equipment/16-more-ways-cut-energy-waste-data-center/containmentenclosures
Scientific Reports (Nature), Limitations to air free cooling in data centers under rising heat and humidity — https://www.nature.com/articles/s41598-026-56926-3
Vertiv, Freecooling, Evaporative and Adiabatic Cooling Technologies in Data Center Applications — https://www.vertiv.com/48ee5c/globalassets/products/thermal-management/free-cooling-chillers/freecooling-evaporative-and-adiabatic-cooling-technologies-in-data-center-applications.pdf
Vertiv, Calculating the Impact of Water Usage on Data Center Costs and Sustainability — https://www.vertiv.com/4ad73a/globalassets/documents/white-papers/vertiv-water-usage-and-sustainability-wp-en-na-web_317594_0.pdf
Vertiv, How rear door heat exchangers (RDHx) support high-density rack cooling — https://www.vertiv.com/en-asia/insights/articles/educational-articles/how-rear-door-heat-exchangers-rdhx-support-high-density-rack-cooling/
Motivair, ChilledDoor Rear Door Heat Exchanger — https://www.motivaircorp.com/products/chilleddoor/
DatacenterDynamics, New rear door coolers from nVent for high-density racks — https://www.datacenterdynamics.com/en/product-news/new-rear-door-coolers-from-nvent-to-offer-a-scalable-solution-for-high-density-racks/
Google, Power usage effectiveness, Google Data Centers — https://datacenters.google/efficiency/
Open Compute Project, Water Efficiency at Facebook’s Prineville Data Center — https://www.opencompute.org/blog/water-efficiency-at-facebooks-prineville-data-center
US DOE / EERE, Yahoo! Compute Coop: Next Generation Passive Cooling Design for Data Centers — https://www1.eere.energy.gov/manufacturing/datacenters/pdfs/yahoo_passive_cooling.pdf
Data Center Frontier, KyotoCooling Gains Traction With Service Providers — https://www.datacenterfrontier.com/cooling/article/11431141/kyotocooling-gains-traction-with-service-providers
Upsite Technologies, The State of Data Center Cooling in 2026 — https://www.upsite.com/blog/the-state-of-data-center-cooling-in-2026/
Data Center Knowledge, AI Rack Density’s Real Limits: Power, Cooling, Failure Risk — https://www.datacenterknowledge.com/ai-data-centers/ai-rack-density-s-real-limits-power-cooling-failure-risk

PUE on lo0 — Tech Blog

Cooling the AI datacenter (1/4): the thermal challenge

The law that governs everything: power is heat

AI density breaks the model

The spectrum of solutions

The journey of heat, end to end

The metrics of heat

Water: the new constraint

Waste heat: from problem to resource

What is holding the transition back

Where the market is in 2026

For an inference factory

See also

Sources

Cooling the AI datacenter (2/4): air and its limits

Inside the air-cooled room

The geometry of airflow

The cube law, friend and executioner

Containment: cold aisle or hot aisle

The thermal envelope and class H1

Free cooling and a climate on the move

What it costs in water

The ceiling of air, and who reaches it

Rear doors: the last rung

The real density of the installed base

For an inference factory

See also

Sources