Data centre design reference

Generic reference designs for power, cooling, controls and operations

Cooling and heat reuse

Cooling options for data halls and how the heat can be reused.

On this page
  1. The heat path
  2. Choose the facility-water temperature first
  3. Size the CDUs from flow, approach and filtration
  4. Reject the heat without chillers
  5. Heat reuse
  6. The plant room in 3D
  7. What fails, and what the design does about it
  8. Sources

The heat path

In a liquid-cooled AI hall most of the heat leaves the rack in liquid, not in air. The reference design moves it through three separate loops, each with its own water quality, pressure and failure behaviour.

Reference heat path: cold plates to CDU, facility water, plate heat exchangers and a raw-water heat sink, with a heat-reuse branch Rack cold plates hand heat to a technology cooling loop served by CDUs. The CDUs hand it to a closed, treated facility-water loop with duty and standby pumps on variable-speed drives. Plate heat exchangers hand it to an open raw-water loop with its own pumps and strainers. A heat pump can lift part of the facility-water return to district-heating temperature. Technology cooling loop Facility water loop (closed, treated) Raw-water loop (open) Heat reuse (optional) Racks cold plates CDU N+1 per block TT TT Strainer FW pump duty, VFD FW pump duty, VFD FW pump standby, VFD TT TT PT PT Plate heat exchangers, 3 x 50 % RW pump duty RW pump duty RW pump standby Strainer duty Strainer standby Intake screens Outfall TT TT Heat pump lifts return heat District heating network, about 70 °C 1. Solid lines carry cold water towards the racks, dashed lines warm water away from them. TT, PT: temperature and pressure transmitters feeding the control system. 2. Pump and exchanger counts are a pattern (duty plus standby, 3 x 50 % exchangers), not a sizing; the 3D plant room below shows the smallest case, one duty and one standby pump and a duplex strainer on each loop. Size flows and units for the block load with the sizing calculator. 3. Where no raw water is available, dry coolers take the place of the exchangers and the raw-water loop. 4. Not drawn: the raw-water flow transmitter and the strainer differential-pressure transmitters that the alarms on this page need.
Reference heat path: cold plates to CDU, facility water, plate heat exchangers and a raw-water heat sink, with a heat-reuse branch
  • Technology cooling loop. Clean, finely filtered coolant between the cold plates and the CDU. The Open Compute Project's CDU test method runs propylene glycol on this side and water on the facility side.1
  • Facility-water loop. A closed, treated loop from the CDUs to the plate heat exchangers, with duty and standby pumps on variable-speed drives.
  • Raw-water loop. An open loop from a natural heat sink through strainers and pumps to the other side of the exchangers. Raw water carries silt and fouling, so it is kept two heat exchanges away from the IT.

Choose the facility-water temperature first

The facility-water temperature decides almost everything downstream: whether chillers are needed, how large the heat rejection is, and how useful the heat is to anyone else.

ASHRAE TC 9.9 names the facility-water classes by their upper limit, W17, W27, W32, W40, W45 and W+, each with a 2 °C lower limit.2 In 2021 it found that most cold-plate solutions could run on W32 water, and warned that warmer classes get harder to use as chips draw more power.2 One GPU maker now says its newest AI servers accept coolant up to 45 °C, leaving at roughly 55 °C.3

Design position. Choose the warmest facility-water class the IT maker will warrant in writing, with W32 as the design target. Some platforms still ask for colder water, so leave space and electrical capacity for trim cooling. The class is measured at the facility-water supply: a 45 °C coolant limit at the cold-plate inlet means facility water colder by the CDU approach. In favourable, cool climates, warm facility water allows chiller-free heat rejection with dry coolers,34 and a warmer return raises the temperature that can be sold as heat.

Size the CDUs from flow, approach and filtration

The Open Compute Project rates liquid-to-liquid CDUs at 1.5 litres per minute of coolant per kilowatt of IT load and a 5 °C approach temperature difference.1 It distinguishes in-rack from in-row units and sets the technology-loop pressure they must deliver.1 A vendor white paper puts state-of-the-art approach temperatures at 2 to 4 °C.4

Filtration matters more than in air-cooled halls, because cold-plate channels are narrow. The OCP method lists a primary filter of 200 micrometres or finer and a secondary filter of 50 micrometres or finer, and most CDUs offer 50 or 25 micrometre options.14

CDU performance is not yet rated the same way by every maker.14 Ask for test data under one named method, at your own flow and approach, before comparing offers: the OCP method today, or the ASHRAE rating addendum for CDUs once it is published.4

Design choice. N+1 CDUs per power block, each fed from both UPS-backed boards (see the 2N block). A cold plate holds very little heat, so a loss of coolant flow reaches the chips much faster than a loss of room air: the CDU pumps belong on the same protected supply as the racks. This is engineering practice, not a finding the research run tested.

For a quick figure: carrying 1 MW of heat at a 10 K temperature rise takes about 24 kg/s of water, roughly 86 m³ per hour. The sizing calculator does this for a whole hall.

Reject the heat without chillers

Warm facility water makes chillers an exception. A GPU maker says chiller-less operation with dry coolers is possible in favourable climates, with chillers needed for perhaps 1 % of the year in some of them, and a cooling vendor notes that economiser savings are significant in cooler climates.34

The reference design rejects heat to raw water through plate heat exchangers, three at 50 % each, so one can be opened and cleaned while the others carry the load. Where no raw water is available, dry coolers take the exchangers' place.

Watch the common mode. Every exchanger depends on the same raw-water flow. A blocked intake screen or a fouled strainer takes capacity from all of them at once. Fit duplex strainers with a differential-pressure alarm, alarm on falling raw-water flow before the facility-water temperature moves, and decide in the design what the hall does if the heat sink is lost.

Heat reuse

Data-centre heat can carry a town. In Finland, heat from a 15 MW data centre covered 20 GWh, 54 % of the town of Mäntsälä's heat demand in 2018.5 The usual obstacle is temperature: two peer-reviewed studies name the low temperature of the waste heat as a leading barrier to its reuse,67 and the Norwegian government's own data-centre page says the same while noting that several examples already exist in Norway.8

Rules now push in the same direction. In the EU, the Energy Efficiency Directive (EU) 2023/1791 requires data centres of at least 500 kW of installed IT power to publish a set of energy information, and data centres with a rated energy input above 1 MW to use their waste heat unless they show it is not technically or economically feasible.9 The directive is still in the EEA process for Norway,10 but Norway has its own rule: a data centre above 2 MW of supplied electrical power needs an NVE-approved cost-benefit analysis of its surplus heat before construction starts.1112 See Designing in Norway.

Design position. Return facility water at 40 to 45 °C where the IT allows it, and leave a tie-in, space and electrical capacity for a heat pump that lifts it to district-heating temperature. A heat buyer then needs a contract, not a rebuild.

The plant room in 3D

The reference plant room for one block: duplex raw-water strainers and a duty and standby pair of raw-water pumps along the north wall, the facility-water pair beside them, the three plate heat exchangers in a row along the south wall with their headers overhead, and the drive and control cabinets at the west end of the south wall. A walkway runs the length of the room between the two rows. The main items carry a QR-style label for their register key, drawn as a symbol in this model.

Interactive 3D view: needs JavaScript and WebGL.

What fails, and what the design does about it

TriggerWhat followsWhat the design does
Water leaves a closed circuit (a joint, seal or gasket)Static pressure falls, the pump cavitates and trips; if the standby does not start, flow and heat transfer stopPressurisation unit, low-pressure alarm, automatic standby start that is tested on a schedule, isolation valves per circuit
Raw-water flow falls (screen, strainer, intake)Every exchanger loses capacity at the same timeDuplex strainers with differential-pressure alarm, a raw-water flow alarm, a defined response when the heat sink is lost
A CDU or its pump failsThe racks it serves lose flow within secondsN+1 CDUs per block, two pumps per CDU, CDU supply on the UPS-backed boards
Coolant leaks at a rack manifoldWater near live equipmentLeak-detection cable under the manifolds, automatic isolation, drip trays where hoses cross power
A plate heat exchanger foulsApproach temperature creeps upTrend the approach on every exchanger; the third exchanger lets one be cleaned without derating

Sources

  1. Liquid to Liquid CDU Test Methodology and Performance Rating. Open Compute Project, 2024-08. Read 2026-10-18.https://www.opencompute.org/documents/ocp-wp-l-lcdu-test-methodology-performance-rating-r1-pdf
  2. Emergence and Expansion of Liquid Cooling in Mainstream Data Centers. ASHRAE Technical Committee 9.9 (ASHRAE), 2021. Read 2026-10-18.https://www.ashrae.org/file%20library/technical%20resources/bookstore/emergence-and-expansion-of-liquid-cooling-in-mainstream-data-centers_wp.pdf
  3. Hotter Than a Hot Tub: The 45°C Breakthrough to Cool AI’s Biggest Machines. NVIDIA, 2026-06-21. Read 2026-10-18.https://blogs.nvidia.com/blog/liquid-cooling-ai-factories/
  4. Direct Liquid Cooling System Challenges in Data Centers (White Paper 210, Version 1). Schneider Electric. Read 2026-10-18.https://media.datacenterdynamics.com/media/documents/WP210_V1_EN.pdf
  5. District heating from data centre waste heat, Mäntsälä (Sitra case, archived). Sitra (Finnish Innovation Fund), archive, 2019-10-18. Read 2026-10-18.https://arkisto.sitra.fi/en/cases/district-heating-from-data-centre-waste-heat-mantsala/
  6. Future views on waste heat utilization – Case of data centers in Northern Europe. Elsevier, Renewable and Sustainable Energy Reviews (indexed on IDEAS/RePEc), 2018. Read 2026-10-18.https://ideas.repec.org/a/eee/rensus/v82y2018ip2p1749-1764.html
  7. A material social view on data center waste heat: Novel uses and metrics. Frontiers in Sustainability (Frontiers Media), 2023-01-25. Read 2026-10-18.https://www.frontiersin.org/journals/sustainability/articles/10.3389/frsus.2022.1008583/full
  8. The data centre industry - a sustainable industry of the future for the digital Norway (chapter 5). Norwegian Government (regjeringen.no). Read 2026-10-18.https://www.regjeringen.no/en/documents/the-data-centre-industry-a-sustainable-industry-of-the-future-for-the-digital-norway/id3112356/?ch=5
  9. Directive (EU) 2023/1791 on energy efficiency (recast). Publications Office of the European Union (EUR-Lex), 2023. Read 2026-10-18.https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32023L1791
  10. Energieffektiviseringsdirektivet (2023). europalov.no, 2026-04-17. Read 2026-10-18.https://europalov.no/rettsakt/energieffektiviseringsdirektivet-2023/id-29284
  11. Energiloven (LOV-1990-06-29-50). Lovdata. Read 2026-10-18.https://lovdata.no/dokument/NL/lov/1990-06-29-50
  12. Kost-nytteanalyse av overskuddsvarme: § 4 Plikt til å gjennomføre kost-nytteanalyser. NVE, 2025-03-27. Read 2026-10-18.https://veiledere.nve.no/kost-nytteanalyse-av-overskuddsvarme/kapittel-2-plikt-til-a-gjennomfore-kost-nytteanalyse/4-plikt-til-a-gjennomfore-kost-nytteanalyser/