Technology · Liquid cooling · Explainer · updated 1 Nov 2026
In a liquid-cooled data centre the secondary loop, or technology cooling system (TCS), carries coolant from the coolant distribution unit (CDU) to the cold plates on the chips. This page explains how that loop is regulated (temperature, dew point, flow, pressure, coolant chemistry, filtration and leak detection) and which rules and guidelines apply to it. For engineers, operators, buyers and consultants. Facts only, each linked to its source.
Two loopsHow it is regulatedASHRAE classesRulesChecklistQuestions
Photo by Sebastian Morelli-Peyton on Unsplash2 °C
minimum margin above room dew point that the CDU keeps the secondary (TCS) supply at, to stop condensation (ASHRAE TC 9.9 guidance)
~25%
propylene glycol in PG25, the common secondary-loop coolant (24.5% to 27.5% by volume, OCP guideline, Sept 2026)
500 kW
installed IT power from which EU data centres must report water use, reused heat and waste heat temperature every year
Source: EUR-Lex (opens in a new tab)
10%
minimum share of reused energy for German data centres that start operating from 1 July 2026 (EnEfG section 11)
Source: Gesetze im Internet (German Federal Ministry of Justice) (opens in a new tab)
Our drawing, simplified, after ASHRAE TC 9.9's description of a CDU separating the facility water system from the technology cooling system. Real systems add redundancy, expansion tanks and bypasses. ASHRAE TC 9.9: Water-Cooled Servers: Common Designs, Components, and Processes (white paper) (opens in a new tab)
Supply temperature
The CDU holds the secondary supply between two limits. The upper limit is the warmest coolant that still cools the IT at the design flow. The lower limit follows the room dew point: ASHRAE TC 9.9 says the CDU should keep the supply at least 2 °C (3.6 °F) above it.
Flow and pressure
Pumps in the CDU drive the secondary flow, and valves set it for each rack. One rack heat exchanger guide (IBM) sets a 23 to 38 litres a minute flow range, recommends adjustable flow valves on supply lines and requires a relief valve so loop pressure stays below 689.66 kPa (100 psi). Limits differ by product, so use the figures in your own equipment manual.
Coolant chemistry
For PG25 the OCP guideline gives typical values for fresh fluid: chloride below 5 ppm, sulphate below 10 ppm and total hardness below 25 ppm, and says bio-growth may occur if the glycol falls below about 25%. It recommends a qualified lab test of every secondary loop at least once a year.
Filtration
OCP describes sidestream filtration, which passes typically 5% to 15% of the flow through a filter all the time, and full-flow filtration with duplex filters so they can be changed without a shutdown. It recommends filtering fresh coolant during fill, for example with 5 to 10 micron absolute filters.
Leak detection and alarms
The secondary loop runs inside the white space, so leaks matter. Lenovo's rack heat exchanger guide asks operators to consider leak detection on the loop that feeds it, and OCP recommends in-line sensors with set operating limits and actions.
Source: Lenovo Docs: Water specifications for the secondary cooling loop (ThinkSystem Heavy Duty Rack Cabinets) (opens in a new tab); Open Compute Project: Guidelines for using propylene glycol-based heat transfer fluids in liquid cooled racks, revision 2.0 (opens in a new tab)
W classes set the facility (primary) water temperature the IT equipment must accept at full performance. The minimum for every class is 2 °C. W+ (above 45 °C) has no upper limit, so it has no bar. ASHRAE lists it as typically run without chillers, with district heating as the typical heat user.
Source: ASHRAE: Thermal Guidelines for Data Processing Environments, 5th edition: reference card (Table 3.1, liquid cooling classes) (opens in a new tab) Accessed 1 Nov 2026.
| Class | Highest supply temperature | Typical heat rejection (ASHRAE) |
|---|---|---|
| W17 | 17 °C | Chiller and cooling tower, optional water-side economiser |
| W27 | 27 °C | Chiller and cooling tower, optional water-side economiser |
| W32 | 32 °C | Mostly without chillers, some sites still need them |
| W40 | 40 °C | Mostly without chillers, some sites still need them |
| W45 | 45 °C | Usually without chillers, dry coolers where the climate allows |
| W+ | Above 45 °C | Typically without chillers, district heating |
Laws are marked as law, industry documents as guidance. This page covers EU and German rules plus global guidance. It is general information, not legal advice. Checked 1 Nov 2026.
| Where | Rule or guideline | Who it covers | What it asks for | Type | Source |
|---|---|---|---|---|---|
| European Union | Delegated Regulation (EU) 2024/1364 (Energy Efficiency Directive reporting) · our policy page | Operators of data centres with installed IT power of 500 kW or more | Report each year by 15 May: total and drinking water input, waste heat reused outside the site, average waste heat temperature (measured where the fluid that cools the IT enters the heat exchanger at the computer room boundary), refrigerants used, and energy figures. Records of measurement points must be kept for 10 years. | Law in force | EUR-Lex: Commission Delegated Regulation (EU) 2024/1364 on the first phase of a common Union rating scheme for data centres (opens in a new tab) |
| Germany | Energy Efficiency Act (EnEfG) section 11 · our policy page | Data centres with 300 kW or more of non-redundant connected load | New sites from 1 July 2026: PUE 1.2 or lower and at least 10% reused energy (15% for sites from July 2027, 20% from July 2028). Existing sites: PUE 1.5 from July 2027 and 1.3 from July 2030. Warm secondary loops make heat easier to reuse. An amendment bill is before the Bundestag. | Law in force, amendment proposed | Gesetze im Internet (German Federal Ministry of Justice): Energieeffizienzgesetz (EnEfG) section 11: Klimaneutrale Rechenzentren (opens in a new tab) |
| Global | ASHRAE TC 9.9 liquid cooling classes (W17 to W+) | Designers, operators and IT makers | Defines facility water supply temperature classes and the CDU's role in separating the facility water system from the technology cooling system. Industry guidance, not law, but often written into specifications and contracts. | Industry guidance | ASHRAE: Thermal Guidelines for Data Processing Environments, 5th edition: reference card (Table 3.1, liquid cooling classes) (opens in a new tab) |
| Global | OCP guidelines for PG25 heat transfer fluids, revision 2.0 (September 2026) | Design engineers, operators, CDU, cold plate and fluid suppliers | Coolant properties, wetted materials, flushing and fill, filtration, sampling frequency and in-line monitoring for the secondary loop between CDU and cold plates. Guidance, not law. | Industry guidance | Open Compute Project: Guidelines for using propylene glycol-based heat transfer fluids in liquid cooled racks, revision 2.0 (opens in a new tab) |
| European Union | Pressure Equipment Directive 2014/68/EU | Makers and suppliers of stationary pressure equipment above 0.5 bar maximum allowable pressure | Covers design, manufacture and conformity assessment. Whether a given CDU, manifold or pipework assembly falls under it depends on its pressure, volume and fluid, so ask the supplier for its conformity assessment. | Law in force | European Commission: Pressure Equipment Directive (2014/68/EU) (opens in a new tab) |
| European Union | F-gas Regulation (EU) 2024/573 | Operators of listed equipment containing fluorinated gases, relevant to two-phase loops that use a fluorinated refrigerant | Leak checks every 3 to 24 months depending on charge and whether leak detection is fitted, records, certified technicians, and a phase-out of placing HFCs on the EU market by 2050. Single-phase water or PG25 loops contain no F-gas. Check how the rules apply to a specific product and refrigerant. | Law in force | European Commission: F-gases in equipment and products: stakeholder obligations (Regulation (EU) 2024/573) (opens in a new tab) |
| Global | OCP Project Deschutes facilities specification and MQD/MQDB coupling specification | Buyers, designers and suppliers who adopt them | Voluntary Open Compute Project specifications for liquid cooled facilities and quick disconnect couplings. They bind only when written into a contract. | Industry guidance | Open Compute Project: Project Deschutes: data center facilities specification (opens in a new tab) |
Section 11(2) of the Energy Efficiency Act (EnEfG) as in force on 1 November 2026. Sites have two years after commissioning to reach the share as an annual average. The 15% and 20% steps are planned shares for sites starting later, not targets for existing sites. A government bill (Bundestag printed paper 21/8027) would give four years instead of two and count some internal heat use. It is not law yet.
Germany
Source: Gesetze im Internet (German Federal Ministry of Justice): Energieeffizienzgesetz (EnEfG) section 11: Klimaneutrale Rechenzentren (opens in a new tab) Accessed 1 Nov 2026.
| Data centre starts operating | Minimum reused-energy share |
|---|---|
| From 1 July 2026 | 10% |
| From 1 July 2027 | 15% |
| From 1 July 2028 | 20% |
Proposed changes are tracked on the EnEfG amendment bill page and the EU's next step on the EU data centre rating scheme page.
Our checklist, drawn from the guidance above. It is not a design standard.
Official videos from the companies' own channels, shown side by side for balance. CoolPower Index does not endorse either. They load from YouTube only when you press play.
It is the closed pipe loop that carries liquid straight to the hot computer chips and back, like the coolant in a car engine.
Why it matters A heat exchanger in the coolant distribution unit (CDU) passes the heat to the building's own water loop, without the two liquids mixing.
Source: ASHRAE
A machine called the CDU keeps the liquid at the right temperature and flow, filters it and raises alarms if something goes wrong.
Why it matters It keeps the liquid just warm enough to stop water droplets forming on the electronics, like mist on a cold glass.
Source: ASHRAE TC 9.9, OCP
Not for the loop itself. But EU and German rules make larger data centres report water, waste heat and energy use.
Source: EU Delegated Regulation 2024/1364, German EnEfG
Usually PG25: water mixed with about a quarter propylene glycol (a kind of antifreeze) and chemicals that stop rust.
Source: OCP
They are labels for the warmest water the computers can accept: 32, 40 or 45 °C. The warmer the water allowed, the less need for power-hungry chillers.
Why it matters Chillers work like a fridge. Warmer classes can often cool with outside air instead.
Source: ASHRAE
Coverage limits: product limits (flow, pressure, temperature) differ by maker, so the IBM and Lenovo figures are examples only. This page does not cover national rules outside the EU and Germany. Photo: Unsplash. Flags: flag-icons (MIT licence). Charts: Apache ECharts (Apache 2.0). Related: Coolant distribution units (CDUs) · Direct liquid cooling · Warm-water cooling loops · Heat reuse · Policy tracker.
