Facility Water Temperature for GPU Liquid Cooling: How to Check Compatibility
Your facility water can support a liquid-cooled GPU deployment only if the selected CDU can deliver the required coolant temperature and flow at the specified IT inlet, under the operating conditions your project must support.
Comparing the facility supply temperature directly with the server’s maximum inlet temperature is insufficient. A liquid-to-liquid CDU needs a temperature difference to transfer heat. Distribution effects and operating tolerances consume additional margin.
We recommend resolving this question before selecting a CDU by nominal cooling capacity. The assessment should produce one of three results:
- Suitable: the proposed system meets the temperature and hydraulic requirements across the agreed operating conditions.
- Suitable with conditions: compliance depends on defined measures such as auxiliary cooling, a restricted load or a different maintenance arrangement.
- Not demonstrated or unsuitable: essential performance evidence is missing, or the calculated operating conditions exceed the equipment requirements.
That distinction helps separate an existing plant that needs modest changes from one that requires additional cooling infrastructure.
Establish the Requirement at the Correct Inlet
The starting point is the approved coolant specification for the exact server or rack configuration.
“GPU inlet temperature” may refer to a rack manifold, a compute tray connection or inlet air. The relevant boundary must appear on the project drawings and in the acceptance requirements.
If the server manufacturer defines a temperature limit at the rack connection, evaluate compliance there. If the limit applies at individual server connections, a CDU outlet measurement alone does not establish compliance.
Request the following together:
- Continuous coolant inlet temperature range.
- Required flow and permitted pressure conditions.
- Approved coolant composition and concentration.
- Applicable hardware configuration and power mode.
- Condensation requirements.
- Permitted temperature excursions and rates of change, where specified.
A maximum inlet temperature is meaningful only with its accompanying operating conditions.
Recent warm-water platform announcements do not replace those requirements. NVIDIA’s discussion of 45°C cooling, for example, concerns its Rubin architecture and links chillerless operation to climate and system design. It does not establish a universal facility water limit for other GPU installations.
Calculate the Facility Water Limit from the IT Requirement
For this assessment, the Facility Water System (FWS) delivers cooling water to the CDU. The Technology Cooling System (TCS) circulates approved coolant between the CDU and IT equipment.
In a conventional liquid-to-liquid CDU, a heat exchanger separates these circuits. Without refrigeration, the CDU cannot continuously deliver TCS supply coolant colder than the entering facility water while rejecting a positive heat load.
The supply-side approach is commonly expressed as:
Approach temperature = TCS supply temperature − FWS inlet temperature
Both measurement locations must be defined. A heat-exchanger-port rating and a complete-CDU-connection rating may treat pump heat differently.
For preliminary screening:
Allowable FWS inlet temperature = IT inlet limit − required CDU approach − downstream allowance − operating reserve
The critical term is required CDU approach. Use the value supported by the supplier’s selection at the specified heat load, fluids and flow rates.
Do not automatically use the temperature difference observed during ordinary operation. A controller may deliberately maintain TCS supply warmer than the minimum achievable temperature. In that case, a larger measured difference does not by itself indicate poor heat exchanger performance.
Vertiv’s CoolChip CDU 100 application guide illustrates the selection relationship using different approach temperatures, facility flows and inlet conditions. This supports treating approach as a duty-dependent requirement, rather than a constant attached to every operating state.
Example: A 40°C Server Limit Does Not Permit 40°C Facility Water
The following values are hypothetical. They explain the method and are not specifications for a particular GPU server or XINKE CDU.
| Temperature budget item | Illustrative value | Basis required in a real project |
|---|---|---|
| Maximum IT coolant inlet temperature | 40°C | Approved equipment requirement |
| Required CDU approach | 4°C | Selection at actual load, coolant and flows |
| Downstream positive temperature allowance | 1°C | Distribution calculation or measurement |
| Operating reserve | 2°C | Defined control and measurement allowances |
| Maximum FWS inlet temperature | 33°C | 40 − 4 − 1 − 2 |
At 35°C facility supply, the estimated IT inlet would be:
35 + 4 + 1 = 40°C
This reaches the equipment limit without preserving the specified reserve. It therefore fails this illustrative design basis, even though the facility water is cooler than the server limit.
A CDU requiring only a 2°C approach could potentially close the gap. However, the supplier must substantiate that performance at the available facility flow and pressure conditions.
The subtraction identifies the temperature target. A coupled thermal and hydraulic selection must establish whether the equipment can achieve it.
The 1°C and 2°C allowances above are not standard industry values. Distribution coolant may gain or lose heat depending on its surroundings. Identify credible positive temperature effects and avoid counting pump heat, sensor uncertainty or control excursions twice.
Verify What the Facility Delivers at the CDU
A plant setpoint does not establish the conditions delivered to every CDU branch.
For an existing facility, the useful evidence is a time-aligned record of FWS inlet temperature, flow and available differential pressure at the proposed connection. Include concurrent plant load and operating mode.
The assessment should cover credible combinations such as:
- High IT load during the hottest relevant weather.
- Simultaneous demand from other users on a shared water network.
- Cooling plant changeover.
- The maintenance configuration the project intends to support.
- Initial partial-load operation and planned expansion.
An average temperature can conceal a short period that determines compliance. Conversely, combining unrelated worst-case values can create an unrealistic design condition.
Where monitoring history is incomplete, state what it establishes and what remains to be supported by plant calculations. A few mild-weather measurements cannot establish peak-summer capability.
For a new installation, specify the FWS temperature and hydraulic delivery envelope at the CDU connection, with responsibility assigned for maintaining it.
Confirm Flow Before Accepting the Temperature Result
An acceptable FWS inlet temperature is necessary, but it is not sufficient.
On the facility side, inadequate flow can prevent the heat exchanger from achieving its selected performance. On the IT side, inadequate branch flow can leave individual servers outside their cooling requirements even while the CDU supply temperature remains acceptable.
The heat balance is useful:
Liquid heat load = mass flow × specific heat capacity × coolant temperature rise
However, it does not establish the manufacturer’s minimum flow through a server or replace the pump and network calculation.
For each proposed duty, confirm:
- Facility flow through the CDU at the available differential pressure.
- TCS flow delivered against the installed distribution resistance.
- Required flow at the supported racks or servers.
- Maximum operating pressure as a separate limit.
- Fluid properties at the specified concentration and temperature.
Increasing flow is not an unlimited remedy. It changes pressure losses and pumping requirements and must remain within equipment limits.
Our CDU sizing guide explains the wider capacity assessment. For temperature compatibility, the essential point is to validate temperature, flow and pressure together at the same duty.
Check the Lower Temperature Limit
Colder facility water can improve the available heat-transfer margin, but the TCS must still meet minimum temperature and condensation requirements.
Lenovo’s SC777 V4 Neptune documentation, for example, links the permitted inlet temperature to dew-point conditions. This demonstrates why a maximum-temperature check alone is incomplete.
For exposed IT-side components, establish a project-specific lower temperature boundary using the equipment requirements and local humidity conditions. The dew point at the relevant equipment location matters; a remote room sensor may not represent every exposure condition.
The CDU must maintain its selected TCS target between that lower boundary and the upper operating target. Confirm controllability at low load and cold FWS conditions, including valve range and bypass arrangements where provided.
Facility-side piping requires its own condensation assessment if it operates below the surrounding dew point.
If the acceptable lower and upper boundaries overlap poorly—or do not overlap at all—the solution requires changes to humidity control, operating conditions or system design. A new setpoint alone cannot resolve an incompatible operating window.
Include the Outdoor Heat-Rejection Temperature Difference
For a facility served by a dry cooler, the temperature budget extends to the outdoor unit.
During steady heat rejection in dry mode, the leaving fluid must remain above the entering air temperature by the approach the selected cooler can achieve. The CDU then requires another temperature difference.
A preliminary relationship is:
IT inlet temperature ≈ dry-cooler entering air temperature + dry-cooler approach + CDU approach + distribution effects
For illustration, assume:
- Dry-cooler entering air: 32°C.
- Required dry-cooler approach: 5°C.
- Required CDU approach: 4°C.
- Positive distribution effect: 1°C.
The estimated IT inlet is 42°C before operating reserve.
This does not mean dry cooling is unsuitable generally. It means this particular combination would not support a 40°C inlet limit.
For a dry cooler selection, specify the actual entering air condition, including credible discharge-air recirculation, together with fluid temperatures, mixture, flow and fan operating condition.
If the duty depends on adiabatic assistance or mechanical cooling, identify that dependency explicitly. A wet-assisted selection cannot serve as proof of dry-mode performance.
Modify the Part of the System That Causes the Shortfall
A failed temperature budget does not automatically require replacing the whole cooling plant.
| Assessment result | Modification to evaluate | Evidence needed |
|---|---|---|
| Facility water is cool enough, but CDU output is too warm | Restore facility flow or reassess heat exchanger performance | Measured flows, temperatures and supplier selection |
| Required CDU approach exceeds the available budget | Select a lower-approach configuration | Confirmed duty at available flows and pressure conditions |
| Facility water becomes too warm seasonally | Add heat rejection or mechanical cooling assistance | Performance at the relevant seasonal conditions |
| CDU supply is acceptable, but particular IT inlets are not | Correct distribution, mixing or local flow problems | Measurements at the affected connections |
| TCS becomes too cold at light load | Improve temperature control within the approved range | Demonstrated low-load operation |
| Normal operation passes, but maintenance operation fails | Revise redundancy or the supported maintenance load | Remaining thermal and hydraulic capacity |
An in-row CDU should be assessed against the required approach and hydraulic duty, rather than selected by its headline kilowatt rating.
Where the FWS cannot deliver sufficiently cool water, an integrated cooling station may provide the additional cooling required, subject to project-specific selection.
Prove Steady Operation and Transients Separately
A steady-state temperature reserve does not establish performance during a pump transfer, power interruption or cooling-source changeover.
Transient behaviour also depends on circulating fluid volume, retained flow, control timing and the IT load response. A fixed temperature allowance cannot represent all these effects.
We recommend separating acceptance into two parts:
- Steady operation: demonstrate the required temperatures and flows at the agreed load and FWS conditions.
- Specified transitions: demonstrate that the agreed switching or disturbance events remain within approved limits, or trigger the agreed protective response.
| Acceptance item | What the agreement should define |
|---|---|
| Equipment and load | Hardware configuration, liquid heat load and supported rack quantity |
| FWS delivery | Inlet temperature, flow and available differential pressure |
| TCS delivery | Coolant, required flows and temperature limits |
| Measurement locations | CDU connections and the specified IT inlet boundaries |
| Measurement quality | Calibration, accuracy and logging suitable for the event being assessed |
| Test scenarios | Steady loads, maintenance states and selected transitions |
| Acceptance basis | Continuous limits and separately approved transient criteria |
Factory testing can establish CDU performance at the tested conditions. A simulated heat load does not validate the installed rack network or actual server behaviour.
Site testing must address those installation effects. Untested seasonal conditions should remain identified as calculation-supported conditions until appropriate evidence is available.
The strongest acceptance record shows FWS conditions, CDU delivery and IT inlet conditions together under the same operating load.
Order & Project Support
For a temperature compatibility review, send us the server or rack model, approved coolant requirements, initial and future liquid heat loads, FWS temperature range, available flow and differential pressure, and proposed layout.
Include the load that must remain supported during maintenance and any existing summer operating records.
These inputs allow the assessment to identify the allowable facility water temperature, the required CDU duty and any additional cooling or distribution changes that need evaluation. Share your project information through our project consultation page.