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Choose cooling as a whole-facility system, not as a hunt for the highest product efficiency rating. Start with the present and planned IT load, rack density, climate, water constraints, availability target and operating hours. Then correct airflow, model full- and part-load performance, compare air, water, economizer and liquid options, and verify the controls and maintenance plan. The most efficient design is site-specific; the U.S. Department of Energy (DOE) Federal Energy Management Program (FEMP) says no single design is best for every data center.
Start with the data a cooling design actually needs
A cooling specification is only as good as its design assumptions. Assemble a current baseline and a growth case before requesting equipment proposals. Record the following for each operating zone:
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- Measured and forecast IT power, including minimum, typical and peak demand.
- Rack count, rack power density and the types of servers, storage and accelerators to be installed.
- Daily and seasonal load variability, planned maintenance windows and hours at each load level.
- Required availability, redundancy and the effect of a cooling component outage on IT service.
- Supply-air and chilled-water temperatures currently used or permitted by the IT equipment.
- Outdoor dry-bulb and wet-bulb conditions, altitude, air quality, humidity and the number of hours suitable for economizing.
- Water availability, treatment requirements, discharge limits and the local cost or risk of consumption.
- Contamination concerns, flood and leak exposure, available plant space, electrical capacity and service access.
- Whether recovered heat has a useful sink, such as nearby buildings or industrial processes.
DOE’s Best Practices Guide for Energy-Efficient Data Center Design (July 26, 2024) treats the IT equipment and its environmental conditions as an upstream priority. A change that lowers the heat produced or permits a wider operating envelope can reduce mechanical and electrical requirements downstream.
Turn the baseline into design cases
At minimum, model a current operating case, a near-term expansion case and the maximum credible future case. Include low-load nights and weekends rather than sizing only for a nameplate peak. Identify which loads are constant and which can be scheduled, throttled or moved. This profile determines whether full-load efficiency or part-load efficiency has the greater effect on annual energy use.
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Set the boundary for every metric
Decide whether a result covers a room, a cooling plant, a building or the entire site. State the reporting period and weather data used. A plant coefficient or chiller rating cannot be compared directly with a whole-facility metric unless the boundaries match.
Fix airflow before buying more cooling
Air management is usually the least expensive place to remove wasted cooling capacity. Arrange racks so cool supply air reaches equipment intakes and hot exhaust returns to the cooling units without mixing. Use a hot-aisle/cold-aisle layout, containment where appropriate, blanking panels in unused rack spaces, sealed cable openings and a defined return-air path.
Look for bypass and recirculation
Bypass air travels around IT equipment and returns to the cooling unit without removing useful heat. Recirculation sends hot exhaust back to an intake. Both make sensors and control loops react to misleading temperatures, encouraging unnecessary airflow or lower setpoints. During a survey, inspect open rack spaces, missing floor tiles, unsealed penetrations, gaps in containment and obstructions that force supply air away from intakes.
Use temperature and humidity ranges deliberately
Check the thermal envelope specified for every installed equipment class against the applicable ASHRAE guidance and the manufacturer’s requirements. Do not copy a single universal temperature or humidity setpoint: permissible conditions vary with equipment class, altitude and other factors. DOE’s January 9, 2019 cooling-water guidance warns that excessively low temperature and unnecessarily narrow humidity control increase chiller and cooling-tower demand. Separate decentralized humidity controls can also work against one another; coordinate them through a plant-level sequence.
Understand the potential energy effect
DOE FEMP’s 2019 page, citing its Best Practices Guide, reports that aisle separation can permit higher chilled-water temperatures and reduced airflow, with a potential 20% reduction in chiller energy. That is an attributed opportunity, not a guaranteed saving for a particular facility. Commission the airflow changes and verify fan, chiller and room energy before assigning a project benefit.
Match chillers to the loads they will actually see
For electric chillers, compare air-cooled and water-cooled arrangements at the required capacity and under the annual operating profile. A chiller that looks excellent at one rating point may be inefficient during the many hours when the data center is lightly loaded. DOE FEMP’s electric-chiller acquisition guidance, updated October 2024, requires performance measurement under AHRI 550/590 procedures and pairs full-load and integrated part-load value (IPLV) criteria.
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Read EER and IPLV together
Energy efficiency ratio (EER) describes a defined full-load rating condition. IPLV combines performance at several part-load points using the weighting specified by the rating procedure. Use both numbers and confirm that the weighting resembles the facility’s operating pattern. A full-load-optimized selection is not automatically the best annual-energy choice for a plant that spends most of its hours at partial load.
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| DOE FEMP category | Capacity and optimization | Full-load EER | IPLV | How to use the value |
|---|---|---|---|---|
| Air-cooled chiller | Below 150 tons, full-load optimized | 10.890 | 13.700 | Federal procurement table criterion for the defined category. |
| Air-cooled chiller | Below 150 tons, part-load optimized | 9.700 | 16.860 | Federal procurement table criterion for the defined category. |
These figures are October 2024 federal acquisition guidance values, not universal market rankings. Apply the table that matches the chiller type, capacity and load profile, and verify the current edition before writing a purchase specification. Do not compare a value measured under a different procedure or category as if it were equivalent.
Model staging and redundancy
Ask suppliers to model each chiller, pump and tower at the loads and ambient conditions expected throughout the year. Include lead-lag rotation, minimum turndown, standby equipment, condenser-water reset and the energy of pumps and fans. A redundant unit that rarely runs still affects capital cost, controls and maintenance; a plant that cannot unload cleanly may waste energy during ordinary low-load operation.
Use lifecycle cost instead of first price
FEMP defines an efficient product as cost-effective when lifetime energy savings exceed the upfront premium. Recalculate that test with the site’s tariff, demand charges, climate, runtime, financing assumptions, maintenance cost and expected replacement date. FEMP’s illustrative calculations use stated assumptions; they should not be generalized to another utility or operating schedule. Require the model files or transparent inputs so procurement can test tariff and load sensitivities.
Choose economizers and heat rejection for the local climate
Economizing can reduce compressor hours, but its value depends on weather, air quality, humidity tolerance and controls. Compare annual eligible hours rather than relying on a label such as “free cooling.”
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An air-side economizer uses suitable outdoor air to cool the space or a separated air loop. Evaluate filtration, contamination monitoring, acoustics, pressure control and humidity limits. Smoke, dust, salt and industrial pollutants may make untreated outside air unacceptable. Include the fan energy and the transition logic between economizer and mechanical modes.
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Water-side economizers
A water-side economizer uses a heat exchanger and condenser-water path to bypass or unload the chiller when outdoor conditions permit. Check approach temperatures, heat-exchanger fouling, pump power, freeze protection and the sequence that returns to mechanical cooling. It can be valuable where outdoor air cannot enter the data hall, but it still carries water-treatment and maintenance obligations.
Cooling towers and water balance
Evaporative towers reject heat efficiently but consume water through evaporation and blowdown. Review cycles of concentration, filtration, chemical treatment, drift control and discharge limits with the water-treatment engineer. DOE FEMP’s 2019 guidance reports that increasing cycles of concentration from three to six reduced tower makeup water by 20% and blowdown by 50% in the cited best-management practice. The result depends on water chemistry, treatment capability and equipment limits; verify that the change is safe before adopting it.
Water availability can be a hard design constraint even when electricity is inexpensive. Compare annual withdrawal, makeup, blowdown and drought restrictions alongside kWh. A dry climate may favor an air-cooled or hybrid rejection strategy despite a higher electrical penalty, while a water-rich site may accept evaporative rejection to reduce energy. State the trade explicitly instead of optimizing one resource in isolation.
Evaluate direct liquid cooling as a complete system
Direct liquid cooling captures heat at compatible IT equipment and carries it through a recirculating loop. A coolant distribution unit (CDU) commonly separates or conditions the facility loop before heat reaches the IT-side loop. This can support high rack densities that would require extreme room airflow, but it does not automatically eliminate air cooling.
Map every heat path
Determine which components are liquid-cooled and which still reject heat to room air. Memory, storage, power supplies, networking equipment and non-liquid racks may leave a residual room load. Some designs use computer-room air handlers (CRAHs) for that remainder; others connect the CDU to a chilled-water loop or another heat-rejection system. Size the residual air system and its controls instead of assuming the liquid loop carries 100% of facility heat.
Check compatibility and maintainability
- Confirm server and accelerator cold-plate compatibility, allowable supply temperature and coolant chemistry.
- Define leak detection, isolation valves, drip management and a response procedure.
- Provide filtration, water-quality monitoring and a documented fill and drain process.
- Specify pump, CDU and heat-exchanger redundancy consistent with the IT availability requirement.
- Plan access for hose, manifold and CDU service without disrupting adjacent racks.
- Integrate liquid-loop alarms with the building-management and IT monitoring systems.
Hybrid systems add control loops that must be commissioned, operated and monitored. DOE FEMP’s 2019 page describes a National Laboratory of the Rockies hybrid thermosyphon installation with reported PUE of 1.06 and WUE of 0.7. Those are metrics for that named case, not a promise for another site; reproduce the boundary, weather, load and reporting period before making a comparison.
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Compare energy, water and carbon outcomes separately
One metric cannot express every cooling tradeoff. Use the measures that match the decision and disclose their boundaries.
| Metric | Definition | Best use | Important qualification |
|---|---|---|---|
| PUE | Total facility energy divided by IT equipment energy. | Shows non-IT energy overhead, including cooling and distribution. | State whether the period is hourly, monthly or annual and what facilities are included. |
| WUE | Site water use in liters divided by IT energy in kWh. | Compares water intensity where towers, evaporative systems or drought risk matter. | Define which water sources and discharge flows are counted. |
| ERE | Energy reuse effectiveness, used when waste heat is recovered. | Assesses whether useful heat offsets another energy demand. | Document the recovered-energy boundary and the recipient load. |
| CUE | Carbon usage effectiveness, used to put energy consumption in emissions context. | Shows the effect of grid mix or other energy sources. | State the emissions factors, location and accounting period. |
DOE’s December 2024 design discussion highlights ERE for heat reuse and CUE for carbon context. Report PUE and WUE together when water is material; a lower PUE can coincide with higher water use, and a lower WUE can require more electricity.
A practical selection sequence
- Define constraints. Document IT load cases, rack density, availability, climate, altitude, water, contamination, space, heat-reuse opportunities and service capability.
- Measure the existing plant. Trend IT power, cooling power, temperatures, humidity, fan speeds, pump speeds, valve positions, chiller loading, tower water and alarms across representative seasons.
- Correct airflow. Implement rack orientation, containment, blanking and sealing improvements; verify supply and return paths before increasing mechanical capacity.
- Set an approved environmental envelope. Reconcile ASHRAE guidance, equipment warranties, altitude effects and humidity-control interactions. Use the widest safe range rather than an unnecessarily tight setpoint.
- Create at least two plant concepts. For example, compare air-cooled and water-cooled chillers, or mechanical cooling with an economizer-assisted design. Add direct liquid cooling when rack density and hardware compatibility justify it.
- Model annual operation. Use hourly weather and the measured load profile. Include part-load efficiency, staging, pumps, fans, towers, economizer transitions, standby equipment and control deadbands.
- Quantify water and risk. Calculate makeup, blowdown, treatment and drought exposure. Test a water-constrained case even if current supply appears ample.
- Price the lifecycle. Include equipment, plant modifications, electrical upgrades, commissioning, controls, water treatment, maintenance, replacement and energy over the analysis period.
- Specify resilience. Test failure of a chiller, pump, CDU, control network, water-treatment component and power source against the required IT service level.
- Commission and verify. Require functional tests at full and partial load, economizer changeover, sensor calibration, alarm response, liquid leak scenarios and documented operator training.
Controls and operations decide whether the design performs
Efficient equipment can waste energy if sensors drift, sequences conflict or operators force conservative manual settings. Write the control narrative before issuing the equipment order. It should define chilled-water reset, supply-air reset, chiller staging, pump and fan speed control, economizer changeover, tower approach, humidity coordination and alarm priorities.
Use trend data, not snapshots
Trend temperatures, humidity, valve and damper positions, compressor loading, fan and pump speeds, water meters and IT power. Correlate changes with outdoor conditions and rack load. A short commissioning test can show that equipment works; seasonal trending shows whether the sequence remains efficient.
Make maintenance part of the design
Provide safe access to filters, coils, strainers, towers, pumps, chillers and CDUs. Define water-treatment sampling, filter replacement, sensor calibration, leak-detection tests and firmware or controls change management. Keep a current sequence of operations and a rollback procedure for control changes. Hybrid liquid/air systems need especially clear ownership because a fault can cross mechanical, electrical and IT teams.
Common selection mistakes and how to correct them
Choosing by headline efficiency
Symptom: A proposal highlights one excellent rating point but provides no annual model. Correction: Require AHRI 550/590-compliant full-load data, IPLV, auxiliary power and hourly results at the site’s expected loads.
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Sizing from nameplate IT power only
Symptom: Cooling is oversized for normal operation and cycles inefficiently. Correction: Supply measured minimum, typical and peak profiles, future growth timing and the redundancy case to the designer.
Lowering setpoints to hide airflow faults
Symptom: Operators reduce temperature or tighten humidity because some racks run hot. Correction: Inspect containment, bypass, recirculation, blanking and sensor placement before changing plant capacity or environmental limits.
Ignoring water as an operational risk
Symptom: A tower saves electricity but cannot operate during restrictions or suffers fouling. Correction: Model makeup, blowdown, treatment, drought limits and the failure mode of water-treatment equipment.
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Assuming liquid cooling removes all room cooling
Symptom: CRAHs or room sensors remain heavily loaded after a liquid retrofit. Correction: Inventory every air-cooled component and size the residual-air system, CDU and facility loop together.
Adding controls without an O&M owner
Symptom: Alarms are acknowledged manually, sequences are overridden and efficiency degrades. Correction: Assign ownership, train operators, trend the critical points and test alarm and failover procedures during commissioning.
How to compare bids fairly
Give every bidder the same load cases, weather file, utility rates, water assumptions, redundancy requirement and analysis period. Request a table that separates IT energy, cooling energy, pumping and fan energy, tower or dry-cooler energy, water use and maintenance. Ask for full- and part-load data, minimum stable load, staging logic, economizer hours, acoustic and air-quality limits, control points, service response and assumptions about future expansion.
Score proposals against the same axes:
- Annual and part-load efficiency, not only full-load ratings.
- Capacity, turndown and expansion path.
- Climate fit and credible economizer hours.
- Water consumption, treatment complexity and local water risk.
- IT hardware compatibility and rack-density capability.
- Resilience, maintainability and access during service.
- Controls, commissioning and operator complexity.
- Capital cost and lifecycle cost under multiple utility scenarios.
- Potential for useful heat recovery.
No design wins all of these categories. A documented trade study is more defensible than selecting the product with the largest single efficiency number.
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Select the system that meets the IT thermal and availability requirements with the lowest verified lifecycle impact for the site’s climate and resources. In practice, that means: fix airflow first; compare chiller performance at the loads the facility will actually run; treat economizers and towers as climate- and water-dependent systems; use liquid cooling where compatible rack density warrants the added loop; and fund controls, commissioning and maintenance as part of the cooling equipment rather than as optional extras.
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