How to Compare CDU Suppliers: 15 Technical Questions to Ask Before Selection

The cooling capacity may have been calculated at different Facility Water System (FWS) temperatures. The quoted pump may deliver the required flow only at a low pressure drop. “N+1” may refer only to pumps while the controller, heat exchanger or electrical path remains common. FAT may mean a basic functional check rather than verification at the specified duty point.

For a data center owner, EPC contractor or server integrator, these differences matter more than the number printed on the first page of the quotation.

When we compare CDU suppliers, we first put every proposal on the same engineering basis:

thermal duty + hydraulic duty + TCS/FWS temperatures + coolant + pressure limits + redundancy + controls + acceptance testing

Only after these items are aligned does a price comparison become meaningful.

If the project requirements have not yet been formalized, our guide to preparing an RFQ for a data center liquid cooling system explains what should be defined before quotations are requested.

The following 15 questions are intended for the next stage: technical bid evaluation.

1. What Exact Conditions Produce the Quoted Cooling Capacity?

Never compare CDU capacity without its rating conditions.

A supplier quoting 1,000 kW should also state:

Parameter	Required Supplier Data
TCS supply / return temperature	°C / °C
TCS flow	m³/h
TCS coolant	Water, glycol or specified fluid
FWS supply / return temperature	°C / °C
FWS flow	m³/h
Heat exchanger approach	°C
Cooling capacity at these conditions	kW

If two suppliers use different temperatures or coolant properties, their “1 MW” ratings are not directly comparable.

What to ask for: a performance sheet at the project duty point.

Red flag: a capacity number with no stated FWS/TCS conditions.

2. What Capacity Is Guaranteed at Our Actual Duty Point?

The catalogue rating is less important than performance under the project’s real conditions.

Suppose the server loop requires a specific TCS supply temperature but the site has relatively warm facility water. The temperature approach across the heat exchanger may become one of the limiting conditions.

Ask the supplier to confirm the guaranteed performance using your:

• TCS supply and return temperatures;
• FWS supply and return temperatures;
• coolant type and concentration;
• required heat load.

The supplier should also state the expected approach temperature and any operating limitation.

A useful CDU proposal says:

X kW at the specified FWS/TCS temperatures and flow.

It should not rely only on:

Maximum cooling capacity: X MW.

Our CDU sizing guide covers the relationship between heat load, temperature difference and coolant flow in more detail.

3. Can the TCS Pump Deliver the Required Flow at the Actual System Pressure Drop?

Maximum pump flow is not a useful comparison by itself.

The TCS circuit may include:

• cold plates;
• rack manifolds;
• quick disconnects;
• hoses;
• filters;
• control valves;
• distribution headers;
• fittings.

All of these create pressure loss.

If the project requires 100 m³/h at 200 kPa external pressure drop, the supplier should demonstrate that operating point on the pump curve.

Ask for:

• pump curve;
• design operating point;
• available external differential pressure;
• minimum stable flow;
• variable-speed operating range;
• pump quantity and control sequence.

Red flag: “Maximum flow: 120 m³/h” with no corresponding pressure.

Thermal duty and hydraulic duty must be verified together.

4. What Does the CDU Require From the Facility Water System?

The same question applies to the FWS side.

Ask each bidder to state:

• required FWS flow;
• FWS inlet and outlet temperatures;
• CDU primary-side pressure drop;
• minimum available differential pressure;
• maximum working pressure;
• connection size.

This information should be checked against the actual facility pumping system.

It is particularly important in retrofit projects. An existing chilled-water plant may have enough spare thermal capacity but insufficient hydraulic margin at the proposed CDU connection.

The facility requirement should therefore be confirmed before the CDU is awarded—not after installation begins.

5. What Coolants and Wetted Materials Are Approved?

“Compatible with glycol” is not a complete coolant specification.

Ask which fluids and concentrations are approved on both sides of the CDU and request a wetted-material list.

Review:

• piping material;
• heat exchanger material;
• pump wetted parts;
• valve materials;
• seals and gaskets;
• sensor wetted materials.

The supplier should also define relevant coolant limits such as:

• pH;
• conductivity where applicable;
• glycol concentration;
• corrosion-control requirements;
• water-quality limits.

This information has to be compatible with the server manufacturer’s requirements as well as the CDU.

Red flag: no documented coolant or water-quality operating range.

6. How Are Filtration, Filling, Flushing and Air Removal Managed?

This question is often overlooked during procurement and becomes important during commissioning.

The supplier should explain:

• filtration rating;
• full-flow or side-stream filtration;
• filter differential-pressure monitoring;
• filter replacement procedure;
• filling connection;
• venting and air removal;
• expansion-volume management;
• drain arrangement;
• coolant sampling.

Also clarify who is responsible for flushing the external TCS piping before servers are connected.

Modern high-capacity CDU systems may use different filtration stages for the main loop and side stream, so filtration should be treated as part of the system design rather than as a minor accessory.

Red flag: the supplier specifies the CDU filter but provides no requirement for system cleanliness before commissioning.

7. How Is TCS Pressure Controlled and Protected?

A CDU must provide enough pressure to circulate coolant without exceeding the allowable limits of cold plates, hoses, manifolds and quick disconnects.

Ask how the design handles:

• normal supply pressure;
• differential-pressure control;
• maximum operating pressure;
• minimum pressure;
• expansion;
• blocked flow;
• pump start/stop;
• valve closure;
• pressure relief.

The supplier should explain where pressure is measured and what control variable the pump follows.

For long distribution systems, pressure at the CDU and pressure at the most remote rack can be significantly different.

Do not evaluate only the CDU’s maximum pressure rating. Evaluate the complete TCS operating range.

8. What Exactly Does “N+1” Mean in This Proposal?

This is one of the most important supplier-comparison questions.

“N+1” frequently means N+1 pumps. It does not automatically mean the complete CDU has no single point of failure.

Ask suppliers to identify the redundancy of:

Component	Configuration
TCS pumps	Duty / standby / parallel
Power feeds	Single / dual
Controllers	Common / redundant
Heat exchanger	Common / redundant
Control valve	Common / redundant
Sensors	Single / multiple
Filters	Online service possible?
Complete CDU	N, N+1, 2N or other

Then ask:

What is the maximum IT cooling load lost after one credible failure?

That question is more useful than simply asking whether the CDU is N+1.

For larger deployments, CDU-level redundancy and cooling-zone design may be more important than adding another pump inside one cabinet.

9. What Happens When a Component Actually Fails?

A list of alarms is not a failure-response strategy.

Ask the supplier to explain the sequence following:

• one pump failure;
• temperature-sensor failure;
• pressure-sensor failure;
• loss of FWS flow;
• loss of TCS flow;
• abnormal pressure;
• leak detection;
• controller fault;
• communication loss;
• loss of one power feed.

For each condition, determine whether the CDU:

• continues operating;
• switches to standby equipment;
• reduces capacity;
• changes setpoint;
• closes a valve;
• generates an alarm;
• shuts down.

A technically mature proposal should be able to describe these responses before detailed commissioning.

Red flag: “Alarm will be provided” with no explanation of what the equipment does next.

10. How Does the CDU Control Rapid Changes in IT Load?

AI and HPC loads can change much faster than conventional building cooling loads.

The question is not whether the CDU includes a PLC and PID control. The useful question is:

What is being controlled, and how does the CDU respond when rack flow or heat load changes?

Ask about:

• TCS supply-temperature control;
• differential-pressure control;
• pump-speed control;
• FWS control-valve response;
• minimum-flow protection;
• multiple-CDU coordination.

Also ask what happens when several rack valves open or close within a short period.

The supplier should be able to explain the control philosophy in engineering terms rather than only listing controller brands.

11. What Instrumentation Is Included, and What Can the Operator Actually See?

Two CDUs may both offer “full monitoring” while providing very different levels of visibility.

Request the instrumentation and BMS point list.

Useful values typically include:

• TCS supply/return temperature;
• FWS supply/return temperature;
• TCS flow;
• FWS flow;
• supply/return pressure;
• differential pressure;
• pump speed and status;
• filter differential pressure;
• valve position;
• coolant level;
• leak status.

For critical measurements, also ask about sensor range, accuracy and replacement access.

This matters because a CDU cannot be properly commissioned or diagnosed if operators cannot distinguish between thermal, hydraulic and sensor-related problems.

12. How Will the CDU Integrate With the BMS or DCIM?

“Modbus supported” is only the beginning of the discussion.

Confirm:

• Modbus TCP or RTU;
• BACnet, if required;
• physical communication interface;
• available read/write points;
• alarm mapping;
• remote setpoint capability;
• communication-loss behavior.

The project team should also decide who owns operating setpoints.

For example, will differential pressure be controlled locally by the CDU, or will an external supervisory system send the setpoint?

These details should be defined before site commissioning.

Red flag: communication protocol confirmed, but no point list or controls responsibility defined.

13. What Are the Real Electrical and Physical Installation Requirements?

A technically suitable CDU can still be a poor fit for the site.

Request:

• dimensions;
• dry weight;
• operating weight;
• electrical voltage and frequency;
• full-load current;
• number of power feeds;
• pipe connection sizes and locations;
• drainage requirements;
• front/rear/side service clearance;
• component-removal path.

Maintenance access deserves particular attention.

Can a pump, filter, controller or heat exchanger be replaced without moving adjacent IT racks or disconnecting major distribution piping?

The answer may influence whether the project is better suited to rack-mounted, sidecar or in-row equipment. Our Rack-Mounted vs Sidecar vs In-Row CDU comparison covers those architecture differences separately.

14. What Exactly Will Be Verified During FAT and SAT?

“100% factory tested” does not explain what was tested.

Before issuing a purchase order, agree on the Factory Acceptance Test scope and acceptance criteria.

Depending on project requirements, FAT may include verification of:

• flow;
• pump performance;
• pressure control;
• temperature control;
• leak/pressure integrity;
• redundant pump changeover;
• sensor operation;
• alarms;
• PLC sequences;
• BMS communication;
• electrical-feed failure.

For critical projects, ask which tests can be performed near the specified operating duty.

A functional test at no meaningful hydraulic or thermal load does not prove the same thing as performance verification.

SAT should then confirm that the CDU works correctly with the actual facility-water and TCS installation.

Red flag: FAT is included in the quotation, but there is no agreed test procedure or acceptance criterion.

15. How Will the System Expand Beyond Phase 1?

Do not select the CDU only for the first rack group if more liquid-cooled racks are already planned.

Ask:

• Can additional CDUs operate in parallel?
• How is load shared?
• How are common headers sized?
• How many units can the controller coordinate?
• Can another CDU be commissioned without interrupting operating racks?
• Is space reserved for future pipes and electrical feeds?
• What happens to redundancy as the load increases?

For example, a rack-mounted CDU strategy can be practical for a small deployment, while a larger multi-rack expansion may justify sidecar or in-row architecture.

The right comparison should therefore use:

Phase 1 load + final planned load + required redundancy

not Phase 1 alone.

Put Every CDU Bid Into One Comparison Sheet

After the technical clarification, normalize each supplier into one table.

Comparison Item	Supplier A	Supplier B	Supplier C
Guaranteed capacity at project duty
TCS flow at required external dP
FWS flow / pressure drop
Heat exchanger approach
Coolant compatibility
Pump redundancy
CDU-level failure domain
Dual power feeds
BMS point list
FAT at agreed conditions
Future parallel expansion
Service access / critical spares

We recommend resolving four items before commercial ranking:

1. Thermal duty is proven.
2. Hydraulic duty is proven.
3. Coolant and pressure compatibility are confirmed.
4. The defined failure condition can be supported.

If one proposal cannot pass these four checks, a lower purchase price does not make it equivalent to the other bids.

Common Red Flags in a CDU Quotation

A technical clarification is usually needed when:

• cooling capacity is stated without rating temperatures;
• maximum flow is shown without a pump curve;
• external available pressure is unclear;
• “N+1” does not identify which components are redundant;
• coolant chemistry limits are missing;
• filtration is specified but flushing requirements are not;
• BMS compatibility is stated without a point list;
• failure alarms are listed without a sequence of operation;
• FAT has no agreed acceptance criteria;
• the supplier has not asked for FWS conditions or TCS pressure drop.

These issues do not automatically disqualify a supplier. They mean the proposals are not yet technically comparable.

Do Not Compare CDU Price Until the Engineering Scope Is Equal

The lowest quotation may contain a smaller heat exchanger, lower pump head, fewer redundant components, less instrumentation or a narrower FAT scope.

That is not necessarily wrong. It may simply be a different technical solution.

The buyer’s task is to identify those differences before comparing price.

For projects where the CDU is connected to a dedicated outdoor heat-rejection loop, the same principle should also be applied to the facility side. Our guide to sizing a dry cooler for a liquid-cooled data center explains why outdoor equipment should also be compared at common temperature and ambient conditions.

Order & Project Support

A useful CDU supplier comparison should answer one engineering question:

Can this CDU remove the required heat, deliver the required coolant flow at the real system resistance, maintain the specified operating conditions and continue supporting the required load after the defined failure?

If the quotation cannot answer that clearly, supplier selection is premature.

For a technical comparison, provide the same design basis to every bidder:

liquid cooling load + TCS temperatures + required flow + external pressure drop + coolant + FWS temperatures + FWS conditions + redundancy + controls + final expansion load

Once those values are fixed, differences in heat exchanger selection, pumping, redundancy, controls, testing and price become much easier to evaluate.