Rear Door Heat Exchangers: How EC and DC Centrifugal Fans Fit In

What Is a Rear Door Heat Exchanger, and Why Are Data Centers Adding Them?

A rear door heat exchanger (RDHx) mounts to the back of a server rack and cools server exhaust air as it passes through a liquid-cooled coil before that air re-enters the data center. Passive RDHx units rely on the server’s fans to move air through the coil; active units add integrated fans to overcome the coil’s resistance and control airflow.

Fan selection comes down to required airflow at your actual static pressure, power architecture, control interface, redundancy, and service access, and that’s what the rest of this guide walks through. ASHRAE describes RDHx as a hybrid approach that reduces room heat load short of full liquid adoption, and that framing explains most of why the device exists as a category. As rack power density rises, conventional room-level cooling can struggle to maintain stable airflow and temperature without additional rack-level cooling capacity.

Rack density keeps rising because GPU-heavy workloads pack more compute into the same footprint. That density can create hotspots that conventional raised-floor and room-level cooling systems may struggle to manage efficiently, and sustained heat shortens the working life of the electronics sitting in that rack. As room-level cooling approaches its practical limits, cooling moves closer to the heat source: onto the back of the rack itself. That’s the RDHx.

Vertiv reports a typical RDHx rack-density range of 20 to 80 kW, though the number that actually matters for your project is your own heat load, not a vendor headline figure, and actual capacity still depends on the complete door and coolant conditions. Ablecom’s recent AbleCool RDHx portfolio launch, spanning 30 to 100 kW models depending on the door, is one example of the density band vendors are now designing around. It’s a useful signal for where demand sits even though the specific product isn’t the point of this guide.

RDHx isn’t the only rack-level response to that density problem; later in this guide we put it next to in-row and immersion cooling for context. Of the options that keep cooling close to the rack, RDHx is the one where integrated fans become a critical part of airflow control once the door moves from passive to active operation. Get the door category right and there’s still a fan decision underneath it, and that’s what this guide spends most of its length on.

We’ve found that the RDHx question worth spending time on isn’t “what is it.” It’s “what’s actually inside the door, and does that part hold up once the rack is running hot.” The next section covers active versus passive designs, because that split decides whether a fan is even part of the equation, and if it is, what job it has to do.

Active vs. Passive RDHx: Where the Fan Actually Comes In

Rear door heat exchangers split into two designs, and the difference comes down to whether the door has its own fans or borrows airflow from somewhere else.

A passive RDHx has no fans of its own. It relies entirely on the server’s internal fans to push exhaust air through the cooling coil. That works fine at moderate rack loads, because most of the pressure the airflow needs to get through the coil is small enough that the servers’ own fans can supply it without much strain.

That simplicity is exactly why passive still gets specified on plenty of new builds, not just legacy ones. No independent fans means no additional power draw, nothing extra to fail, and no control system to integrate with the rest of the rack’s monitoring setup. At moderate density, none of that costs the room noticeable cooling headroom, which is why passive isn’t automatically the wrong answer just because active exists.

The problem shows up as density climbs. A coil adds resistance to the airflow path, and every server fan has a curve where output airflow drops as the static pressure it has to work against goes up. Push a passive RDHx past its comfortable range and the operating point on that curve slides toward lower airflow right when the rack needs more of it, not less. The exhaust air backs up, the coil can’t pull enough heat out fast enough, and the rack runs hotter than the nameplate numbers would suggest. That’s the failure mode worth watching for in a retrofit especially: the door looks like it should work because the frame and coil match, but the server fans behind it were never sized to fight that much additional resistance, and the shortfall doesn’t show up until the rack is already under load.

An active RDHx solves that by adding its own integrated fans to the door. Those fans provide additional pressure capability across the coil and reduce how much cooling performance depends on the server fans alone, which gives the RDHx more control over airflow as rack load and server fan speed change. That’s the reason active designs show up on the high-density end of the RDHx range and passive designs stay in the lower band.

Once a project lands on active, the question moves fast from whether there’s a fan in the door to what kind of fan it is, and whether that fan can hold the static pressure the coil demands at your rack’s real duty point, not just its rated maximum. That’s where EC and DC centrifugal designs start to look meaningfully different from each other.

EC vs. DC Centrifugal Fans in Active RDHx: What Actually Differs

Outside the fan industry, EC and DC often get treated as interchangeable shorthand for “not AC.” They aren’t the same choice, and in an active RDHx the difference shows up in three places that matter for a real installation: control protocol, power infrastructure fit, and how the fan gets serviced once it’s running.

Control protocol is the most visible split. Proventofan’s EC centrifugal line runs on 230V AC power up through the 450mm frame, stepping up to 380V three-phase on the two largest sizes; PWM and 0 to 10V control are standard across the line, and Modbus RS-485 is available from the 220mm frame up (the three smallest frames, 133mm through 190mm, don’t offer it). The DC centrifugal line covers 24V, 48V, 85V, and 110V across the product line, with PWM and 0 to 10V control as the default; Modbus RS-485 comes standard on some series and is available on request for others. Neither protocol set is objectively better. The question is which one matches the control system already sitting in the rest of your rack.

The wiring and configuration side of PWM, 0 to 10V, and Modbus setup is its own topic; if you need that level of detail, our guide to controlling an EC fan walks through it. The same control concepts carry over whether the fan runs on AC or DC power, but the wiring, signal voltage limits, PWM frequency, and Modbus register map still need to be checked against the specific fan’s datasheet rather than assumed from the protocol name alone.

That question usually gets answered by power infrastructure, and this is where the EC-versus-DC decision stops being abstract. Ablecom’s recent AbleCool CDA42 door was built specifically for 48V DC infrastructure and telecom environments, and that pairing isn’t a coincidence. Telecom-grade equipment and edge sites have run on 48V DC for decades, and a DC centrifugal fan slots into that power architecture without a conversion stage. An EC fan, by contrast, expects AC power and fits naturally into a facility already built around a 230V distribution scheme.

If your rack or your customer’s rack is already 48V DC, a DC fan is the fan that doesn’t fight the infrastructure. If it’s AC-fed, EC is the more direct match. That’s a narrower question than EC versus AC in general, though: if you’re weighing EC against AC fans for a project outside the RDHx context, the efficiency and retrofit-economics side of that comparison is covered separately in our EC fan vs AC fan guide.

Serviceability is the piece that gets skipped most often, and it shouldn’t be. Uptime Institute’s own reliability guidance on close-coupled cooling puts it directly: “Airflow when a door is removed from service for either a planned event or due to a failure is a major consideration.” That guidance is about the system-level risk created when an entire RDHx door is pulled from service: in a room where RDHx cools the whole space, the affected racks can end up blowing hot exhaust into the racks behind them until the door is back in service.

Replacing a single fan inside an active door is a separate, smaller-scale question, and whether that’s disruptive depends on whether the RDHx provides redundant or hot-swappable fan modules in the first place. Some active RDHx designs use redundant fan modules, including N+1 configurations, but redundancy has to be verified at the complete-door level: airflow distribution across the coil face, backflow through a failed fan position, and control response can all affect how much cooling capacity actually remains. If you want the basic principles behind sizing multi-fan redundancy, our AHU fan array retrofit guide covers them, though the specifics differ for a coil-mounted RDHx array.

None of this is a case for one motor type over the other in the abstract. It’s a case for matching the fan to the power architecture and control system you’re actually running. Either way, the fan moves air through the liquid-cooled coil, while the coil transfers heat from the server exhaust air into the coolant loop, and the fan’s real-world performance is what decides whether that liquid cooling investment holds up once the rack is under load. For other EC fan applications beyond RDHx, our EC Fan Solutions hub is the place to start.

Attribute	EC Centrifugal	DC Centrifugal
Standard voltage	230V AC (380V on the 500mm/560mm frames)	24V / 48V / 85V / 110V DC
Control signals	PWM, 0-10V, Modbus RS-485 from 220mm up (133/175/190mm: PWM/0-10V only)	PWM, 0-10V standard; Modbus RS-485 standard on some series, on request for others
Best infrastructure fit	AC-distributed facilities	48V DC / telecom power architectures
Typical RDHx pairing	AC-fed active doors	DC-powered or telecom-adjacent installations

Table source: Proventofan product specifications. Voltage and control figures reflect standard catalog configuration; confirm against the current datasheet for a specific frame size before specifying.

If you’re weighing this decision for a specific retrofit or a new build, the next section walks through what to check before you lock in a spec, using a real evaluation we ran for a customer as the example.

What to Check Before Specifying Fans for an RDHx

Vendors describe RDHx performance in kilowatts of cooling capacity, but a door-level kW rating alone isn’t enough to select an individual fan, because installed airflow also depends on coil resistance and the required operating point. The fan is exactly where that gap tends to hide, because catalog fan ratings and installed fan performance are not the same thing. Maximum airflow is normally reported near free-air conditions, while input power and efficiency vary with fan speed and the actual operating point. An RDHx coil adds real resistance that pulls the fan’s actual operating point away from that free-air number, so selection should be based on the fan’s full performance curve against the system’s resistance, not the maximum airflow figure alone.

Here’s how that plays out in practice. We ran a technical evaluation for a Vietnamese telecommunications and systems-integration company that needed a 48V DC centrifugal fan to match their incumbent unit, an ebm-papst R3G355-RP23-33, with a requirement that any replacement match at least 95% of the original’s performance and physical footprint. The first candidate we looked at seemed close on paper: same frame size, same voltage class, a similar control interface, and a maximum airflow rating that looked competitive.

But its performance curve fell below the customer’s two real duty points: 2,495 m³/h at 350 Pa and 1,710 m³/h at 450 Pa. We didn’t put it forward. Recommending it on the strength of a matching frame size and voltage alone would have meant the customer discovering the shortfall themselves, after the fan was already installed in their enclosure.

The candidate we did put forward is a 355mm-frame, 48V DC fan (36 to 57V operating range) rated up to 1,700 RPM, with maximum airflow of 4,101 m³/h and maximum static pressure of 522 Pa, running 0 to 10V and PWM speed control with FG tachometer feedback and a 183mm installation depth. Against the same two duty points, it comfortably clears both, with airflow and pressure headroom to spare rather than a marginal pass.

What follows is specific to RDHx retrofits, where the coil and the door frame are already fixed and the fan has to work around them; if you’re selecting a DC fan for a different application where more of the design is still open, our general DC fan selection guide covers that broader process. Here’s what to run through before you specify or replace a fan in an RDHx:

• Do confirm the fan’s airflow and static pressure at your actual duty point, not the maximum free-air rating on the datasheet. A number that looks close to what you need can still fall apart once the coil’s resistance is factored in.
• Do check control interface compatibility against your existing system: voltage, signal type (PWM, 0 to 10V, or Modbus), and any feedback signals your monitoring setup expects.
• Do verify mounting dimensions and hole pattern against the fan you’re replacing, especially in a retrofit where the enclosure isn’t changing.
• Do confirm certification coverage (UL, CSA, CE, or whatever your target market requires) applies to the specific model, not just the product family.
• Don’t assume an active RDHx needs no ongoing upkeep. Check fan-module access, coolant-loop water quality and filtration, leak detection, and control system health, not just the fans themselves. Some designs, like nVent’s RDHx line, support hot-swappable fans that can be replaced without shutting the whole door down, but that’s a design-specific feature, not a given, and it’s a question worth asking the manufacturer directly before you commit to an array configuration.
• Don’t assume a fan is a drop-in match because the frame size and voltage line up. That’s how the candidate above would have failed the customer, and it’s the single most common gap we see in fan replacement evaluations. Frame size and voltage are the two specs that are easiest to check and the two that tell you the least about whether the fan will actually hold up once the coil’s resistance is in the picture.

This checklist isn’t specific to one project. If you’re evaluating a fan for an RDHx retrofit or a new specification, this is the sequence that catches the gap before it becomes a field problem instead of a spec review, and it’s the same sequence we ran on the case above before we told the customer which fan to use.

RDHx vs. Other Rack-Level Cooling: A Quick Frame

RDHx isn’t the only way to cool a high-density rack, and it’s worth knowing roughly where it sits relative to the alternatives before you commit to it. Traditional CRAC-based room cooling is still the baseline most facilities start from: it uses familiar, widely available facility equipment, but it tends to run out of headroom first in high-density deployments, since it’s cooling the whole room instead of targeting the rack. In-row cooling moves the units between racks rather than behind them, pulling heat out at the row level instead of waiting for it to mix into the room air the way CRAC does. That gets you closer to the rack than a room-based system without the door-mounted hardware RDHx needs, though it doesn’t sit as close to the heat source as an active RDHx does. Immersion cooling skips air entirely and submerges hardware in a dielectric fluid, which can support higher densities than air-based options generally allow, but it changes almost everything else about how the rack is built, serviced, and inspected, and it’s a bigger infrastructure commitment than most retrofits are looking for.

RDHx sits in a practical middle position between those extremes: it moves cooling to the rack without the full infrastructure change that immersion demands, and it handles higher densities than room-based CRAC cooling typically supports, without asking a facility to rebuild its racks around a fluid bath. None of these approaches makes the others obsolete. The right one depends on your density target, your existing infrastructure, and how much of the facility you’re willing to redesign to get there. Note that RDHx cooling still has to end up somewhere once it leaves the rack, typically a chilled water loop or, in facilities that reject heat straight to outside air, a dry cooler array; that’s a separate design question from which rack-level method you pick, and it’s covered in our guide to dry coolers for data centers. For most retrofits, though, the rack-level choice is the real decision, and the fan question covered above only comes up once you’ve settled it.

Frequently Asked Questions

What is a rear door heat exchanger (RDHx)?

An RDHx is a cooling device that mounts to the back of a server rack and uses a liquid-cooled coil to capture hot exhaust air before it re-enters the data center. It moves heat rejection to the rack level instead of relying on room-based air conditioning alone.

What’s the difference between active and passive RDHx?

Passive RDHx has no fans of its own and depends on the server’s internal fans to push air through the coil. Active RDHx adds its own integrated fans, supplying additional pressure capability instead of relying on the server fans alone, which lets it handle higher rack densities.

Do all rear door heat exchangers need fans?

No. Passive RDHx units have no fans and depend on server airflow. Only active RDHx units include their own fans, and that’s the design covered in this guide.

Should an active RDHx use EC or DC centrifugal fans?

It depends on your power infrastructure and control system. DC centrifugal fans fit naturally into 48V DC and telecom-style power architectures. EC centrifugal fans fit AC-distributed facilities and offer PWM, 0 to 10V, and Modbus control built into standard AC power.

Can rear door heat exchangers be installed on existing racks?

Many RDHx product families offer retrofit options for standard rack frames, but compatibility has to be confirmed case by case: rack dimensions, hinge loading, door clearance, hose or coil connection routing, and service access all vary by model.

What cooling capacity can a rear door heat exchanger handle?

Industry figures commonly cite a 20 to 80 kW per rack range, and newer active product lines now extend that further, with some flagship models rated up to 100 kW. Actual capacity depends on the specific unit, the fan driving it, and your facility’s chilled water or refrigerant supply.

How much power do RDHx fans draw?

It varies by fan size, voltage, and how hard the fan is working against the coil’s resistance at your actual operating point, not the free-air rating on the datasheet. Confirm power draw against the manufacturer’s data at your specific duty point rather than the headline maximum.

Can active RDHx fans be serviced without shutting down the door?

Some designs support hot-swappable fans that can be replaced without taking the whole door offline, but this depends on the specific unit. Ask the manufacturer directly whether the model you’re specifying supports this before you lock in an array configuration.

What should I check before specifying or replacing fans in an RDHx?

Confirm airflow and static pressure at your actual duty point, verify control interface compatibility, check mounting dimensions against the existing fan, and confirm certification coverage applies to the specific model. See the checklist above for the full sequence.

Who should evaluate fan compatibility when retrofitting or replacing an RDHx unit?

This works best as a technical evaluation involving whoever owns the facility’s electrical and control specifications, not a spec-sheet comparison alone. In our experience, the gap between “looks compatible” and “is compatible” usually shows up in the duty point data, not the headline numbers, which is why we treat every fan-matching request as a real evaluation rather than a lookup.

References

• ASHRAE. AI Data Center Energy Performance Framework: Energy and Thermal Efficiency. https://www.ashrae.org/technical-resources/ai-data-center-framework/energy-and-thermal-efficiency
• Uptime Institute. Close Coupled Cooling and Reliability. https://journal.uptimeinstitute.com/close-coupled-cooling-reliability/
• Ablecom. Join the Green Computing: Ablecom Unveils AbleCool RDHx Portfolio (Data Center Dynamics, August 25, 2026). https://www.datacenterdynamics.com/en/product-news/join-the-green-computing-ablecom-unveils-ablecool-rdhx-portfolio/
• Vertiv. How Rear Door Heat Exchangers (RDHx) Support High-Density Rack Cooling. https://www.vertiv.com/en-us/insights/articles/educational-articles/how-rear-door-heat-exchangers-rdhx-support-high-density-rack-cooling/
• nVent. Rear Door Heat Exchangers. https://www.nvent.com/en-us/data-solutions/rear-door-heat-exchangers

Disclaimer

The RDHx cooling capacity ranges and system-level figures in this guide come from publicly available industry sources, not from Proventofan lab testing of complete RDHx systems. Proventofan manufactures the EC and DC centrifugal fans referenced here, and fan-level specifications reflect our own product data. Actual installed fan performance depends on your specific coil resistance, ducting, and operating conditions. Confirm fan selection against your real duty point and the current datasheet before finalizing a specification, and consult the RDHx manufacturer for door-level design and certification questions.

Ready to Match a Fan to Your RDHx Project?

Send us your fan’s frame size, voltage requirement (AC 230V or DC 24/48/85/110V), control signal needs (PWM, 0-10V, or Modbus), and any certification requirements your project needs. We’ll confirm whether an existing Proventofan model matches, or scope a custom configuration and get back to you with a technical evaluation.

Get a Fan Compatibility Evaluation → Reach out through our contact page, or browse our EC centrifugal fan and DC centrifugal fan ranges, or the full EC Fan Solutions hub.