TL;DR
- A rear-door heat exchanger (RDHx) replaces the rack's perforated rear door with a finned-coil water-to-air heat exchanger that captures hot exhaust before it enters the room.
- Passive RDHx units rely on server fans to push air through the coil; active units add their own fans for higher heat capture and tighter outlet-temperature control.
- Practical capacity is 40-60 kW per rack, making RDHx the simplest path to retrofit a traditional air-cooled hall into a moderately dense AI environment.
- Often deployed alongside direct-to-chip liquid cooling: DLC captures the GPU heat, RDHx captures the residual chassis-air heat, and the room itself stays neutral.
Overview#
Rear-door heat exchangers are the least disruptive way to push a data hall beyond the 15-20 kW per rack ceiling of conventional air cooling. The unit looks like a normal rack rear door but contains a copper-and-aluminium coil through which chilled or warm facility water flows. Server exhaust passes through the coil on its way out of the rack; depending on water temperature and flow rate, the air leaves the back of the rack at close to room ambient.
Because the room never sees a hot aisle, hot-aisle containment, CRAH sizing, and air-distribution engineering all simplify dramatically. For colocation operators retrofitting older halls for AI workloads, RDHx is frequently the first step before moving to direct-to-chip.
Passive vs Active RDHx#
The fan question is the central design decision. Passive units have no fans of their own; active units add EC fans to the door.
| Property | Passive | Active |
|---|---|---|
| Fans on the door | None | EC fans, redundant |
| Capacity | Up to ~35 kW per rack | Up to ~60 kW per rack |
| Power draw (door itself) | Zero | 0.5-2 kW per door |
| Failure mode | Server fans pushed harder — recoverable | Door-fan loss reduces capacity; need redundancy |
| Acoustic profile | Same as the servers | Slightly higher |
| Best for | Steady, predictable thermal loads | Bursty or very high-density loads |
Typical Operating Specifications#
| Parameter | Typical range |
|---|---|
| Coolant supply temperature | 12-22 °C (chilled) or 25-30 °C (warm-water) |
| Coolant flow | 30-80 L/min per door |
| Heat capture | 90-100 % of rack exhaust |
| Outlet air temperature | Within 1-2 °C of room ambient |
| Pressure drop (air-side) | 10-30 Pa |
| Door weight (filled) | 150-250 kg |
| Rack capacity supported | 30-60 kW (active), up to 35 kW (passive) |
RDHx pairs naturally with direct-to-chip cooling. Run DLC for the GPUs (~70-80 % of the heat) and RDHx for the residual chassis air (~20-30 %). The two can share the same facility water loop, and the combined envelope handles 100 kW racks without any hot aisle at all.
Trade-offs#
- vs CRAH-only air cooling: 3-4× the density at modest capex; the major shift is plumbing chilled water to each rack rather than chilled air to each aisle.
- vs DLC: simpler — no cold plates inside the server, no quick-disconnects, no per-vendor thermal qualification.
- vs immersion: shares only the 'liquid is involved' headline; thermal capacity is an order of magnitude lower and PUE is materially higher.
- Door weight and depth: an RDHx adds 200-400 mm to rack depth and significant weight; check aisle clearance and floor loading before committing.
- Pump and CDU sizing: many RDHx deployments need a dedicated CDU per row to isolate the door loop from the facility loop and to manage filtration.
When to Use RDHx#
- Retrofitting a legacy air-cooled hall to support 30-60 kW racks without rebuilding the air-handling system.
- Edge or regional sites where a full DLC build is over-engineered but conventional air is under-engineered.
- Mixed-workload rooms where some racks are AI-dense and others are general compute — RDHx scales per-rack without changing the room.
- Phased migrations toward DLC — install RDHx today, add cold plates inside the servers later, reuse the same secondary loop.
Operational Pitfalls#
- Condensation: if facility water is below room dew point, the coil sweats. Either insulate everything and run drip pans, or commit to keeping supply temperature above local dew point.
- Door swing clearance: rear-door coils are heavier and thicker than perforated doors. Aisle width and adjacent-rack interference need to be checked at design time.
- Server-fan curves: passive RDHx adds back-pressure. Servers with weak or BMC-curve-fixed fans may run hotter than expected. Validate during commissioning.
- Water-side filtration: any debris in the coil compromises heat transfer. A row-level filter and periodic flow checks pay for themselves.
- Single-door failure: if a door is removed for service, that rack reverts to room-air cooling. Plan for either temporary spot cooling or maintenance windows during low-load periods.
References
- ASHRAE TC 9.9 — Thermal Guidelines · ASHRAE
- Uptime Institute — High-Density Cooling Strategies · Uptime Institute
- Open Compute Project — Advanced Cooling Solutions · OCP