Rear door heat exchangers (RDHx)
A rear door heat exchanger (RDHx) replaces the back door of a rack with a water coil that pulls the heat out of the server exhaust before it reaches the room. Published ratings run from 75 kW per rack (Motivair ChilledDoor) and 83 kW (nVent RDHX PRO) up to 200 kW on Legrand’s USystems ColdLogik CL23.
| Technologies | |||||||
|---|---|---|---|---|---|---|---|
| Legrand (USystems ColdLogik) | 2001 | 2001 | RDHx | — | Legrand | Europe | 2026-11-09 |
| nVent | 833 | — | CDU · RDHx · Manifold | 2,0001 | — | Europe | 2026-11-09 |
| Motivair by Schneider Electric | 752 | — | CDU · D2C · RDHx · Sidecar | 2,5001 | Schneider Electric | North America | 2026-11-09 |
| Dell Technologies (PowerCool) | — | 5041 | CDU · D2C · RDHx · OEM | — | — | North America | 2026-11-09 |
- Technologies
- CDUCoolant distribution unit
- D2CDirect-to-chip cold plate
- D2C-2φTwo-phase direct-to-chip cold plate
- IMM-1φSingle-phase immersion tank
- IMM-2φTwo-phase immersion tank
- RDHxRear-door heat exchanger
- SidecarLiquid-to-air CDU
- ManifoldRack and row manifolds
- QDQuick-disconnect couplings
- OEMServer maker with liquid-cooled racks
- FluidCoolant fluid supplier
Passive, active and where a rear door fits
How ratings are readPassive or active
A passive door uses the servers’ own fans to push air through the coil. An active door adds fans and controls and handles more heat. Legrand sells both: the ColdLogik CL21 Smart Passive, and the CL20 ProActive, which reaches 93 kW with 14 °C inlet water.
Rear doors with direct-to-chip
Rear doors pair well with cold plates. The plates take most of a GPU server’s heat as liquid, and the door catches what the fans still blow out, so far less heat reaches the room air. Dell sells exactly that pairing as PowerCool, with rack CDUs and an enclosed rear door, and Delta’s GoCool CDUs feed both direct-to-chip and rear-door loops.
Plumbing
A rear door needs chilled or facility water at the rack, usually through a CDU that keeps the coil above the room’s dew point. Vertiv specifies its CoolChip CDUs for rear-door loops and Delta its GoCool units; see the CDU table.