RDHx (Rear-Door Heat Exchanger)

A rear-door heat exchanger (RDHx) is a liquid-cooled panel mounted on the back of a server rack that removes heat from IT equipment exhaust air before it re-enters the room, bridging traditional air cooling and full liquid cooling. Passive units rely on chilled water and server fans alone to push air through the coil, while active units add their own EC fans to boost heat rejection. Published vendor capacities vary widely: standard passive RDHx units typically handle 20-30 kW per rack, while active designs from vendors such as Motivair and Vertiv are rated up to 75-200 kW per rack. This makes RDHx a common retrofit path for colocation operators supporting GPU racks without a full re-platform to direct-to-chip liquid cooling, since it captures heat at the rack rather than the room and raises the practical density ceiling of existing air-cooled halls.