Definition
A Rear-Door Heat Exchanger (RDHx) is a cooling coil built into the back door of a server or miner rack. As hot exhaust air leaves the equipment, it passes through a chilled-water or coolant-filled coil mounted on the door, which absorbs the heat so the air re-entering the room is close to neutral temperature. By capturing heat right at the rack, an RDHx removes the load before it ever mixes into the room, eliminating hot aisles and the need to over-cool the whole hall.
Passive versus active
A passive RDHx relies entirely on the equipment's own fans to push exhaust through the coil, adding no extra electricity and no extra noise. An active RDHx adds its own fans to pull air across the coil, supporting higher heat loads at the cost of some fan power. Capacities span a wide range, from roughly 20 kW per rack on passive units up to 60-75 kW on active high-density doors.
Efficiency advantages
Because the coil meets air that is already hot, an RDHx performs well at warm chilled-water set-points, which lets the plant run chillers less aggressively or skip them entirely. In favourable climates the door can be fed by treated water from a cooling tower through a plate-and-frame exchanger, removing chiller energy from the equation. That makes RDHx a pragmatic retrofit path for raising rack density without rebuilding the room's air handling.
The door coil is itself a Heat Exchanger, and it is often fed through a Coolant Distribution Unit (CDU).
In Simple Terms
A Rear-Door Heat Exchanger (RDHx) is a cooling coil built into the back door of a server or miner rack. As hot exhaust air leaves…