Rear-Door Heat Exchanger (RDHx)

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 — a genuinely appealing property in a room full of already-loud machines. 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, so the door can be matched to how hard the rack behind it is being pushed rather than sized for a worst case that never arrives.

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 its chillers less aggressively or skip them for part of the year. 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 altogether. That combination — heat captured at the source, plus warm-water operation — is what makes an RDHx a pragmatic retrofit path for raising rack density without rebuilding the room's air handling from scratch.

A natural fit for heat reuse

Capturing heat into water at the rack does more than cool the room; it collects the waste heat into a medium that is easy to move and reuse. Warm water leaving the doors can preheat a building, feed a district loop, or serve any of the heat-reuse ideas that align with running a mine as a productive heat source rather than a pure cost. For a sovereign operator thinking about heating a workshop or home alongside mining, capturing heat into a liquid loop is far more useful than blasting it into the air.

Why it suits dense hardware

Mining and AI-compute racks pack a lot of heat into a small footprint, and traditional room cooling struggles to keep up as density climbs. The RDHx meets that heat where it is densest, at the rack outlet, rather than diluting it across the room and paying to cool the mixture. For an operator upgrading an existing space rather than building fresh, it is often the least disruptive way to add cooling capacity to a rack that has outgrown its aisle.

Adopting rear-door exchangers also changes what the room around the racks needs to be. Because heat is captured before it escapes into the aisle, the surrounding space can run closer to a comfortable ambient rather than being chilled aggressively to fight a wall of exhaust, which cuts the energy spent conditioning the room itself. The flip side is that the doors bring liquid — water or coolant — into close proximity with electronics, so leak detection, drip trays, and quick-disconnect fittings become part of the design rather than optional extras. That is a familiar trade in high-density cooling: you accept the discipline of managing a liquid loop in exchange for moving far more heat than air alone can carry. For an operator scaling a room up in density without rebuilding it, the rear door is often the gentlest first step onto that ladder.

The door coil is itself a Heat Exchanger, it is often fed through a Coolant Distribution Unit, and for the densest loads it gives way to full immersion cooling.