# Rear-Door Heat Exchanger A rear-door heat exchanger is a water coil built into the back door of a rack. Hot exhaust passes through it on the way out, gives its heat to the water, and returns to the room at roughly the temperature it started. It is the least disruptive way to bring liquid into an existing building, and it remains the standard companion of racks whose components are not all liquid-cooled. > [!note] The one-sentence version > Put a radiator in the rack's back door, and the room never sees the heat. ## Why it exists Two problems share one answer. An existing air-cooled hall asked to host denser equipment cannot always be replumbed or rebuilt, and its cooling units and airflow were sized for a lighter load. Separately, even a fully liquid-cooled accelerator rack exhausts real heat to air from memory, network interfaces, power conversion, and drives. A coil in the door addresses both: it intercepts heat at the rack rather than letting the room handle it, which multiplies the density a hall can support without touching the servers. ## Prerequisites - **Passive and active variants:** a passive door relies on the servers' own fans to push air through the coil; an active door adds its own fans, which raises capacity and tolerates higher airflow resistance. - **Approach temperature:** how closely the leaving air temperature approaches the entering water temperature; it determines how much heat the coil can actually capture. - **Facility water:** the same loop that serves coolant distribution units; a rear door is usually fed from it and often sized for warm water, which widens the hours when outdoor conditions alone can reject the heat. - **Condensation control:** supply water held above the room's dew point so that no water forms on the coil; this single rule makes the installation dry by design. - **Room-neutral operation:** the design intent that air leaving the door is close to room temperature, so the hall's own cooling units see little load from that rack. ## How it works Water enters the door's coil, air from the rack's exhaust passes across it, heat transfers, and the warmed water returns to the facility loop or a coolant distribution unit. Capacity depends on water temperature, flow, airflow, and coil surface; a rear door can absorb heat in the region of tens of kilowatts per rack, which is precisely the scale that extends air cooling past its ordinary ceiling. Piping is run overhead or under floor to each rack position with isolation valves, and leak detection is placed beneath the connections. A failed door degrades gracefully: the door's airflow path is open, so the rack continues to exhaust into the room and the room's cooling units pick up the load, which is why the technology is comfortable to retrofit. ## Stack position - **Upstream:** [[wiki/Chilled Water Plant|chilled water]] or a [[wiki/Coolant Distribution Unit|coolant distribution unit]]. - **Downstream:** the room's air system, which it relieves. - **Function and vendor:** a mature product class with several suppliers; it is the conventional conversion path for existing halls and the conventional partner for direct-liquid racks. - **Degraded case:** without it, a hall's density is limited by what its room-level cooling units and airflow can handle, and dense racks must be spread out, which wastes floor and lengthens the network between them. ## Lineage and trajectory Rear-door exchangers entered data centers in the 2000s as blade servers raised rack density beyond what raised-floor airflow could serve, and they have been in continuous use since. Their trajectory is as a component of hybrid halls: as capture at the chip approaches the whole rack, the door's share of the work shrinks while its role stays, because some heat will always reach the air. Warmer water supply is the main development, since it lets the same door run on water cooled by outdoor air alone for much more of the year. ## Relationships - **fed by:** [[wiki/Chilled Water Plant|Chilled Water Plant]] · [[wiki/Coolant Distribution Unit|Coolant Distribution Unit]] - **relieves:** [[wiki/Air Cooling|Air Cooling]] - **partners with:** [[wiki/Direct-to-Chip Liquid Cooling|Direct-to-Chip Liquid Cooling]] - **part of:** [[wiki/Data Center Cooling|Data Center Cooling]] - **source articles:** [[articles/Vertically Integrating an AI Superpower from AI Factory to Citizen|Vertically Integrating an AI Superpower from AI Factory to Citizen]] · [[articles/The Gigawatt Is the New Measure of War|The Gigawatt Is the New Measure of War]] - **collection route:** [[collections/American Infrastructure|American Infrastructure]] · [[collections/American|American Collection]] ## Evidence posture - **Established:** passive and active door variants; dew-point-based condensation control; room-neutral operating intent; capacity in the region of tens of kilowatts per rack; graceful degradation through an open airflow path; use both as a retrofit for existing halls and as the air-cooled partner of direct-liquid racks. ## Sources - Manufacturer technical literature on rear-door heat exchangers and ASHRAE liquid-cooling guidance, as current October 2026.