Rear-Door Heat Exchangers for High-Density Racks
A rear-door heat exchanger replaces a rack’s back door with a water-cooled coil that captures hot exhaust air before it re-enters the room. Passive units cool up to about 40 kW per rack with no fans and no plumbing into the servers — the simplest step up from room air cooling.
What it is
A rear-door heat exchanger (RDHx) is a finned coil built into the back door of a rack. Server fans push hot exhaust air through the coil, chilled or facility water inside the coil absorbs the heat, and near-room-temperature air leaves the back of the rack. Passive designs use only the servers’ own fans; active designs add fans in the door for higher capacity. A passive RDHx can reject up to roughly 40 kW per rack.
The appeal of the rear-door approach is that it is non-intrusive: nothing plumbs into the servers, so it works with standard air-cooled GPU systems and standard 600 mm racks. It rejects heat to a facility water loop — fed by a chiller or dry cooler — which means it can also be a room-neutral design, removing all the heat a rack produces so it adds no load to the CRAC/CRAH plant.
When you need it
The rear-door heat exchanger fills the gap between room air cooling and direct-to-chip liquid. It is the right tool when density has outgrown air but has not yet reached the point where cold plates are unavoidable.
- Rack density in the ~15–40 kW band — too hot for room air (CRAC/CRAH) alone.
- Air-cooled H100 / H200 HGX racks that need extra rejection without server-level plumbing.
- Retrofits — adding density to existing air-cooled halls without re-racking or re-plumbing servers.
- Room-neutral designs — capturing rack heat at the door so it adds no load to room air handlers.
- A stepping stone to liquid: facility water is installed now, ready for direct-to-chip later.
Quick facts & specs
Motivair · Rear-Door Heat Exchanger
| Type | Passive rear-door HX |
|---|---|
| Capacity | Up to 40 kW / rack |
| Coolant | Chilled / facility water |
| Fans | Passive (none) |
| Fit | Standard 600 mm rack |
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Rear-Door Heat Exchanger
passive rear-door · up to 40 kW/rack
From $7,400 / unit
Min order 2 units
Frequently asked
What is a rear-door heat exchanger?
A rear-door heat exchanger (RDHx) is a water-cooled coil mounted in the back door of a rack. Server exhaust air passes through the coil, facility water carries the heat away, and cooled air leaves the rack. A passive unit uses the servers’ own fans and cools up to about 40 kW per rack.
How much heat can a rear-door heat exchanger remove?
A passive rear-door heat exchanger rejects up to roughly 40 kW per rack. Active units with integral fans can go higher. That covers dense air-cooled H100/H200 racks; above ~40 kW/rack you move to direct-to-chip liquid cooling.
Rear-door heat exchanger vs direct-to-chip — which do I need?
Density decides. Up to ~40 kW/rack a passive rear-door HX cools the whole rack with no server plumbing — the simplest option. Above ~40 kW/rack, or for GB200/GB300 systems, you need direct-to-chip liquid cooling with cold plates and a CDU. Many designs use a rear-door HX now and add direct-to-chip later on the same facility water loop.
Does a rear-door heat exchanger need chilled water?
It needs a facility or chilled-water supply routed to the rack, typically fed by a chiller or dry cooler. Because it captures heat at the rack, a rear-door HX can be a room-neutral design — it removes the rack’s heat before it reaches the room, so it adds no load to the CRAC/CRAH plant.