Quick answer
A rear-door heat exchanger replaces a rack’s back door with a water-cooled coil. Hot exhaust air passes through the coil and leaves the rack cooler, so the room sees less heat. Passive doors rely on server fans; active doors add their own fans. RDHx is often used alongside direct-to-chip cooling to capture the heat cold plates do not remove.
Key takeaways
- RDHx cools air leaving the rack using a water or glycol coil.
- Active doors handle more heat than passive ones.
- Often paired with direct-to-chip cooling for the residual air-cooled load.
General information. Confirm the exact fluid, equipment and qualification with your hardware or CDU supplier before specifying.
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How it works
Servers blow hot air out of the back of the rack. With a rear-door heat exchanger, that air passes through a coil fed with chilled or facility water, and much of its heat moves into the water. The water loop returns to a CDU or the building plant.
Passive vs active
| Passive RDHx | Active RDHx | |
|---|---|---|
| Fans | None; uses server fans | Built-in fans |
| Capacity | Lower | Higher |
| Power and noise | None added | Fan power and noise |
Where RDHx fits
- Raising density in an existing air-cooled room without changing servers.
- Capturing residual heat from racks that use cold plates on GPUs but air for other components.
- A step before full direct-to-chip or immersion.
Coolant in RDHx loops
Doors usually run on facility water or a water-glycol mix supplied through a CDU or the building loop. Water quality, condensation control (keeping water above the room dew point) and leak detection matter.
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Get the checklist →Frequently asked questions
What is a rear-door heat exchanger?
A water-cooled coil that replaces a rack’s rear door and removes heat from the server exhaust air.
Do rear-door heat exchangers need a CDU?
Often they are fed through a CDU or secondary loop to control water temperature and quality; check the manufacturer’s design.
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