These are frequently framed as competing options. They are better understood by where they sit in the heat path: a cold plate intercepts heat before it reaches the air, while a rear door treats the air after the rack has heated it.
Cold plates depend on the server
Capture happens on the processor package, so the plate, manifold, and server design have to match. That dependency is why cold-plate deployments track the accelerator roadmap and why factory-integrated liquid-cooled servers simplify the project.
Rear doors depend on the rack
A rear-door unit replaces the rack door with a liquid-to-air coil and is largely indifferent to what is inside the rack. That makes it the more approachable retrofit for a hall that has water but a mixed, changing fleet.
Capture ratio decides the facility work
Cold plates move the majority of processor heat into liquid and leave a residual air load. Rear doors keep the air path but shorten it. The remaining air load is what determines whether the room's existing air handling stays adequate.
- Share of rack load captured to liquid at design conditions
- Residual heat the room must still remove
- Water temperature each option can accept
- Whether the fleet can be standardized on one server design
Airflow and room air still matter with cold plates
A cold plate captures processor heat, not all rack heat. Memory, drives, optics, and power conversion continue to reject heat into the room air, and that residual load is commonly a meaningful share of the rack. Data center cooling designs that assume liquid removes everything end up with hot spots and stranded air-handling capacity. A rear door heat exchanger, by contrast, treats the full exhaust stream, which is why RDHx units are often retained even in halls that have adopted direct-to-chip cooling.
Retrofit scope compared
The two options differ mainly in how much has to change. A rear door replaces a door and needs water at the rack. Cold plates need water at the rack plus qualified servers, manifolds, filtration, and a service procedure for wet work inside the cabinet. For an operating hall with a mixed fleet, that difference in scope frequently outweighs the difference in heat removal.
- Server qualification required, or none
- Wet work inside the cabinet during maintenance
- Water quality and filtration obligations
- Ability to move the equipment when the fleet refreshes
In practice they combine
Several suppliers tracked here offer both, and mixed halls commonly run cold plates on accelerator racks with rear doors on adjacent infrastructure. Specify the pair as one hydraulic system rather than two purchases.