A direct-to-chip liquid cooling retrofit of a legacy data center costs $2–3M per MW of IT load, a capex premium of roughly $2,500–4,500 per kW over air (StackedAI analysis, Cooling Solutions retrofit note, Aug 2026). Data Center Knowledge reports a $1.5–1.6M per MW premium, 20–25% above air-cooled (Data Center Knowledge, Apr 2026). The hardware is rarely the constraint; water and power are.
Which liquid cooling options fit a legacy hall?
Legacy enterprise halls were designed for 5–15 kW per rack, and air cooling with containment tops out near 30–50 kW per rack (StackedAI analysis, Cooling Solutions retrofit note, Aug 2026).
| Option | kW per rack | Cost basis | PUE outcome | Facility piping |
|---|---|---|---|---|
| Rear-door heat exchanger (RDHx) | 20–40 typical; up to 75 (Motivair) or 120 (OptiCool) | Entry about $17K per rack | Modest gain over air | Chilled water to each row |
| Liquid-to-air sidecar CDU | Up to ~70 | Per unit; varies by vendor | Limited; rejects heat to room air | None |
| Direct-to-chip (DTC) | 60–130+ | $50–80K per rack; $2–3M per MW | Toward ~1.15 | Facility water loop plus CDUs |
| Single-phase immersion | 100–250 per tank | Tank and fluid; varies by market | 1.01–1.08 | Warm-water loop to tanks |
| Two-phase immersion | Not recommended (PFAS fluids, supplier exit) | Fluid $50–90 per liter | Not applicable | Yes |
Source: StackedAI analysis, Cooling Solutions retrofit note, Aug 2026.
The common 2025–2026 design is hybrid, about 70% liquid and 30% air (StackedAI analysis, Cooling Solutions retrofit note, Aug 2026): air stays for networking, storage, and lower-density tenants while the liquid loop carries GPU racks.
What does each option cost per rack, per kW, and per MW?
- Per rack: RDHx enters at about $17K; direct-to-chip runs about $50–80K (StackedAI analysis, Cooling Solutions retrofit note, Aug 2026).
- Per kW: CDUs cost $500–800 per kW and the total direct-to-chip premium over air is about $2,500–4,500 per kW (StackedAI analysis, Cooling Solutions retrofit note, Aug 2026).
- Per MW: a broader direct-to-chip retrofit runs $2–3M per MW (StackedAI analysis, Cooling Solutions retrofit note, Aug 2026); a lighter $1.5–1.6M per MW premium, 20–25% above air-cooled, is reported for new AI capacity (Data Center Knowledge, Apr 2026).
Liquid cooling is therefore roughly a fifth to a quarter of StackedAI’s ~$12M per MW AI-ready conversion benchmark (StackedAI analysis, Tier-2 brownfield conversion underwriting framework, Aug 2026); the rest is electrical distribution, switchgear, and CDU plant, which is why JLL’s full AI fit-out reaches $25M per MW (JLL, 2026 Global Data Center Outlook, Jan 2026). See brownfield vs greenfield data center cost per MW.
The return on that spend is PUE. Direct-to-chip pushes PUE toward about 1.15 and single-phase immersion reaches 1.01–1.08 (StackedAI analysis, Cooling Solutions retrofit note, Aug 2026). Under a fixed utility service agreement, every point of PUE recovered is IT load that can be sold, and with vacancy at 1% for three consecutive years (JLL, North America Data Center Report Midyear 2026, Aug 2026) sellable MW is worth more than saved kWh.
Which option applies at 50–100 kW versus 100–200 kW per rack?
The roadmap sets the target: average rack density moved from 16 kW in 2025 to 27 kW in 2026, the AI training band is 50–70 kW, GB200 NVL72 runs 120–130 kW, and Vera Rubin is estimated at 190–230 kW (StackedAI analysis, density roadmap compiled from Goldman Sachs and Dell’Oro data, Aug 2026).
50–100 kW per rack. RDHx at the top of its range, sidecars up to about 70 kW, or entry direct-to-chip, in the hybrid 70/30 design; an existing chilled-water plant is usually adequate with pump and control upgrades.
100–200 kW per rack. Direct-to-chip with a dedicated facility water loop, or single-phase immersion; chilled-water plant, electrical distribution, and floor loading all need engineering review. This is the AI-ready target.
Either way the electrical question comes first: a 10 MW hall at 27 kW average is about 370 racks, at 130 kW about 77 racks with far heavier distribution. The cooling plan is downstream of the switchgear plan.
What are the diligence flags before committing?
Water and power, not the cooling vendor, are the binding constraints (StackedAI analysis, Cooling Solutions retrofit note, Aug 2026).
- Chilled-water plant. Capacity, condition, and headroom; a target with an existing chilled-water plant and electrical headroom warrants a materially higher multiple (StackedAI analysis, Cooling Solutions retrofit note, Aug 2026).
- Make-up water. Rights, supply contracts, and discharge permits for heat rejection.
- Electrical headroom. Service entrance, transformers, and switchgear against the target density; large transformers carry lead times of 80–210 weeks (Wood Mackenzie, Apr 2024), so the target must fit installed gear or gear must be ordered before close.
- Floor loading. Racks at 100 kW and above exceed the loading of many raised-floor halls.
- Tenant-agnostic plant. Rack-scale liquid-cooled systems have a thin secondary market (StackedAI analysis, stranded GPU and neocloud framework, Aug 2026; see GPU depreciation and secondary value), so the landlord should own facility-side CDUs and manifolds that any tenant’s hardware can connect to.
The power side is in how private equity underwrites data center power risk; the investment case is in Tier-2 brownfield data center conversion.
Key terms
- Rear-door heat exchanger (RDHx): a water-cooled coil replacing a rack’s rear door that removes heat from exhaust air before it enters the room.
- Direct-to-chip (DTC): cooling that pumps liquid through cold plates on processors, with heat carried to a CDU and then to the facility water loop.
- Coolant distribution unit (CDU): the heat exchanger and pump package separating the tenant’s cooling loop from the facility water loop.
- PUE (power usage effectiveness): total facility energy divided by IT energy; 1.0 is the theoretical floor.
How StackedAI applies this
StackedAI specifies the cooling path for a conversion only after the utility service agreement, chilled-water capacity, and floor loading are confirmed, because those three items decide which options are available at all. The retrofit is modeled at the rack, kW, and MW level so it reconciles against the tenant’s pod design and the building’s cost per MW. Andre van Zijl’s operating background in colocation over more than 21 years is the basis for the phased-cutover approach on live sites.
Frequently asked questions
How much does it cost to retrofit liquid cooling into an older data center?
A broader direct-to-chip retrofit runs $2–3M per MW of IT load, a capex premium of roughly $2,500–4,500 per kW over air (StackedAI analysis, Cooling Solutions retrofit note). Data Center Knowledge reports a $1.5–1.6M per MW premium, 20–25% above air-cooled. Per rack, rear-door heat exchangers start near $17K and direct-to-chip runs $50–80K.
What is the difference between rear-door heat exchangers, sidecars, direct-to-chip, and immersion?
Rear-door heat exchangers replace the rack’s rear door with a water coil and handle 20–40 kW typically. Liquid-to-air sidecars are rack-adjacent CDUs needing no facility piping, up to about 70 kW. Direct-to-chip runs coolant through cold plates on the processors for 60–130+ kW. Single-phase immersion submerges servers in dielectric fluid for 100–250 kW per tank.
What PUE does a liquid cooling retrofit achieve?
Direct-to-chip retrofits push PUE toward about 1.15, and single-phase immersion reaches 1.01–1.08 (StackedAI analysis, Cooling Solutions retrofit note). Legacy air-cooled halls sit well above those figures. Every point of PUE recovered is utility capacity that can be sold as IT load instead of consumed by fans and chillers, so PUE affects revenue as well as opex.
Which cooling option applies at 50–100 kW per rack versus 100–200 kW?
At 50–100 kW per rack, a hybrid design of about 70% liquid and 30% air using rear-door heat exchangers, sidecars, or entry direct-to-chip is sufficient and preserves the air plant. At 100–200 kW, covering GB200 NVL72 at 120–130 kW and Vera Rubin at an estimated 190–230 kW, direct-to-chip with facility water or single-phase immersion is required.
What are the diligence flags before committing to a liquid cooling retrofit?
Water and power, not the cooling vendor, are the binding constraints. Confirm chilled-water plant capacity and make-up water rights, electrical headroom at the service entrance and switchgear, floor loading for heavier racks, and that the utility service agreement supports the target density. A site with a chilled-water plant and electrical headroom warrants a materially higher multiple.
Sources
- StackedAI analysis, Cooling Solutions retrofit note, Aug 2026
- StackedAI analysis, density roadmap compiled from Goldman Sachs and Dell’Oro data, Aug 2026
- StackedAI analysis, Tier-2 brownfield conversion underwriting framework, Aug 2026
- StackedAI analysis, stranded GPU and neocloud framework, Aug 2026
- Data Center Knowledge, Neocloud Storm Gathers as Data Center Deals Stall Over Credit Risk, Apr 2026, https://www.datacenterknowledge.com/cloud/neocloud-storm-gathers-as-data-center-deals-stall-over-credit-risk
- JLL, 2026 Global Data Center Outlook, Jan 2026, https://www.jll.com/en-us/insights/market-outlook/data-center-outlook
- JLL, North America Data Center Report Midyear 2026, Aug 2026, https://www.jll.com/en-us/newsroom/data-center-demand-exceeds-expectations-in-h1-2026
- Wood Mackenzie, Supply Shortages and an Inflexible Market Give Rise to High Power Transformer Lead Times, Apr 2024, https://www.woodmac.com/news/opinion/supply-shortages-and-an-inflexible-market-give-rise-to-high-power-transformer-lead-times/