Waste Heat Reuse from Immersion-Cooled Data Centers
Immersion cooling captures server heat in a liquid. That makes reuse easier – and EU rules increasingly expect it.
Updated 9 November 2026 · Published by IsoTec
Why immersion makes heat reuse easier
In an air-cooled data hall, heat is spread across large volumes of warm air. In immersion cooling, nearly all of the heat of the servers is captured in the fluid and leaves the tank at a higher temperature than the exhaust air of an air-cooled room. A liquid-to-liquid heat exchanger transfers it to a water circuit. From there it can be used directly for low-temperature applications or raised with heat pumps to the temperature level of district heating networks.
Possible uses
- District heating networks, usually via heat pumps
- Space heating of offices and buildings with low-temperature systems
- Pre-heating of domestic hot water
- Greenhouses, aquaculture and drying processes
- Industrial low-temperature process heat
What EU rules require
- Reporting: under Article 12 of the Energy Efficiency Directive (EU) 2023/1791 and Delegated Regulation (EU) 2024/1364, operators of data centers with an installed IT power demand of at least 500 kW report key data every year to a European database – including the amount of waste heat reused and its temperature.
- Waste heat use: Article 26 of the directive requires data centers with a total rated energy input above 1 MW to use their waste heat or other waste heat recovery applications unless this is shown not to be feasible. New and substantially refurbished installations have to assess this in a cost-benefit analysis.
- Rating scheme: the EU is preparing a rating scheme for data centers based on the reported indicators.
Member states transpose the directive into national law and may set stricter rules. Check the requirements in your country.
What warmer operation means for the fluid
Higher fluid temperatures make heat reuse more attractive, but they also accelerate fluid ageing. As a rule of thumb, chemical reaction rates roughly double for every 10 K. What helps:
- An oxidation-stable fluid – synthetic esters are very resistant to oxidation
- Covered or sealed tanks to limit air contact
- Low water content to prevent hydrolysis of esters
- Regular analysis of acidity and viscosity
- Checking the temperature limits of all server components
The lower viscosity at higher temperature improves heat transfer and reduces pump power. See natural vs synthetic esters and fluid maintenance.
Heat exchanger and system design
The smaller the temperature difference between fluid and water circuit, the more of the heat can be used at a high temperature. This requires clean heat exchanger surfaces on the fluid side – another reason for filtration and fluid monitoring.
Frequently asked questions
Do data centers in the EU have to reuse waste heat?
Under Article 26 of the Energy Efficiency Directive, data centers with a total rated energy input above 1 MW must use their waste heat or other recovery applications unless this is not feasible. National rules may go further.
Which data centers must report energy data in the EU?
Data centers with an installed IT power demand of at least 500 kW report annually to the European database, including waste heat reuse.
Does a higher fluid temperature shorten fluid life?
It accelerates ageing. An oxidation-stable fluid, limited air contact, low water content and monitoring keep this under control.
Planning warm operation with heat reuse?
We help select an oxidation-stable ester fluid for your operating temperatures. IsoTec products are supplied to customers outside Germany.