Why choosing the right refrigerant matters in data centres

AI and high-performance computing are increasing data centre cooling demands, making refrigerant strategy a more important design, compliance, and investment decision. As compressor-driven systems return to focus and F-Gas and PFAS regulations tighten, developers and operators need to choose refrigerants that support performance today while remaining viable over the long-term life of the asset.  

AI has caused several design choices to change when constructing data centres. An element that hasn’t faced scrutiny is the choice of refrigerant, but this should not be overlooked. The answer is no longer simply to select the most efficient option available. It means choosing a refrigerant and cooling system that can meet rising thermal demands while remaining viable under tightening international regulations that are addressing the use of PFAS (per- and polyfluoroalkyl substances), otherwise known as ‘forever chemicals'.  

For years, operators focused on removing compressors from the cooling chain. This was driven by a rise in permissible IT inlet air temperatures and improved evaporative cooling technologies that helped to lower power usage effectiveness (PUE). However, the rapid expansion of AI workloads and high-performance computing (HPC) has pushed data centres back towards greater reliance on compressor-driven systems.  

Refrigerant strategy is not just a mechanical decision. It is a decision that spans design, compliance, resilience, and finance, so developers, operators and investors all need to be aware of the topic.  

Why refrigerants have become a strategic issue

With the proliferation of AI processors, thermal design power (TDP) has risen. This changes how we design for cooling. Direct-to-chip liquid cooling is now more common, but coolant temperatures have not increased as much as expected. This means compressors are still necessary to maintain cooling stability, particularly during high external temperatures, in summer, or in hotter climates.  

Beyond the IT load, compressors are also needed in ancillary systems, including:  

• Direct expansion (DX) splits typically used for battery rooms due to tighter temperature control.  
• Direct expansion computer room air conditioners (DX CRACs) or variable refrigerant flow (VRF) systems for other electrical rooms that require tighter temperature control.  
• Heat pumps for heat recovery or export.  
• This renewed reliance on compressors brings the choice of refrigerant back to the forefront. It will affect not just energy efficiency, but also compliance, long-term maintainability, and reliance on the assets themselves.  

Regulatory pressure is increasing

The UK retained most EU F-Gas (fluorinated gases) legislation after Brexit. However, since then, the EU has introduced steeper HFC phasedown timelines. The UK is now able to operate under its own F-Gas regime, but this is unlikely to promote significant divergence. As we have seen with many other regulations, EU laws will continue to influence UK legislation, supply chains, and investment.  

For developers, refrigerant selection now affects how long an asset is likely to remain compliant and operational. For operators, it will influence how often maintenance must take place and will also impact supply chain resilience. In both cases, the best approach is not to focus on what is best now, but on what will be usable over the life of the key assets in the data centre.  

What is viable by cooling application?

Water-cooled chillers:  

Current EU rules allow high-capacity chillers to use refrigerants with a GWP below 750. As such, several synthetic options remain viable in the medium term. These include:  

• R-1233zd  
• R-1366mzz  
• R-514A  
• R-1234ze  
• R-513A    

Operationally, the main considerations are:  

• These refrigerants are widely available and supported by major equipment manufacturers.  
• A2L classification introduces additional safety requirements in some applications.
• Low-pressure options can offer strong efficiency but are less commonly manufactured.  

In the long term, natural refrigerants such as ammonia (R717) or propane (R290) should be considered. This is particularly relevant as PFAS restrictions tighten. Both require early investment during design, especially regarding plantroom safety and emergency ventilation. However, natural refrigerants may provide a more sustainable alternative in the future by moving away from PFAS chemicals that are found in common replacement options.

Air-cooled chillers:  

Air-cooled chillers are subject to the same regulatory GWP ceiling of 750 or lower. The main options are:  

• R-1234ze  
• R-513A  
• R-515B  

Because air-cooled chillers are usually external, A2L safety constraints are less restrictive. R-1234ze(Z) is therefore often preferred for its comparatively low GWP, while ammonia and propane remain strong futureproofing alternatives.  

Water-to-water heat pumps:  

Here, EU rules are significantly tighter. By 2030, GWP must be below 150. This narrows the options to:  

• R-1233zd  
• R-1366mzz  
• R-1234ze  

At high temperatures and when waste heat recovery is implemented, CO2 (R744) can be a good replacement, due to its high compressor discharge temperatures, which make it well suited to heat pump applications operating at 60-90°C. However, its high operating pressure requires more robust plant, which drives up cost.  

DX CRACs, splits and VRF systems:  

These systems are facing the most intensive HFC phasedowns in the EU:  

• Sub-12kW units must reach GWP below 150 by 2027.  
• All fluorinated gases are banned in VRF systems with a capacity of 12kW or less by 2035.  

This will push the market towards two options:  

• Ultra-low GWP synthetics in the short term.  
• Natural refrigerants in the long term.  

For developers, standard VRF-based cooling might become harder to justify, particularly for new builds. Designs for projects currently under development will need to address the long-term compliance risks and the potential cost of replacing equipment that may no longer be viable from a regulatory and commercial perspective in the future. This could be driven by changes in the legislative landscape and the impact this would have on supply chains.  

How to design for a PFAS-restricted future

One conclusion is clear: many of today’s synthetic refrigerants should be viewed as transitional rather than long-term solutions.  

As F-Gas regulations and PFAS restrictions tighten, the industry will likely move towards natural refrigerants for cooling technologies. While the most appropriate choice will depend on the application, this shift would drive:  

• Greater separation between refrigerant and occupied spaces.  
• More centralised cooling strategies that simplify future plant replacement.  

In both the UK and the EU, operators planning for AI need to take a long-term view when considering their refrigerant strategy. Changing legislation targeting the 2030s is fast approaching and will affect the data centres that are being built now. Organisations involved cannot afford to treat refrigerant selection as a late-stage issue. For those planning new facilities or upgrades to existing infrastructure, reviewing current refrigerant strategy now may save a great deal of time and money in the future.

At Cundall, our data centre team have specialists who can help you assess your refrigerant strategy and pathways. Our full whitepaper, which breaks down the different options, will be available soon.