The Rules Narrowed the Menu
For two decades, most mid-size cold storage ran on HFC blends because they were familiar, non-toxic, and easy to service. The EPA limits that took effect January 1, 2026 ended that. New cold storage warehouse systems with 200 pounds or more of charge are capped at a global warming potential of 150, and smaller systems at 300. The practical choices now come down to a short list, and each makes sense for a different building. The permitting side is covered in our cold storage permits piece.
Traditional Central Ammonia
Ammonia is still the efficiency benchmark for large industrial refrigeration, especially with flooded evaporators. It is cheap, it has a global warming potential of zero, and engineers have run it for more than a century.
The catch is toxicity. A large central plant easily exceeds 10,000 pounds of charge, which puts the facility under OSHA Process Safety Management and EPA Risk Management Planning, with the permanent paperwork, audits, and trained staff those programs require. Best for large distribution centers with a professional refrigeration team.
Low-Charge Packaged Ammonia
These self-contained rooftop or penthouse units hold a small fraction of a traditional ammonia charge, often low enough that each unit stays well below the thresholds that trigger PSM and RMP. You get most of ammonia's efficiency without a compliance program, and the refrigerant stays out on the roof, away from occupied spaces.
The costs are a higher upfront price per ton than a central plant and more individual units to maintain. For mid-size conversions, this has become the default choice. Best for 20,000 to 150,000 square feet of converted space.
Transcritical CO2
CO2 has a global warming potential of 1, faces no phase-down under the AIM Act or the Kigali amendment, and is non-toxic at normal exposures. It is the most future-proof option on the list.
Its weakness is heat. Above about 31°C, CO2 runs "transcritical," rejecting heat through a gas cooler rather than a condenser, which historically cost efficiency in hot climates. Multi-ejector modules now raise system efficiency by roughly 15 to 25 percent and have made CO2 workable in warm markets. It runs at very high pressures, so you need technicians trained for it, and that pool is growing but still thin in some regions. Best for cold-climate sites and owners who value regulatory certainty.
Hybrid and Cascade Systems
Hybrids put ammonia on the high side and circulate CO2 or glycol to the rooms. That keeps the ammonia charge small and out of spaces where people work, while getting close to ammonia efficiency. An ammonia-CO2 cascade is especially strong for low-temperature freezers. The trade-off is design complexity and two sets of expertise. Best for multi-temp buildings with a large freezer and an occupied dock.
Self-Contained and Split Units for Small Rooms
Below warehouse scale, factory-built condensing units and packaged systems using low-GWP refrigerants fit individual cooler and freezer rooms. They are simple, fast to install, and easy to service. They get inefficient once you multiply them across a big floor plate. At the very small end, a farm walk-in can run on a window AC with an override controller, the approach in our DIY walk-in cooler guide. Best for single rooms, pilots, and small food businesses.
Side-by-Side Summary
- Efficiency: central ammonia first, then hybrids and packaged ammonia, then CO2 (strong in cold climates, closing the gap in warm ones), then small self-contained units.
- Regulatory burden: central ammonia is heaviest. Packaged ammonia is moderate. CO2 is lightest.
- Future-proofing: CO2 and ammonia both sit at the natural-refrigerant end, with GWP near or at zero. Synthetics are exposed to future rule changes.
- Upfront cost: self-contained units are cheapest for small rooms. Central ammonia is cheapest per ton at large scale. CO2 and hybrids sit in between.
Defrost and Controls Matter as Much as Refrigerant
Whatever you choose, pick the defrost strategy on purpose. Hot gas, electric, and water defrost behave differently, and defrost is where freezer energy use and ice problems concentrate. Specify staged compressor loading, floating head pressure, variable frequency drives, and remote temperature logging. Those features cut demand charges and produce the records your auditors will ask for, which is why they appear throughout our cost breakdown.
Related Reading
- Dry Warehouse to Cold Storage: How to Tell If Your Building Can Make the Jump
- Cold Storage Permits and the 2026 Refrigerant Rules
- Warehouse and Big-Box Conversions Ranked
Plan the whole project with the dry warehouse to cold storage guide, and price your system choice in the cost calculator.