Safety And Tools 

Start with safety. Wear proper personal protective equipment (PPE), especially eye protection. CO₂ systems run at significantly higher pressures than HFC or HFO systems, and high-side working pressure typically falls between 1,100 and 1,400 psig. 

As a result, all tools and test equipment must be rated for these conditions. A manifold set should be rated as 120 BAR (1,740 psig), and the same applies to hoses, vacuum pumps, and service couplers. A good rule of thumb is that if it touches a sealed system, confirm it’s approved for CO₂ service. 

Piping And Fittings

It’s easy to mix copper fittings — even at the supply house — as CU and CUfe2P tees and elbows look about the same. However, it’s critical to never install standard Type L copper, as ferrous alloy fittings are required for CO₂ pressure. A quick magnet test will confirm the correct fitting: The magnet will stick to the ferrous version. Double-check every tee and elbow before brazing to avoid long-term reliability problems. 

Most CO₂ unit coolers arrive as bare coils, so the electronic expansion valve (EEV), temperature sensor, and pressure transducer must be installed on site. Verify placement of the air thermistor in the airstream and the pipe thermistor at the correct location of the suction line. The pipe sensor and pressure transducer work together to maintain superheat, so accuracy is critical.  

Review the installation instructions before placing components, as assumptions can lead to control issues once the system starts.  

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Pre-Startup 

Do not begin charging until the system passes all pre-startup checks. Be sure to perform a full pressure test. Pull a deep vacuum — using multiple pumps if needed — and confirm electrical connections, grounding, and correct voltage at disconnect. 

Pressure relief valves (PRVs) should be blanked off during leak testing, and the system should hold test pressure for at least 24 hours. While these steps matter on all sealed systems, they’re essential on CO₂ equipment because pressures are nearly double those seen in HFC/HFO equipment. Always follow the manufacturer’s startup checklist. 

Charging Procedures 

Use dry instrument-grade CO₂. Moisture is just as harmful here as in synthetic refrigerant systems.  

Charging must begin with vapor. CO₂ reaches its triple point at about 75.1 psia, where it can form dry ice if the pressure is too low. Two cylinders are required: A liquid cylinder with a dip tube and a liquid one without a dip tube. Begin with the vapor charging. As system pressure climbs past 100 psig, dry ice cannot form, and switching to liquid charging becomes safer and much faster.  

Balancing The Rack 

On a transcritical booster rack, every compressor has its own off-on switch, and all switches should start in the off position. Medium-temperature compressors must start first, because the low-temperature discharge feeds into the medium-temperature suction header.  

Without medium-temperature compressors running, the low-temperature discharge has nowhere to go. Watch the flash tank sight glass during startup. When liquid appears in the middle sight glass, start the low-temperature compressors, then balance the system charge. Large systems take time to stabilize, so be patient.  

CO₂ carries a GWP of 1, making it a long-term, future-proof refrigerant. The more technicians work with it, the more predictable the installation and startup process becomes. Following these practices helps avoid early mistakes and ensures the system performs the way it was designed.