Non-Condensable Gases in a Refrigeration System
Key Takeaways
- Non-condensables defined: Gases like air or nitrogen that cannot condense during the refrigeration cycle, taking up valuable space in the condenser
- Primary symptoms: High discharge pressure (400+ psi), elevated condenser split, increased compression ratios, and higher subcooling
- Critical evacuation tip: Always energize solenoid valves during evacuation to prevent air pockets between components
- Troubleshooting approach: Check for non-condensables when experiencing unexplained high head pressure after proper installation
Most of my hard-core learning came in the early days of my career. I was thrown into many situations I probably had no business being involved in and, like most young apprentices, there were more than a few royal screw-ups.
I remember vividly my first encounter with non-condensables in a system, and you guessed it, I put them there—or more accurately, failed to remove them.
The Job
We were installing a 5-ton Liebert Challenger for a small server room. The condenser was outfitted with an OROA flood back control to build condenser pressure in low ambient conditions.
The job went well for the most part, but it did include a fall from a ladder, a compound fracture, a hospital visit, and a fusible plug that was accidentally torched (melting the solder within it). But those stories are for another day.
On the lighter side, myself, Carlos, and Mike were the install crew. When I told the boss about the fusible plug incident, he asked who torched it.
Growing up with the phrase “snitches get stitches,” I told him I’d rather not say. But like most bosses, he demanded to know. My response was, “Well, it wasn’t me or Carlos.” Technically speaking, I fed Mike to the wolves, but I never mentioned his name once!
The Start-Up
We were on the back nine approaching the 18th hole (evacuation of the system). After pulling a vacuum overnight, we added a holding charge, pounding liquid into the liquid line until the system stopped accepting it. The power supply was checked and verified as correct.
Upon initial start-up: instantaneous high-pressure fault. And no, it wasn’t a faulty pressure switch. The pressure was actually 400+ psi on the R22 system. After a few hours of this and that, I finally made the call to the office. I needed some help.
The Grumpy Senior Tech
The grumpiest but smartest of the bunch—our “super tech”—showed up, and I was thankful because I knew he would get the situation sorted out.
After an hour or so of surveying the site and installation, he went straight to the condenser mounted on the roof, stuck a hose on the service fitting at the highest location, and blew off about 30 seconds’ worth of gas (which I am not recommending anyone do).
We went back down to the indoor unit and started it up. No longer did it instantly pop the pressure control. It ran… but not so well. He explained to me that there was still air in the system (non-condensables). I was dumbfounded because of our 14-hour evacuation.
It turns out that on a system with a flood back control and liquid line solenoid valve, we must energize the solenoid during evacuation. If not, we will be unable to remove the air trapped between the two devices. The entire charge was removed, and the evacuation was executed once more with the solenoid energized. A fresh batch of R22 was charged into the system, and we called it a day… a long day!
What Are Non-Condensables?
Non-condensables are gases such as air or nitrogen that cannot be condensed during the refrigeration cycle. They migrate into the condenser and cause issues within a system, but lucky for us, those issues have distinct symptoms.
Symptoms of Non-Condensables
The elevated discharge pressure we experienced occurs because non-condensables take up needed space within the condenser coil. This elevated pressure in the condenser will cause higher-than-normal compression ratios as well.
Due to the increased condenser pressure and saturated condensing temperatures, the condenser temperature difference (or condenser split) will also be high—that’s the difference between condensing temperature and ambient temperature.
Other symptoms to watch for:
- Possible increase of suction pressure
- High compressor amp draw
- Higher than normal subcooling
- Reduced system capacity
- Increased energy consumption
Use best practice evacuation procedures to ensure all non-condensables are removed for a healthy operating system.
Prevention and Best Practices
Beyond proper evacuation, preventing non-condensables requires attention to detail throughout the installation process. Always verify that all system components are properly sealed before evacuation. When working with systems that have bi-flow TXVs or specialized metering devices, ensure you understand their operation during the evacuation process.
For commercial systems with complex controls, consider scheduling your preventive maintenance during the September sweet spot to catch these issues before the heating season begins.
Learning from Experience
Some lessons can’t be learned from books. You must breathe in the mistakes and embrace them as learning experiences. This particular incident taught me the importance of understanding not just the basic procedures, but how different system components interact during critical processes like evacuation.
When dealing with high-pressure issues in the field, always consider non-condensables as a potential culprit, especially on new installations or after major repairs where the system has been opened.
