Non-condensables in a residential split air-conditioner can be, for most technicians, a pandora's box to diagnose. Fortunately, there is a simple process that can identify this condition without spending hours, and hundreds of dollars in virgin refrigerant. I think we all are aware that R-22 has hit an all time high, so getting this right is more important than ever - if you want to retain your customer.
There are several ways air and/or water vapor can enter a sealed system. First, improper evacuation practices appear to be the most common these days. If you are not currently evacuating a system using the Triple Evacuation Method, I highly recommend it - along with the manufacturer of the equipment you are installing if you want to keep that warranty! So do a quick search on the web, read it, it's simple.
Second, careless service by the technicians. When connecting your gauges to a closed system, be sure to purge the air (non-condensables) from your refrigerant lines. This seems like such an easy thing, but hurried technicians working in extreme conditions tend to make mistakes, particularly after a long week of such.
The final way that I am aware of air entering a system is one with a leak, so bad that the suction line pulls into a vacuum, pulling air into the system. When a technician finds this situation, the first instinct is to add refrigerant and then diagnose the issue. There is a leak, do not knowingly vent refrigerant into our environment by adding! Use dry nitrogen to pressurize an empty system, then use the Triple Evacuation Method once the leak is fixed!
Non-condensables can cause many problems in a working system. The problems arise when that air settles in the condenser coil, taking up room and board without pulling it's weight! Since air cannot be condensed, it remains stuck there in the condenser or accumulator, increasing your head pressure. Since the area of your condenser becomes reduced for the refrigerant to reject heat, the head pressure elevates to compensate, now relying on temperature difference instead of surface area of the coil. When this happens, you will get higher discharge/condensing temperatures and reduced capacities. Based on the only value I could find over the years, "a 10 psi increase in condensing pressure will increase power consumption of compressors by 6%." ("Air Tech Notes". T. Quello, 2004) If there is enough air in there, you could cause the amperage of the compressor to raise to a point of internal overload.
If you think you have non-condensables in a working system, because some joker is keeping you gainfully employed, be sure to verify it is not any of the following conditions causing your high head pressure:
- Clean Condenser Coil
- Proper condenser airflow
- Recirculation of condenser air
- Overcharged; Check Subcooling
Then use this process to verify your unfortunate situation:
- Turn off the condenser using the disconnect switch (you still need a call for cooling).
- Remove the wires to the compressor, so that only the condenser fan is operable.
- Apply voltage to condenser by turning disconnect back on. Only the condenser fan should run.
- Measure the temperature of the air entering the condenser.
- After a few minutes, use your Pressure/Temperature (PT) Chart for your selected refrigerant to identify the saturation pressure - based on the Condenser Entering Ambient.
- Your Liquid Line Pressure should equal the Saturation Pressure, based on the PT Chart. This is within a couple of degrees of the outdoor ambient, to allow for tool accuracy.
This process works because of the Pressure/Temperature relationship, and Dalton's Law of Partial Pressures: Total pressure of a mixture of gases is equal to the sum of their individual pressures.
For instance, if there was non-condensables in a system, your actual pressure will be higher than your supposed saturation temperature. If your gauges were supposed to read 155# for R-22 because it is 85F outside, but instead they read 175#, then you have 20# of said non-consensable air. This could increase compressor power consumption by as much as 12%!
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 f@#k 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 fusible plug that was accidentally torched that melted the solder within it, but those stories are for another day. On the lighter side, myself, Carlos and Mike were the install crew and 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, "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 plus 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 "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 air still 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, 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 can not be condensed during the refrigeration cycle. They move into the condenser and cause issues within a system, but lucky for us, those issues have symptoms.
Symptoms Of Non Condensables
We talked about the elevated discharge pressure, this is due to the fact that the non condensables are talking up needed space within the condenser coil. The 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 (difference between condensing temperature and ambient temperature). Others things to look for...possible increase of suction pressure, high compressor amp draw and higher than normal subcooling. Use best pratice evacuation procedures to ensure all non condensables are removed for a healthy operating system.
Some lessons can't be learned from books, you must breathe in the mistakes and embrace them as learning experiences. Happy HVACing!
Check out the link to my YouTube channel for more tips, tricks, and troubleshooting videos and check out the The HVAC Know It All podcast here or on your favourite podcast app.
Gary McCreadie
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