Hopefully you have read my previous posting and understand how a simple refrigerant cycle operates.  Now I will go into some diagnostics of the refrigerant side of a system.

The first thing to remember is that refrigerant does not wear out!  It does not disappear inside the sealed system. It does not need to be changed like antifreeze!  The only problems inside the refrigerant side of a system are over charged, under charged, and non-condensibles. If a system has not had a history of leaks and has worked fine in the past; if care has been taken when putting on gauges to assure no contaminants enter the system, the last thing you ever want to do is to start adjusting the charge.

So, what can cause the pressures  or super-heat, or sub cooling to be abnormal?  Too often we tend to think that the air conditioner is the box sitting in the back yard.  An air conditioner is a SYSTEM!  There is a compressor, condenser coil, metering device, evaporator coil and, of course, the line set. Add to that an outdoor fan and an indoor fan to provide proper air flow to condense and evaporate the refrigerant.  Any problem with any of these parts of the system can cause your pressures to read “incorrectly”.

Let’s look at an example: If someone had their gauges on a system and saw low suction pressure and the evaporator coil was freezing and they did not take any other readings or look at the other components in the system – too often, servicers would add refrigerant to the system to bring up the pressure — right?  WRONG!

What causes low suction pressure and an icing coil? — Dirty filters, improper indoor air flow, dirty evaporator, metering device not feeding properly, even a light load in the home (no heat to transfer to the refrigerant) can cause this.  Now, if you first added refrigerant to the system without correcting these issues first, you have just made the system worse and have probably over-charged the unit.

Could this cause the coil to freeze?

could this cause the coil to freeze?

Or how about seeing high head pressure on you gauges — does that mean the unit is overcharged? If you don’t remove the panel and check the condenser coil and start removing charge, you are making the system worse.  And don’t forget to see if the coil is a split condenser or is a double row coil.  You could have a clean outside coil but the inner coil is “packed” with dirt. Yes, it takes a little more time to verify, and if necessary, clean an inner coil, but the end result will be a properly operating system.

could this affect the high pressure readings on your gauges?

I know the above pictures are extreme examples but I use them to make the point that if you do proper diagnostics and really find the cause of the problem, it is usually not charge related.  There are too may external things that can cause refrigerant pressures and temperatures to be abnormal and only three (3) things that can be wrong on the refrigerant side of a system. You have to look at the whole picture, make the proper diagnostic, and make the proper repair.

To make a proper diagnostic, you not only have to take both suction and liquid pressures, you need to calculate both super-heat and sub-cooling. Taking an amp draw on the various electrical components, especially the compressor, also aids in making the proper diagnosis. You need to know if there is enough air flow across the coils (both evaporator and condenser).

By knowing all the various readings, applying them to the system, and looking for where the problem actually is, you will find in most cases that it is not the “charge” that is the problem, but more than likely, something “external” to the system causing the incorrect readings.

Attached at the end of this posting is a handy chart that you can print out and use for reference.  It will let you look at the various scenarios of high super heat, sub-cooling, high suction pressure, high liquid pressure (arrows up) or low super heat or sub-cooling, low head pressure or low suction pressure (arrows down).  But be careful, don’t jump at the first one you see.  There may be additional problems that can have similar diagnostics so you need to look at ALL the diagnostics that have the same high/low temperature/pressure readings.  You could have multiple problems.

It is also good to take your time and fill in the “critical data sheet” (attached at the bottom of this posting) so you can record all of your readings and use this to help you diagnose the problem.  If you still cannot figure it out, then you will have data necessary that you can send to a “factory rep” and ask for their assistance.  In most cases, once you record all the data and use the diagnostic chart, the problem becomes apparent and you can then make the necessary repair.

I actually carry this chart and the critical data sheet with me when I go out to assist a contractor in the field.  Just something more for your tool box that will make you look like a champion on the service call.

Refrigerant Diagnostic Chart



About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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