Common things like a restricted metering device or a clogged filter drier fall under refrigerant circuit restrictions. There are cases that might be tough to diagnose especially kinked liquid lines, poorly connected compressor or blocked evaporator feeder tube as well as a discharge line filled with solder. Here is what you need to know about diagnosing and finding a refrigerant restriction. The Symptoms When a restriction occurs in the refrigerant, it will back up against it. Therefore, there will be more refrigerant before the restriction and less after it, something like a jam. The restriction causes a pressure drop where a lot of pressure is found in the inlet and the outlet side has lower pressure. First, you need to know that the restriction exists and that’s where you notice the symptoms. Some of the common restrictions on HVAC equipment liquid lines, especially those without receivers include the following.
  • Low suction pressure
  • Very high subcooling
  • A lot of superheating
With these symptoms, you can be assured immediately that it’s not low on charge due to the subcooling reading. On the other hand, it’s not a simple evaporator airflow issue because of the superheat. Therefore, it can only be a restriction. First, you need to take a good look at the history of the system and conduct a visual inspection. It’s easy enough to find many restrictions without doing a fancy diagnosis. Without facing any challenges, you can easily connect a gauge to the system in any location and find the exact pressure drop. However, there might be a big challenge where you have only 2 or 3 connection points which might not be sufficient to identify the exact restriction. One thing to note is that whenever the pressure drops, there will also be a consecutive drop in temperature. However, therein lies the problem because by the time there is a notable drop in temperature enough to measure using a thermometer, it might be tough to find the minor restrictions. You might also have a lot of problems, when the metering device might be a suspect since it is a designed restriction. It is designed to drop the pressure but it might be causing a restriction on its own. Therefore, it is very challenging to find a restriction yourself. Look Out For The Common Issues You need to know the history of the system so teach yourself everything there is to it. You need to know how old it is and anything that has been done on it recently. For instance, has the refrigerant circuit been exposed to open air? On the other hand, if there was a burnout compressor recently, it’s likely that the liquid line and suction drivers might have a restriction. On the other hand, if your system has been in operation for 7 years without any issues, there is a likelihood that the TXV element tube has worn out and slammed down. If you recently repaired a leak on the distributor, there’s a chance that the feeder tubes might be filled with solder. You can use your mind to solve some of these issues immediately. A Drop In Temperature Get your line temperature clamp and start measuring everything on the possible restrictions such as filter driers as well as the liquid line. If you notice any drop in temperature across a line drier, there’s a possible restriction but you need to check thoroughly before making the diagnosis. Note that, across a typical liquid line, the temperature will only drop for a few degrees but you need to know the ambient temperature beforehand as well as the line length and the condensing temperature. The Freeze Test You might have a hard time locating the exact point where the temperature has dropped. You’re allowed to do a freeze test if you suspect everything from the inlet screen, feeder tubes, distributor, evaporator or the metering device. Start by disconnecting the blower then watch the frosting patterns. If the system is working fine, the ice will start frosting at the outlet on the metering device then move forward to the feeder tubes and work evenly through the coil and the piping route. Therefore, look for any deviation from this pattern and you will find the restriction present. For instance, does the frost start right before the metering device instead of after it? The restriction will be on the inlet screen. You need some experience to track the patterns on the tubing because you might confuse a coiled pipe might be restricted while it’s just the design. You need to be aware that there is a designed drop in pressure of the metering devices that contain feeder tubes and a distributor which is cumulative from the restriction points. You should expect more frost after the distributor than what’s present between the distributor and metering device. Thermal Imaging If you want to find any restrictions, you need to consider using a thermal imaging camera. Here you can see the restrictions in real time and identify the exact area where the change in temperature begins. You can take advantage of the thermal imaging camera to find any elusive restrictions such as those in the discharge line often caused by issues in the condenser feeding, poor brazing practices and any restrictions in the evaporator. There is a simple process for finding restrictions which involves
  • Looking at your readings carefully to prove that you actually have a restriction.
  • Performing a visual inspection and applying your common sense.
  • Take all the necessary temperature measurements till you find the restriction.
  • Use a thermal imaging camera to identify the restriction immediately.
Note that, you might have a hard time diagnosing a restriction if your system has a receiver since it might hold a lot of excess WWW.BLUONENERGY.COM refrigerant. In this case, the restriction in the liquid line will simply look like a low charge. You might also have a hard time finding the restrictions in the minor suction line because there will be a very low temperature reading. That’s why it’s prudent to take into account all the readings before the diagnosis.