When an undesigned restriction occurs, refrigerant will “back up” against the restriction, resulting in more refrigerant being present before the restriction and less afterwards than designed. Think of it like a refrigerant traffic jam with the refrigerant “road” being congested before the restriction and free and clear afterwards. This restriction will result in a pressure drop across the restriction with higher pressure being on the inlet side and lower pressure on the outlet side of the “traffic jam”.

First we must be aware that a restriction exists in the first place. In the case of the most common liquid line restrictions on HVAC equipment (with no receiver) we will see low suction pressure, high superheat, and normal to high subcooling. In cases like this we know it is not simply “low on charge” because of the subcooling reading, and we also know it isn’t just a an evaporator airflow issue because of the high superheat. This leaves us in the realm of restriction.  Like anything else, some common sense, a look at the system history and a visual inspection can find many restrictions without any fancy diagnosis, but sometimes you have to put on your thinking cap, grab a pipe or a cigar, and go to work.

In a perfect world we could just connect a gauge anywhere in the system and we could find the pressure drop, in the real world we only have two or maybe three points on connection and they are not sufficient for us to pinpoint a restriction. Luckily we have temperature drop as a proxy for pressure drop, whenever the pressure drops there will also be a temperature drop. The trouble is, by the time the temperature drops enough for us to reliably measure it with a thermometer it is usually pretty bad, making minor restrictions hard to find.  It can also be challenging when the metering device itself is a suspect (and it often is), because the metering device is a designed restriction. This means that a pressure drop is it’s very purpose, but is it restricting too much?

So to actually find a restriction you are left with a few tools in your arsenal.

Common Sense

Get acquainted with the history of the system. How old is it? What has been done on it recently? Has the refrigerant circuit been open to atmosphere?  If you recently had a burnout compressor then it is very likely that suction and liquid line driers could be restricted. If the system has been running just fine for 7 years it is more likely that that TXV element tube rubbed out and now the TXV is slammed down. If the distributor just a leak repaired on it, it is very possible that they accidentally filled one of the feeder tubes with solder when they made the repair. A little common sense can save a lot of random hypothesis. Any experienced technician will agree with the problem solving principle called Occam’s Razor that states: “With all things being equal, simpler explanations are generally better than more complex ones.” This certainly hold true when looking for restrictions.

Temperature Drop 

Grab your most accurate line temperature clamp and start making measurements across possible restrictions like line filter driers and the liquid line itself. If you find any confirmable temperature drop across a line drier than you can knows it’s restricted, just make sure to double check. Across a typical liquid line you will generally only see a few degrees of temperature drop but it does depend on the ambient temperature, condensing temperature and the line length.

Freeze Test

Sometimes the exact point of temperature change can be tough to locate. In these cases when the metering device, distributor, feeder tubes, inlet screen or evaporator are all suspects you can do the freeze test. Disconnect the blower and watch the frost patterns. On a properly functioning system the ice will start right at the outlet of the metering device and extend forward though the feeder tubes and work its way fairly evenly through the coil on the coil piping route. Look for inconsistencies in the pattern and you can often find a restriction.

If for example, you see that the frost is starting before the metering device instead of after, you can bet the restriction is an inlet screen. This test is finicky and requires a trained eye to track the tubing patterns, otherwise you might think a coil is restricted when it’s just the way it’s piped. Also be aware that the designed pressure drop of metering devices that also contain a distributor and feeder tubes is cumulative across all of those restriction points. This means that in some cases you may get more frost after the distributor than you do between the metering device and the distributor, this is to be expected.

Thermal Imaging 

The holy grail of finding restrictions is the thermal imaging camera. You are able to see restrictions in real time and pinpoint the exact location where the temperature change begins. Thermal imaging can even be used to find illusive restrictions like discharge line restrictions caused by poor brazing practices, condenser feeding issues, evaporator restriction etc…

So the process for finding restrictions is –

1. Prove you have one by looking carefully at your readings
2. Use some common sense and perform a visual inspection
3. Take lots of temperature measurements until you find it
4. Whip out the fancy pants thermal imaging camera and spot that sucker in no time flat and be the hero with throngs of adoring fans

Keep in mind it get’s even trickier to diagnose when you are working on a system with a receiver, because the receiver can usually hold a lot of excess refrigerant, often making a liquid line restriction appear more like a low charge in the readings. Also, minor suction line restrictions like a kinked suction line can be very difficult to find because the temperature drop will usually be unmeasurably low.

This is why taking all the system readings in conjunction with some common sense and knowledge of the systems history are your best allies. And when in doubt… get a thermal imager from TruTech tools .

I told you it wasn’t easy.

Publication date: 12/18/2017

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