EVALUATING COMFORT COOLING SYSTEM PERFORMANCE.
By Jim Johnson

Copyright © 2014: Technical Training Associates


This is the first in a series of articles on the subject of HVACR system performance, and I think it’s appropriate that we begin by taking a nuts-and-bolts look at some of the simple tests technicians can accomplish with standard test equipment when they are checking a comfort cooling system for proper operation and optimum performance. As we all know, the “beer can cold” suction line test went by the wayside long ago, and refrigeration and air flow system performance analysis requires a more sophisticated approach. The illustration in Figure One shows the data collection points I’ll be discussing.

Of course, one of the most common method employed to evaluate a system is a standard set of compound gauges attached the high and low pressure access valves (shown to the left of our illustration at the outdoor segment of a typical split system). Unfortunately, this is often the only step taken to find out if a comfort cooling system is operating as efficiently as possible, however there’s much more beyond this basic process that should be accomplished in the way of performance checks. Let’s start with one that is very simple and easy to do, yet tells you a lot about the system.

You’ll note that in addition to the pressure measurements that can be taken at the outdoor unit, we are also explaining that you should test the temperature of the discharge line. This is a temperature test that should be accomplished about 6 inches from the compressor, and the maximum reading you should get is 225 Degrees F. The bottom line on this test is simple: If you’re getting a 225-degree reading at this point on the discharge line, then the temperature inside the compressor itself is going to be approximately 300-Degrees. And, a temperature higher than that in a compressor crankcase will mean that some of the oil is vaporizing, which affects the lubrication process. Of course, the ultimate result of this situation is premature compressor failure.

How does a compressor discharge temperature get too high? Again, unfortunately, a system overcharge is often the culprit….a system overcharge that is the result of poor service procedures like “adding a little gas” in an effort to increase the performance of the equipment. The percentage of overcharged residential comfort cooling systems is literally staggering, and they are not only failing prematurely, requiring sealed system service that increases the incidences of refrigerant discharge into the atmosphere, but they are operating inefficiently due to increased current draw….a far cry from proper system performance.

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When it comes to accomplishing this temperature test, a quality, professional-grade digital meter that employs a pipe clamp thermocouple accessory, such as we’re showing in Figure Two, is one way to obtain the accurate information you need.
This device will also give you another temperature reading that provides you with insight into the operation of the equipment…the liquid line temperature. In our data collection point illustration, you can see a point near the indoor unit where you should take this temperature reading. A good rule of thumb to keep in mind about this test is that the temperature of the liquid line should not be more than 15 degrees higher than the ambient temperature. If it exceeds this differential, then the temperature of the refrigerant entering the metering device is too high, which will lower the overall efficiency of the equipment by causing a drop in the net refrigeration effect….in essence a reduction in the transfer of heat from the conditioned space to the outdoors.

Another liquid line test you can accomplish is to test the temperature differential at two points: First at the liquid line near the outdoor unit, then near the indoor coil. If the temperature drop is too great, it indicates either a kink in the line, a restriction for some other reason, or a design or installation problem relative to the liquid line, that will prevent the system from performing properly.

In addition to refrigerant piping temperature tests, a Type-K thermocouple device that connects to a digital meter (shown in Figure Three) will allow you to accomplish dry bulb temperature tests, and the same kind of device equipped with a wet sock (Figure Four) will provide wet bulb temperature information. Using these accessory devices as we’re showing in our data collection points illustration on the return and supply ducts will tell you if there is a problem in the duct system that is affecting performance. A significant rise in the dry bulb temperature between the return air grille and the duct near the indoor unit indicates air leakage into the return, or insufficient insulation. Likewise, a significant dry bulb temperature rise between a location in the supply duct near the indoor coil and a supply register will indicate duct leakage or insulation problems in the supply side of the air handling system.

And, taking wet bulb readings at these locations, and then applying the dry bulb and wet bulb information to a psychometric process will not only allow a technician to calculate humidity that will prove duct leakage if there is a increase in humidity between the two test points, it will also serve as the basis for calculating superheat, which is a subject I’ll discuss in the next segment in this series.

Jim Johnson is the Director of Training for Technical Training Associates, a Tucson-based firm that develops HVACR technician training DVD’s, soft skills training for technicians, and provides on-site training. To reach Jim, call 520-625-6847, visit www.techtrainassoc.com or email directly to jim@techtrainassoc.com.