Troubleshooting Refrigeration Systems

In this specific troubleshooting situation, we are dealing with a residential split system, and it's the middle of July, which means we have just moved into the warmest temperatures of the cooling season. The customer's specific complaint is that "the house is too warm", and when you arrive, you find that the temperature inside is higher than it should be, and that the indoor fan motor is operating, and the air flow through the return and supply system is normal.

When you ask the customer for information on the service history of this 5-year-old, R-22 system, they tell you that they had the same problem at the beginning of the season, and the technician who did the repair told them that a leak was found in the suction line of the connecting tubing, and he repaired it.

When you check on the operation of the condensing unit, you find that the compressor and outdoor fan motor are operating, but the suction line is colder than it should be, and the liquid line (on which there is no filter-drier) is very hot.

You also note that this unit is not equipped with a high-pressure or low-pressure switch. Before you can proceed with a more detailed evaluation of the system, the compressor kicks off on overload, and the outdoor fan motor continues to run. After a delay, the compressor on this high-efficiency, capillary tube metering device system cools down enough to start again. With your gauges attached, and noting that the outdoor ambient temperature is 95-degrees, you are able to determine that the high-side pressure goes up to 290 PSIG before the compressor shuts down again. When you check a temperature/pressure chart, such as the one shown below, you arrive at a diagnosis.

Figure One

Figure One

What you've determined is that there is air in the sealed system. When non-condensibles are occupying the upper section of a condenser coil, the operating size of the condenser is diminished, and the higher-than-normal high side pressure you measured with your gauges (on a high-efficiency system the normal high side operating pressure would be approximately 243 PSIG, which we would calculate by adding 20-degrees to the ambient temperature....95+20=115, and reading across from that temperature to the pressure shows it should be 242.8), coupled with a hot liquid line tell us this is the case. In order to solve this situation, we need to recover the refrigerant, install a new filter-drier on the liquid line, and evacuate the system properly before re-charging. 

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Troubleshooting HVAC Pages

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PO Box 2259
Green Valley, AZ 85622-2259
Phone: 520-625-6847

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