A restriction of refrigerant flow is caused by the formation of ice from moist air in the system. Air can penetrate into the system from a very tiny hole in the tubing, perhaps caused by an improperly sealed joint, flared connection, or failure to properly vacuum the system if previously repaired.

This restriction occurs in the capillary tube, TEV or AEV, because they are the narrowest passages through which refrigerant flows in a sealed system. When the capillary tube becomes restricted, the most common complaint is that “the unit sometimes seems to cool and sometimes it doesn’t cool at all. It acts crazy!” This is due to the fact that the formation of ice in the capillary tube temporarily disrupts the flow of refrigerant to the evaporator causing the unit to stop cooling. As the evaporator temperature goes up, the ice melts, causing the circulation of refrigerant to be restored and the evaporator to cool again until the temperature drops low enough to refreeze the circulating moisture in the capillary tube.

1. Unit runs at warmer than normal temperature with little or no frost on evaporator coil.
2. Low-side pressure reads partial vacuum.
3. High-side pressure reads higher than normal.
4. Unit runs continuously.
5. Higher than normal wattage draw.
6. The condenser, capillary tube, or drier feels cool.
7. Low-torque compressors used in residential and light commercial applications cycle on overload and take longer than three or four minutes to restart after being shut off. This is due to high-pressure refrigerant being trapped and separated in the system by the restriction. Equalization of high- and low-side pressures through the capillary tube is difficult to achieve during off cycles.

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