Which of the following would indicate a restriction, such as dirt or scale, in the liquid line?

Identifying a restriction in the liquid line of a refrigeration or HVAC system can be vital for proper diagnosis and maintenance. When a restriction occurs due to dirt, scale, or another obstruction, it often results in specific observable effects.

Frost immediately after the restriction is a key indicator because it occurs when refrigerant experiences a drop in pressure as it passes through the blockage. This pressure drop causes the refrigerant to cool rapidly, and as a result, moisture in the air condenses and freezes on the line, creating frost buildup. This phenomenon can be directly correlated with the presence of a restriction in the liquid line, making it a reliable sign for technicians to recognize.

In contrast, while warm air from the vents, low pressure readings, and condensation on the line can present challenges in system performance, they do not specifically indicate a restriction in the same way that frost buildup does. Warm air may result from various issues, low pressure could be symptomatic of a different problem altogether, and condensation may simply suggest humid conditions or temperature changes not directly related to blockages in the liquid line. Thus, observing frost specifically indicates the presence of a restriction, providing direct evidence of a problem that needs to be addressed.