CSLB C-20 HVAC ContractorCooling and RefrigerationHard
A technician suspects a restricted liquid line filter drier on an operating system. Which field test would best confirm this diagnosis?
- AMeasuring a significant temperature drop (often with visible frost) across the filter drier, with the outlet noticeably colder than the inlet
- BChecking for bubbles at the compressor discharge port
- CMeasuring identical temperatures on the inlet and outlet fittings of the filter drier
- DMeasuring superheat only at the evaporator outlet
Show answer & explanationAnswer & explanation
Correct answer: A. Measuring a significant temperature drop (often with visible frost) across the filter drier, with the outlet noticeably colder than the inlet
A restriction in the filter drier causes a pressure drop across it, which lowers the saturation temperature of the refrigerant downstream, producing a measurable (sometimes frost-visible) temperature drop between the inlet and outlet fittings — a classic sign of a clogged drier.
Why the other options are wrong
- B. Discharge bubbles are not a valid or accessible test point for this diagnosis.
- C. Equal temperatures indicate free flow with no restriction, ruling out the suspected fault.
- D. Evaporator superheat alone doesn't isolate the restriction to the filter drier specifically.
Restricted Filter Drier Diagnosis
A restricted filter drier causes a pressure drop that lowers refrigerant saturation temperature downstream, producing a measurable temperature drop (often with frost) across the component.
- Restriction causes pressure drop = temperature drop across component
- Frost may form at/after the restriction
- Confirmed by comparing inlet vs outlet temperatures with a thermometer
Memory trick: A cold spot after the drier means it's clogged.