CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
During a service call, a technician measures the liquid line temperature leaving the condenser at 95°F. The high-pressure gauge corresponds to a saturation temperature of 105°F. What is the subcooling value?
- A200°F
- B10°F
- C5°F
- D15°F
Show answer & explanationAnswer & explanation
Correct answer: B. 10°F
Subcooling is calculated as the saturation temperature minus the actual liquid line temperature: 105°F − 95°F = 10°F. This value is compared against manufacturer specifications, especially for TXV-equipped systems, to verify proper refrigerant charge.
Why the other options are wrong
- A. This incorrectly adds the two temperatures instead of subtracting.
- C. This value does not match the correct subtraction of the given temperatures.
- D. This value does not match the correct subtraction of the given temperatures.
Subcooling Calculation
Subcooling is the difference between the condensing saturation temperature and the actual liquid line temperature, showing how much the liquid has cooled below its boiling point.
- Subcooling = saturation temp (from high-side pressure) − liquid line temp
- Used primarily to charge TXV systems
- Low subcooling suggests undercharge
- High subcooling suggests overcharge or restriction
Memory trick: Sub means below — subcooling is below boiling point on the liquid side.