CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
A technician charges a residential split system on an 85°F day but notices head pressure is significantly higher than the pressure-temperature chart predicts for that ambient temperature, even after verifying correct charge weight. What is the most likely cause?
- AThe system is undercharged
- BThe evaporator coil is dirty and restricting airflow
- CNon-condensable gases, such as air, are trapped in the system
- DThe thermostatic expansion valve is overfeeding the evaporator
Show answer & explanationAnswer & explanation
Correct answer: C. Non-condensable gases, such as air, are trapped in the system
Non-condensable gases like air do not condense in the condenser and add to the total pressure in the system, causing head pressure to read higher than the pressure-temperature chart would predict for a given ambient temperature. This typically results from improper evacuation before charging.
Why the other options are wrong
- A. Undercharge typically lowers pressures, not raises them abnormally above the chart.
- B. A dirty evaporator coil reduces airflow and suction pressure but does not typically cause head pressure to deviate from the PT chart.
- D. An overfeeding TXV would primarily affect superheat and suction pressure, not cause a head pressure/PT chart mismatch.
Non-Condensable Gases
Gases like air or nitrogen that remain in vapor form inside the refrigeration system and add extra pressure that doesn't correspond to refrigerant saturation temperature.
- Usually caused by improper or incomplete evacuation
- Causes head pressure higher than the PT chart predicts
- Fixed by proper deep vacuum evacuation before charging
Memory trick: Air won't condense — it just adds pressure and offends.