Non-Condensable Gases in the System

Why air trapped in a system raises head pressure, and how to confirm and correct it.

Troubleshooting

Advanced

#diagnostics#refrigeration#evacuation#charging

Safety

Recover refrigerant to an approved cylinder rather than purging to atmosphere.

Symptom

High head pressure and high condensing temperature with a clean condenser coil and good airflow.

Symptom

Subcooling and pressure readings that do not agree with each other.

Possible causes

A system charged without a proper evacuation.
A system opened for repair and closed without pulling a vacuum to the target.
A leak on the low side that runs into a vacuum during off cycles, drawing in air.
Refrigerant added from a contaminated cylinder or hoses that were not purged.

What to check

1Shut the system off and allow pressures and temperatures to equalize with ambient, ideally overnight or at least long enough to stabilise.
2Measure the standing pressure and the ambient temperature at the condenser.
3Convert the standing pressure to saturation temperature for the refrigerant in the system.
4Compare. If the saturation temperature is meaningfully above ambient, non condensables are present.

Condenser coil and fan operation

Rule out the far more common causes of high head pressure before concluding non condensables

Standing pressure versus ambient

The definitive field test for non condensables

Service history

A recent repair without evacuation is the usual origin

Low side pressure during off cycles

A system that goes into a vacuum when off can draw air in through a leak

Next step

The correction is to recover the charge, replace the filter drier, pressure test and repair any leak, evacuate properly to the target micron level with a stable rise test, and weigh in a fresh charge. Purging from the top of the condenser is not a reliable field repair on modern blends because it changes the composition of the remaining charge.