Gases such as air or nitrogen in a refrigeration circuit that will not condense at operating conditions.
Non-condensables collect in the condenser, take up space that refrigerant should condense in, and raise head pressure. They usually get in through poor evacuation, open lines, or a leak on the low side that ran into vacuum.
A clue is head pressure higher than expected without a matching rise in subcooling (compare with an overcharge, which raises both). The fix is to recover the charge, evacuate properly and recharge.
Exam work areas
F · Maintenance, service and troubleshooting