Non-condensables are gases that will not condense into a liquid within the operating temperatures of the refrigeration system. Air and nitrogen are the most likely non-condensables you will see. Air can enter a refrigeration or A/C system when improper service and evacuation procedures are not followed or when you have had a leak on the low side.
Nitrogen can enter the system in other ways. The new Dry R22 A/C units and most refrigeration equipment are shipped from the factory with a holding charge of dry nitrogen, not refrigerant. Once the refrigeration lines are connected, the service valves need to be opened and all of this nitrogen holding charge must be removed. The condensing unit should then be evacuated to 500 microns along with the lines and evaporator coil.
When working with precharged equipment like R410A, care needs to be taken during leak checking procedures. Some A/C manufacturers require leak testing by pressurizing the system with nitrogen to 400 or even 500 PSI. These high pressures can potentially push nitrogen past the stop valves on the condensing unit and into the condensing unit refrigerant charge. To prevent this from happening, make sure to tighten down the stop valves completely before pressure testing. The stop valves may be sealed enough to hold the R410A in but not to keep the high-pressure nitrogen out. Always use a nitrogen regulator with an accurate gauge to ensure you don’t over-pressurize your system. Make sure to check the manufacturer's recommendations before pressure testing.
If you do get nitrogen in the system, you will have high head pressure with high sub-cooling (cool liquid line) before you get the system fully charged. Suction pressure may be low and system performance will be poor. In most cases, you will have high evaporator superheat and a high suction line temperature, but superheat may be normal. To confirm, shut the system off and let the pressures equalize. Allow the condenser to cool to the outdoor air temperature. Check the pressure on the high side gauge and convert it to temperature using a PT chart. This saturated temperature should be within a couple degrees of outdoor air temperature. If this saturation temperature is any higher, you have nitrogen or non-condensables in the system.
The entire charge should be removed into a clean and dry recovery cylinder. The system should be evacuated to 500 microns and recharged using new refrigerant. The old refrigerant can be reused after purging the nitrogen or non-condensables from the recovery cylinder.
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