Auto A/C Terms - Glossary
Foaming

In auto A/C systems, foaming is a term that is used to refer to the formation of small bubbles in the refrigerant oil. That typically happens when there is a rapid boiling of the refrigerant that is mixed with the oil.

This condition can be seen on A/C systems that use a liquid line receiver drier that includes a sight glass. Liquid line receiver driers are mounted in the line between the A/C condenser and the expansion valve inlet. When foam is present in the sight glass, it is typically an indication of a very low refrigerant charge in the system.