A.S.H.R.A.E. (American Society of Heating, Refrigeration, and Air-conditioning Engineers) has studied this problem. A.S.H.R.A.E. estimates that this thermal barrier accounts for up to a 30% reduction in efficiency within the first 5 years of the equipments life. As studied by A.S.H.R.A.E., during the refrigerant compression process, oil migrates from the compressor to the coil system contaminating the metals that transfer heat. Compressors that use oil for lubrication do not prevent oil from fouling the coil system. Even the smallest amount of oil in the coil system creates a film or coating that will hinder heat transfer reducing the efficiency of the unit.