Your thermal valve controls the flow of oil from the sump tank to the oil cooler. The valve is integral when the compressor is cold from being stopped or when it is running in very cold conditions. When the compressor oil is cold, the valve is bypasses the oil cooler so no oil flows through the cooler. This is done to heat up the oil as soon as possible to remove water condensate. Low temperatures are the number 1 enemy of screw compressors. Air Compressor Services' 128786-180 replacement thermostatic valve was specifically engineered to replace the Quincy branded valve part number 128786-180.