A thermal valve unit is provided for use with an oil filled screw compressor. The thermal valve unit can be affixed to an exterior pipe segment and can include a thermally responsive element in thermal communication with compressed air traversing through the pipe segment. The valve unit can include a movable valve member actuated by the thermally responsive element that opens and closes oil passages to selectively route oil either to a heat exchanger prior to being delivered to the oil filled compressor, or directly to the compressor without passing through the heat exchanger.