A direct gas-fired process air heater assembly that utilizes a premix burner applied to a shrink wrap conveyor oven for uniform dissemination of heated air into the a recirculating air stream of a circulating blower, said air circulating blower directing the heated air into the shrink wrap chamber or tunnel for shrinking polymer film around packaged goods for shipment and/or storage. According to various aspects, exemplary embodiments are disclosed of the direct gas-fired packaged process air heater assembly construction features including the application of premix burner technology.