A heat exchanger system, for transferring waste heat from a stream of exhaust air produced in a dryer to a stream of fresh air directed to flow into the dryer, including a retriever, or retriever, comprising an array of a plurality of substantially mutually parallel, substantially vertical heat-transfer conduits having square cross-sections arrayed in alternating staggered rows relative to a perpendicular to the general direction of the flow of the stream of fresh air into the dryer, wherein each heat-transfer conduit is oriented to have one corner facing in the general direction of the flow of the stream of fresh air into the dryer. Associated structure for housing the retriever, supporting it in the flow of fresh air, and directing dryer exhaust air into the heat-transfer conduits is also included. The heat exchanger system is adaptable to a wide variety of dryers, and may be built-in.