The air-to-air heat exchanger (a fan powered ventilation device that recovers heat from stale outgoing air) is explained in this six-part publication. Topic areas addressed are: (1) the nature of air-to-air heat exchangers and how they work; (2) choosing and sizing the system; (3) installation, control, and maintenance of the system; (4) heat exchange controls; (5) cost effectiveness factors; and (6) state of the art assessment of heat recovery technology. Appendices contain a discussion of current ventilation standards and lists of suppliers, suggested readings, and selected Department of Energy Appropriate Technology Small Grants Program awards under these headings: agricultural use of air-to-air heat exchangers, residential applications for air-to-air heat exchangers, residential use of heat recovery, and commerical use of heat recovery. (ML)