In any laboratory facility using fume hoods, relatively large quantities of air must be circulated through the hoods to prevent the escape of fumes to the laboratory. Under safety and building code restrictions, this contaminated air cannot be recirculated but must be exhausted outdoors. This exhaust air also contains a considerable amount of heating energy; recovery of the heat - if economically feasible - is an attractive concept for laboratory building owners. The key to understanding an air-to-air heat pump is to recognize that the heating cycle requires the cooling of exhaust air being discharged to the atmosphere. By cooling the exhaust air stream - using it as a heat source - and transferring the extracted heat plus the heat of compression, the building gains useful energy. By using this reverse cycle perspective, the heat pump concept becomes a simple heat transfer problem.