In this article, a new design for reversible air-air heat pumps is proposed. This design is not based on the refrigerant changeover, as is proposed by ASHRAE, but rather is based on the inversion of the air flow on the ducts. Thanks to this modification it is possible to use any air-air refrigeration machine for air conditioning as a reversible heat pump. The new design is more efficient, especially when the building requires greater air quality and therefore a larger amount of outdoor air. It also increases its efficiency as the outdoor temperatures become more extreme. This improvement will make it possible to use heat pumps, with acceptable performances, in more rigorous climates than those in which they are used today. (C) 2007 Elsevier B.V. All rights reserved.