This article presents the concepts of an air-source heat pump for simultaneous heating and cooling (HPS) designed for hotels and smaller residential, commercial and office buildings in which simultaneous needs in heating and cooling are frequent. The main advantage of the HPS is to carry out simultaneously space heating and space cooling with the same energy input. Ambient air is used as a balancing source to run a heating or a cooling mode. The second advantage is that, during winter, energy recovered by the subcooling of the refrigerant is stored at first in a water tank and used subsequently as a cold source at the water evaporator to improve the average performance and to carry out defrosting of the air evaporator using a two-phase thermosiphon. Unlike conventional air-source heat pumps, defrosting is carried out without stopping the heat production. A R407C HPS prototype was built and tested. Its performance on defined operating conditions corresponds to the data given by the selection software of the compressor manufacturer. The operation of the high pressure control system, the transitions between heating, cooling and simultaneous modes and the defrosting sequence were validated experimentally and are presented in the second part of this article [1]. (C) 2010 Elsevier Ltd. All rights reserved.