The optimum energy consumption in an air conditioning system depends on many factors. Some of these are the following: the use of high performance devices, the utilization of these in the optimum efficiency range, a correct control strategy, a proper maintenance, etc. In air conditioning facilities most of the energy consumption corresponds to the thermal production, this being heat and cold. This thermal supply is commonly accomplished with boilers, chillers and heat pumps. In this article the efficiency of the reversible water-water heat pumps is analyzed, proposing a design that optimizes these elements. This improvement hardly requires modifications at manufacturing and operation levels, and it also contributes towards optimizing the energy consumption of these machines. (C) 2006 Elsevier B.V. All rights reserved.