The use of a low temperature geothermal spring together with the heat energy still contained in waste water from the different therapy systems installed in a spa (shower, jets, bathrooms, Jacuzzis, pools, ventilation processes) can significantly reduce the operating and maintenance costs of the installation, covering part of the air conditioning needs of the building and the heating of thermal water to the appropriate temperature for therapeutic use. The object of the present work is to study the possible energy use of two sources of thermal hot water (spring and waste water) by restructuring of the existing spa so that it is more efficient from both a technical and economic point of view. In this first part, hot water needs are calculated and consumption presented according to the operation schedule on different types of day. Comparison is then made with the contribution that the spring is capable of making and the evolution of the thermal water in the tanks is studied. In a second work, energy and economic analyses will be presented. (C) 2000 Elsevier Science Ltd. All rights reserved.