FACES is a simulation tool for selecting an appropriate heat source system in the early building design stage. Heat source systems have to be studied at an early design stage, because they are closely related to the floor plan. However, in an early design stage, most of the problems are unsolved, so that there is insufficient data for system simulations. In order to enable detailed and accurate studies for various kinds of buildings and heat source systems, FACES utilizes full-scale programs for heat load calculation and system simulations. However, these programs themselves require numerous input data. To bridge the gap between “insufficient data available” and “numerous input data required," automatic designing algorithms and numerous background data are built into FACES. FACES makes it possible to appropriately study the amount of information available in any design stage, providing easy operation and accurate results.