Simulation is a basic part of the decision process .It should be used at all stages of building life cycle .But modelling refinements must be adapted to the problems to be solved
.Simplifications are required ,not for the computer ,but for our own understanding .''Nodal '' and　 ''admitance'' models are good examples of such simplification .HVAC equipment modelling and simulation include not prestigious tasks .It's,among others , necessary to perform a very careful counting of so called ''auxiliary consumption''(pumps fans...). This appears as not auxiliary at all ,with respect to the global energy balance. The most challenging developments for the future are located more or less at both ends of building life cycle :very early design and commissioning .