  In detailled buiding simulation models, airflow modelling and solving are
still open and crucial problems, specially in the case of open buildings as
encountered in tropical climates. As a consequence, wind speed conditioning
indoor thermal comfort or energy needs in case of air conditionning are uneasy
to predict. A first part of the problem is the lack of reliable and usable
large opening elementary modelling and another one concerns the numerical
solving of airflow network. This non linear pressure system is solved by
numerous methods mainly based on Newton Raphson (NR) method. This paper is
adressing this part of the difficulty, in our software CODYRUN. After model
checks, we propose to use Picard method (known also as fixed point) to
initialise zone pressures. A linear system (extracted from the non linear set
of equations) is solved around 10 times at each time step and NR uses this
result for initial values. Known to be uniformly but slowly convergent, this
method appears to be really powerful for the building pressure system. The
comparison of the methods in terms of number of iterations is illustrated using
a real test case experiment.
