problem with the time step

["David DELAUNE" <david.delaune@xxxxxxxxxxxxxxx>]

Dear TRNSYS-Users,

I am modeling a boiler with TRNSYS. At first, the model has been written in
Fortran  and  tested outside TRNSYS. It works with a constant internal time
step  in  order  to  solve  the  differential  equations with a Runge Kutta
method.  Its  results  have  shown  the  required  dynamics to describe the
performance of the boiler.

Into  the  original  program, I used a time step of 0.001h and it delivered
the appropriate results. Under TRNSYS, I declared the internal time step as
a  parameter  and  assigned  it  the same value (0.001h). But it seems that
there  is  a "conflict" between the internal time step and the general time
step  of  simulation.  As a matter of fact, I can't put in the main control
card  a  general time step much bigger than the short internal time step of
the  subroutine. The bigger the ratio between the two values is, the longer
it is to reach the stationary regime of the boiler.

I  also tried to use the Diffeq subroutine from the Trnsys library to solve
the differential equations and avoid using an internal time step. Actually,
the  model  does  not  have  the  required  dynamics either. It reaches the
stationary  regime  but  it  takes a time much longer than the model tested
outside TRNSYS.

Can anyone help me to solve this problem ?

Best regards,

David DELAUNE
Gaz de France - Paris