The accuracy of the calculation is influenced by a number of parameters and measures.

First of all the quality of the basis component data is of paramount importance.

The next is the convergence criterion. The default value is 0.01 K indicating that the iterations continue
until temperature variables of the entire plant is at maximum changing by this value.
It is possible to use other kinds of variables as convergence criterion.
These parameters are set in the accuracy dialogue (illustrated below) entered though the

Then there is the time step when calculating a time span. This should be set depending upon the degree of time-dependent
boundary conditions and controls. You should test the result by altering the time step, and then set it to a proper level.

In case your plant has component(s) with a heat conduction element, then the details of this calculation
will also influence the total accuracy.
The accuracy dialogue also have a section to control the time step of heat conduction calculations.