Unsteady state thermal calculation takes place in a similar way to steady state thermal calculation via the successive use of in accordance with Gl. 20.39 the topological order of the nodes.

The exit temperatures for the new time ti+1= ti + Δt are calculated for the pipes as main causers of the thermally transient procedures in accordance with Gl. 20.32, where for the period between the two calculation times ti and ti+1 the calculated mass flow distribution at time i is accepted as constant.