A goal of the unsteady state calculation of long-distance heating nets is the determination of the time course of the dimension variables of the system with time dependent entry dimensions.

Entry dimensions are the supply requirements and the operating variables.

Under supply requirement the time-dependent heat requirement of the individual consumers is understood. This supply requirement depends on meteorological dimensions, like outside temperature, precipitation, sun exposure, wind velocity and temporal dimensions, like time of day, weekday, season. Usually, the supply requirement is not well-known in advance for individual consumers; usually it is determined by means of static prognosis models for the total network, more rarely for certain consumer groups or large single consumers. The supply requirements of the individual consumers must then be derived proportionately from the summary requirement.

Operating variables are the dimension which the operator of long-distance heating systems can influence during operation, without causing constructional changes. The sum of all current values of the operating variables does not normally uniquely specify the current operating condition of the plant together with the current supply requirement; usually the history of most dimensions must be considered additionally.

The most important operating variables are the entry temperatures and feed performances of the suppliers as well as the rule concepts of all adjustable pressure increase stations and bypasses. If a simulation works non-interactively, then all dimensions mentioned must be present as a time-dependent management plan before beginning the calculation.

The time courses of the variables of state of the system, i.e. the pressures, mass flows and temperatures are the results of an unsteady state network simulation Each entry and exit dimension of the system can be reconstructed by a complete record of system state variables from the beginning of the simulation time up to the current simulation time, in order to continue simulation after an interruption with new inputs, for example.

Usually it is not a priority goal of unsteady state network calculation to determine the thermal and hydraulic variables of state but there are interests in characteristic dimensions of the network such as the total heat losses, the network retention status, the supply run times and the total pump performance. These summary dimensions can be easily derived after network simulation from the variables of state.