SIMULTANEOUS SIMULATION OF BUILDINGS AND MECHANICAL SYSTEMS IN HEAT BALANCE

The current generation of building simulation software is based upon separate building and mechanical system and equipment simulations. This scheme evolved primarily because of memory limitations of the computers which were used to develop the programs . These limitations are no longer important so the separate building and system scheme needs to be reevaluated. This paper will specifically discuss experience resulting from introducing simultaneous system simulations into the BLAST program. BLAST currently uses a linear univariate control profile to describe the heating and cooling provided by the fan system as a function of room temperature during the loads calculation part of the simulation. Control profiles for each thermal zone are used to model the system response during the system simulation. This model of the fan system works very well for systems that provide amounts of heating or cooling that are dependent only on zone temperature. When the output of the fan system is affected by the outdoor temperature or conditions in other zones, the control profile model is no longer adequate. The conditions in the zones must be known in order to calculate the system output, but the system output must be known in order to calculate the conditions in the zones. So a more sophisticated representation of the mechanical systems is needed. This paper specifically discusses the results of doing a complete system simulation within the loads calculation portion of the program by using a shortened time step combined with lagging the two parts of the simulation. The effects of time step length on accuracy and computation time are presented.