The current generation of building simulation software is based upon separate building and mechanical system simulations. While separate simulations are adequate for some buildings, there are many configurations which require a simultaneous simulation to completely model the interactions between the building and mechanical systems. Work is underway to develop a new version of the Building Loads Analysis and System Thermodynamics (BLAST)  energy analysis program which will simulate buildings and mechanical systems simultaneously. Several possible techniques for combining the simulations have been explored. 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. Control profiles for each thermal zone are used to model the system response during the building 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 outdoor temperature or by conditions in other zones, the present 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. Three options have been considered as possible solutions: Generating multivariate control profiles by precalculating the mechanical system response; Simulating the mechanical system in full during each iteration of the building simulation; Adding thermal capacitance to the zone air, using a time step much less than one hour, and calculating system and building conditions without iterating by using information from the previous time step. Each of these approaches has some potential to improve the simulation capabilities of BLAST and other heat balance based energy analysis programs. This paper presents some results of experimenting with these techniques.