Building Energy Simulation (or Dynamic Thermal Simulation) tools are used to study the heat transfer and energy flows that occur during the buildings operation. Heat transfer and bulk fluid flow between the building and the internal and external environments is calculated along with the energy and fluid flow in the HVAC systems and equipment. The outputs of most interest are usually peak heating and cooling demands, room temperatures, overall energy demands, emissions and energy costs. Calculations are often carried out using annual operating and climate data.