Some Building Simulation tools are efficient at calculating average room temperatures for every hour of the year. This does not provide information about how air flows and temperatures vary between one part of a room and another. CFD is used to calculate air velocities, temperatures and contaminant distribution at any point in the room. This may be of particular interest when wanting to study the behaviour of air diffusers, passive ventilation openings, thermal stratification, drafts and local comfort conditions. The fundamental equations of fluid flow and heat transport are solved in CFD calculations. This requires considerable computing power. This often means it is only feasible to calculate room temperatures and flows in this way at a particular point in time. In other words a ‘snap shot’ of the room behaviour is simulated.