This paper studies the flatness-based control of a nonlinear multi-input multi-output heating, ventilating and air conditioning (HVAC) system. First, a novel method for checking flatness property is used to verify that the given HVAC model is flat and that the flat outputs are the temperature and the humidity ratio of the thermal space. Since the flat outputs are also the variables, which one usually wants to control, it is relatively easy to construct a flatness-based feedforward control for the HVAC model. We define the appropriate trajectories for the flat outputs and then the controller is computed from the parametrization of system inputs in terms of flat outputs and their derivatives. The trajectories of flat outputs are chosen such that the temperature and the humidity ratio converge asymptotically from the initial values to the desired values. Simulations are made, which show the effectiveness of the approach. DOI: http://dx.doi.org/10.5755/j01.itc.46.4.17697