Three dimensional mixed convection in an ice rink heated by thermostatically controlled radiant heaters was simulated numerically using the standard k-ε model with wall functions. This large building was modelled under transient conditions by considering the real outdoors atmospheric conditions for a typical spring day in Montréal, Canada. Results indicate the usefulness of the CFD technique as a powerful tool which provides a detailed description of the flow and temperature fields as well as the heat fluxes into the ice. The most important results are: - Heating is needed only during the night (from 22h to 7h) when the outdoors temperature is relatively low. - The On/Off switching of the radiant panels influences the temperature profiles throughout the ice rink; even the ice surface temperature is affected although the view factor between these two surfaces is zero. - The radiation heat flux towards the ice increases significantly when the radiant panels are turned On; this can influence ice quality. - The resurfacing operation increases the ice temperature and, by convection, the temperature of the air immediately above the stands. - The air near the ice surface (1 meter over the ice) is essentially stagnant and significant air velocities can only be found above the stands and near the ceiling above the ice. - The volumetric flow rate and temperature of the air evacuated from the building vary significantly between the different outlets.