A computational research of radiative-convective energy transport in large-scale enclosure with a heat-generating heater under normal room conditions has been conducted. The heater (underfloor heating system) is located at the bottom of the room. Employing the Boussinesq assumption, the control equations have been solved contemporaneously to receive both the velocity fields and temperature patterns. To generate the systems of linear equations using vorticity and stream function, the finite difference technique has been employed. The developed convective-radiative model has been validated through a comparison with several problems. The influence of heater size and location, internal surfaces emissivity from 0 to 1, Ostrogradsky number for a wide range from 0 to 5 on Nusselt numbers and both stream function and temperature distributions has been investigated. The results demonstrate that the influence of the thermal radiation on total heat transfer increases with surface emissivity of walls and heater surfaces.