Low-dimensional discrete linear ventilation models have been studied by Cao and Meyers (2012). In the present study, we investigate the validity and applicability of linear ventilation models for heavy-gas dispersion by employing Reynolds-averaged Navier-Stokes (RANS) simulations. A simple benchmark ventilation case is considered under isothermal condition. Considering large density differences from pollutant gas and fresh air, the effect of buoyancy force has been taken into account in turbulent production term to obtain correct diffusion behaviour. A low-Re k-ɛ model has been selected and the generalized gradient diffusion hypothesis is used for buoyancy source term. It is concluded that the flow equations decouple from the concentration equation when the ratio α of air mass-flow rate m_a to pollutant mass-flow rate m_p is larger than 10^5, which is 10 times higher than the range without density differences (α>10^4). The dependence of the coupling on Richardson number Ri is also studied. It is found that the higher the Richardson number, the larger the α should be to decouple equations.