Thank you for your reply let me explain the problem. I'm simulating the effect of ventilation (air) in a slightly complex mine tunnel that has toxic gas (thoron) coming from the walls. I wrote some macros to introduce the toxic gas in the domain from the cells next to the walls. I'm only interested in the concentration distribution of the gas when there are different ventilation speeds and nothing else. When I switched from steady state to transient I didn't change the default max iterations/time steps which is 20 cause I thought it would be enough. My concern is that the simulation is converging continuously at just 2 iterations/ time steps out of the default 20 max iterations/time steps which I think is too soon. Furthermore when I play around with the time step size by decreasing/increasing it still converges at 2 iterations/time step hence why I think maybe I'm doing something wrong.