In smoldering reactors, a simple change in the way air is supplied to the system directly impacts the entire combustion process. Buoyancy forces may compete with the airflow entering the reactor, resulting in recirculation zones and stagnating the smoldering front. This paper assesses the competition between free convection and vacuum-induced airflow in a shrinking smoldering bed through dimensionless analysis guided by reliable combustion experiments and numerical heat transfer simulations for 22 scenarios. T