Experimental campaigns and numerical simulations were carried out to investigate the effect of air distribution on the performance of a small-scale fixed-bed boiler fed with biomass. Despite the small dimension of the fire pit, temperature measurements just above it showed a large spatial variation, thus indicating that modeling approaches based on perfectly stirred reactor conditions for the biomass bed are not suited. Moreover these approaches, often used for small boilers, suffer of high uncertainty related to the inlet turbulence levels that affect the mixing of reactants and thus reaction rates. Instead the representation of the biomass bed as a porous medium with defined sources and sinks of chemical species and energy, may provide a cheap and effective strategy to take into account the distribution of the primary air and overcome uncertainties on inlet turbulence. Results are encouraging with a good prediction of the trends of the available measurements. Significant discrepancies were noticed in some locations, hence calling for further efforts to improve the description of the kinetics as well as of the biomass dimensional distribution needed for the definition of the porous medium.