Predicting pyrolysate fuel generation rate form a condensed fuel is an important component of fire models. For validating or tuning these subgrid models, mass loss rate (MLR) of small samples exposed to known radiant heat flux is measured in standard devices such as the cone calorimeter and Fire Propagation Apparatus (FPA). In order to accurately predict the MLR, pyrolysis model input parameters should represent actual fuel degradation process. The pyrolysis process is intricately coupled to heat transfer in the fuel. A few experimental studies on non-charring polymers  have shown that these results are sensitive to back boundary, therefore, special care should be taken in interpreting results from these bench-scale instruments. To elucidate the effect of back boundary on the pyrolysis process, here a numerical investigation is carried out for both charring and non-charring fuel material. In this work heat transfer process into the fuel is determined to understand the pyrolysis process. Heat transfer process in the two categories of fuel are quite different. For each fuel category study is carried out for 1 mm and 10 mm thick fuels exposed to low (20kW) and high (100 kW) fluxes. This work aims improve the understanding of burning process in these bench-scale instruments especially for charring substances which have not been addressed in literature in this regard. The study shed important insight for proper use bench tests as a versatile tool for fire safety engineers. © Published under licence by IOP Publishing Ltd.