You can find the model and serial numbers (M/N and S/N) for your furnace by removing the top-front service panel and looking to the left side of the interior cabinet or the top of the blower deck, depending on whether your system is an 80% or 90% AFUE unit. Also, the orientation (horizontal or vertical) and flow direction (down or up) of your furnace may impact the position of this information. For instance, if your furnace is installed horizontally, on its left side, the numbers will be at the bottom of the interior cabinet or left side of the blower deck.