Unremoved ashes cause corrosion. Failure to remove ashes allows the airflow to get blocked and the furnace will not function properly. Stay away from ash augers. Grates will not hold up to high heat in air injection furnaces and they tend to warp. Important point - it takes just as long to shovel ashes from an ash-pan as it does directly from the furnace. Most ash-pans are not insulated underneath and heat loss is tremendous. Ash removal is an ongoing job in an outdoor furnace without forced air draft. If you choose not to buy a furnace with forced air draft, then try to minimize the ash removal chore by looking at furnaces with large doors, and easy access to the firebox.