[Fire chief 2020 epa certified wood burning indoor furnace fire chief 2020 epa certified wood burning indoor furnace, According to the EPA, smoke is the most visible sign of an inefficient wood furnace; hot-burning fires create little smoke. Smoke could indicate a closed damper, dirty furnace or wet wood burning inside the stove. Eliminating the cause of the smoke will make your wood furnace operate more efficiently and keep its heat in the home. (See Reference 2) ]