Ashtrol 3 pack. Used as a furnace firebox pH modifier. This is an absolute must. A small scoop with each load of wood will do wonders with helping to reduce corrosion of your firebox. Ashes are incredibly corrosion and if left unchecked they'll destroy the firebox. Equally important is MolyArmor 350 Corrosion Inhibitor which is added to the water (p/n 2900630). These are basic maintenance items that will absolutely determine how long your furnace will last.