I have an old pot-belly stove in my woodshop. I burn nothing but hardwood in it. Recently I have noticed a few sparks coming out of the top of the pipe outside. It is a double wall pipe going up through an insulated attic in the woodshop. I asked a neighbor if he thought it to be a concern and he said it may be creosote burning off and it wasn't necessarily a bad thing. What do you think?