We have a large house and are operating 3 pellet stoves in different areas. An Enviro Insert, an American Harvest 6039, and a Vogelzang slimline. All three are running the SAME pellets.Okanagan Platinum. The Enviro is exhausted into a 3" or 4" liner through the fireplace chimney. It gets its fresh air from the room. The American Harvest is exhausted directly from 6" pipe into the lower level chimney. It also gets its fresh air from the room. The Vogelzang is exhausted into 4" double walled pipe directly outside. It was getting its fresh air from the room but we recently changed it to get fresh air from outside. (it had an E3POF error and smouldered sending smoke out the fresh air intake filling the house). The Vogelzang is forming a sort of shiny substance almost like varnish in the inside of the stove. I assume it is creosote. Neither of the other stoves has this. Should I be concerned about this? Is there something I should be doing to correct the problem? Since the "smoke" problem I have lost confidence in this stove.