Getting closer and closer to pulling the trigger on an Oslo to replace my VC Encore. The Encore has an oval to round adapter (not sure of the right term) and flows into my 8" chimney pipe. The Oslo is 6" so I'd need to step the chimney down to 6". Is there anything I need to know here in terms of impacts to draft or anything? Is this a common thing? The shop I'll be buying from said it was no big deal but wanted to check with you guys. Thanks.