I am heating my whole house with a woodburning furnace tapped into my existing gas furnace ductwork. What are safe temps for the ductwork? I just stuck my fluepipe surface temp gauge on my main duct. Right now they are fairly warm and the gauge says 120. I have had them a good bit hotter but it's late and the night isn't that cold, so I don't want to really want to crank it up. Also, sometimes my gauge is in the low temp range that is supposed to be creating creasote. But when I go outside and look at my chimney there is no smoke. Does it have to smoke to create creasote?