Saw a huge difference in draft after fixing a hidden offset

I cleaned a chimney in an old house in Burlington last month that had a weak draft for years. The owner said they always had smoke roll back into the room. I ran my camera and found a 30 degree offset in the flue liner about 10 feet up that was almost totally blocked. Came back yesterday after fixing it, and the draft test showed a pull of 15 pascals, up from just 3 before. Has anyone else had a simple offset cause that much trouble?

3 comments

3 Comments

masonbell1mo ago

Wow, that's one stubborn chimney!

lisa51mo agoTop Commenter

Hidden offsets are killers. Found one in a 1920s bungalow last year, just a slight bend. The fireplace would barely draw. Cleaned it spotless, no change. Finally scoped it and that little kink had decades of glazed creosote built up. After we rodded it out, the thing pulled like a jet engine. It's crazy how such a small thing can wreck the whole system.

the_charles1mo agoMost Upvoted

Yeah, that "glazed creosote" bit is the real clue. That stuff isn't from a simple offset, that's from burning at too low a temperature for years. The offset just gave it a perfect spot to catch and bake on. If they don't adjust how they use that fireplace, that hard glaze will just come right back, even after rodding. It's a fuel and burn practice issue more than just a flue shape thing.