Well now that we're doing updates, I'm getting a frustrating smell in the house sometimes.
I'm starting to wonder if I have a leak in my heat exchanger.
The first winter, on occasion I'd get a smell the same as the burning paint/oil during the first couple fires. It was usually right after the damper closed. It didn't smell like wood smoke, but paint/oil smoke, so I figured it was just a momentary hot spot due to closing the damper and the burn mode/zond shifting (or some theory about something that I didn't understand).
Last winter, it still happened a little bit, but not as much. I wondered if the air blowing out of the jacket near the HX door was stirring things up/out of the HX gasket into the basement air, so I put a metal pipe on top of the HX to obstruct airflow and that might have helped a little.
This winter, the first few fires the ductwork (not used last summer) smelled nasty, not super moldy, but more than just dusty. I also would get a fire kind of smell in the house after the damper closed 20-30 minutes into the burn. I putzed around and put a small shim under the damper so it would close 95%, and that seemed to help. But by now I still get a fire smell maybe 1 or 2 burns per week. It didn't used to smell like campfire, but it's starting to smell more that way.
With the recent warm spells, the furnace was cold for a few days, and the bad duct smell was back. So now I have an occasional burning paint smell, and if the system hasn't been hot for a while then I have a dank air smell from the ducts and/or furnace. I wonder if it's coming from whatever causes the paint smell.
I've never had one of the CO alarms make any mention of it, which are 3-5 years old.
I never get the smell when the damper is open.
I popped the sides off the furnace a couple months ago and couldn't find any missing welds like
@brenndatomu had.
Anyways, I'm open to any ideas. I haven't thrown in the towel on this Tundra yet, but things like this make me more likely. Especially because I'm going to start using the ductwork to ventilate a rec room in the summer, but not if that nasty duct smell arrives when the furnace goes cold.