Our furnace did the same thing. 1st trip out in new 20RDSE turned the furnace on about 3am, shortly after it kicked in the smoke detector went off, strong smell of burning wood. Turned it off aired the unit out reset the alarm. Later checking it out found the top of the furnace was covered in sawdust, sides and most likely interior. Attachment 1
Took the cover off under the oven (photo 2) and found there was a cover on the furnace that was never installed, couldn’t have been as the left screw would have gone through the air duct as the screw was too long, just left it on the floor! I cleaned all the sawdust I could get to and found the damaged left side of the furnace (photo 1).
Also had a leak in the shower (never used) from the factory. We called our dealer and were told it would be a month before they would have time to check it out! Not a happy camper.