Basement stove has single wall to ceiling. But then you say the main floor also has single wall? It should be class A chimney pipe after passing through the first ceiling.My manual says 15' mininum between stove top and chimney top. I don't see anything in it about sea level, but I'm at 1755 ft elevation.
I always close the bypass after the needle on the cat probe goes to the active zone.
It appears to be single walled chimney inside the main floor, but then goes to double walled after that, after it goes through the attic.
The single walled would be mostly warmer since it's inside the house, but I suppose with cold air if it goes down the chimney.... But, the initial fire is partly to warm the chimney to increase the draw. After it's warmed up, I would assume it stays warm being inside the house and doesn't have any real cold acting against it.
A few weeks before I got my chimney serviced, I looked up with a flashlight and mirror, it looked pretty good, a bit of ashy soot, grey in colour. But, then after a chimney fire, I noticed a bit of gunked up creosote right at the bottom of the chimney, where it meets the stove. Again, we did have a wicked cold snap here (like everywhere else that got that shocker of the Arctic Dome ooze down from the north).
I do have a chimney cleaning kit, a soot eater and I'm planning on cleaning out the chimney again this coming week to see how it's been doing in the warmer temps.
Another code violation is that furnace intake right next to the stove. There is 10’ separation required and even with that it seems like a bad idea to suck on a room that you’re also trying to feed a stove with.