I'm trying to remove a damaged brass closet flange from a 3" copper drain pipe. It's my understanding that the flange is soldered to the pipe in the same way that copper water lines are soldered, so I'm trying to un-sweat it. But something's happening with the sewer gasses when the pipe starts to warm up, and the flame seems to be drawn down the pipe instead of concentrating at the top. I've spent a lot of time trying to get the pipe/flange hot enough to release with no results. Any tips for dealing with the draft issue? I might also go out and get a MAP torch.