Question: As I type this, I am having a coal burning stove installed in a basement in northern MI. The pipe is single wall except where it penetrates the main floor and the cathedral roof where it is Class A factory double wall insulated. I asked the installers about direction of crimps and sealing which they indicated the double walled factory pipe screwed together OD vs ID and single walled was overlapped and secured with sheet metal screws. Can you please expand on this for me? And, oh yes, I enjoy your Forum. Answer: The installation sounds illegal because you cannot have single wall pipe anywhere except where you are looking at all of it in the room with the stove. In other words, when you go from single to Class A the first time, that's the last time. You may not switch to any other type of pipe later in the run. Class A double wall insulated piping usually does have the crimps (or smaller, inside, flange) downward, but should always be installed in accordance with manufacturer instructions. The advice on crimps is only given for single wall pipe which is installed at the job site.