Hi,

I have a gas furnace that is currently vented horizontally out the foundation wall of the basement. The flue pipe is single-walled pipe. A 5-ft section of the pipe goes through a stud wall, but the home owner left only 1" clearance. I want to replace only that section with Type B (double-walled) flue pipe. Can I do it? How do I connect the new Type B into the existing single-walled pipe?

Yes you can replace the single wall pipe with the double wall pipe, the only way that you can connect to two together is to connect the single wall pipe into the double wall pipe, after you switch to the double wall pipe you can not switch back to the single wall pipe. You will have to use double wall to where the flue terminates.
You make the connection by slipping the single wall into the inside pipe of the double wall pipe, if it does not slip in easily you may have to crimp the single wall with a crimping tool.

Venting of a furnace is one of the most important things to get correct, double wall pipe is cheap compared to your life, go with double wall pipe all the way and make sure it is pitched upward at a 1/4" per foot and 1" from all combustibles , good luck, Mike