The boiler flue and air supply need only plastic pipe to connect to the outside. Most new furnaces use PVC now instead of metal flue. I didn’t want to put another hole in the roof so I ordered a concentric flue that joins both pipes, one inside the other, so only one hole has to go through the roof. The pipe is assembled with pipe glue as used in plumbing. Still it was difficult to line up the pipes and get everything cut to the right lengths. Plus I used leftover pipe from plumbing so had to splice one run. The tape was used to hold the pipe while it was connected. I used the existing metal pipe and cap and bought a vent end that I cut with tin snips to hold the concentric vent. Duck tape was stretched over the sharp edges of the cut metal, the vent was pushed through the hole, and the end was screwed into the bottom of the metal flue.